Download What you still need to know - Macmillan Education South Africa

Solutions for all
Geography
Grade 10
Learner’s Book
L Dube
•
K Marimuthu • A Mthembu
P Ranby • C Vlok
Solutions for all Geography Grade 10 Learner’s Book
© T. Dube, K. Marimuthu, T. Mthembu, P. Ranby, C. Vlok 2011
© Illustrations and design Macmillan South Africa (Pty) Ltd, 2011
All rights reserved. No part of this publication may be reproduced,
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Any person who commits any unauthorised act in relation to this
publication may be liable for criminal prosecution and
civil claims for damages.
First published 2011
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Published by
Macmillan South Africa (Pty) Ltd
Private Bag X19
Northlands
2116
Gauteng
South Africa
Design and typesetting by Resolution
Cover design by Deevine Design
Typeset in 12pt Palatino
Cover image from VMS Images
Illustrations by: Chris Vlok, James Whitelaw, Dominick Mortier, Macmillan
The publishers have made every effort to trace the copyright holders.
If they have inadvertently overlooked any, they will be pleased to make the necessary
arrangements at the first opportunity.
ISBN: 9781431006694
WIP: 3074M000
It is illegal to photocopy any page of this book
without written permission from the publishers.
Photograph acknowledgements:
AAI Fotostock (pp 28, 51, 199, 218, 349, 351, 352)
Digital Source (pp 33, 34, 51, 188, 210, 211, 255, 281, 297, 308, 338)
Gallo Images (pp 1, 218, 219)
GeoEye (p 66)
INPRA (pp 52, 208, 209, 233, 259, 276, 298, 343, 347)
Macmillan (pp 54, 56, 66, 67, 167, 189, 265)
NASA (p 95)
Peter Ranby (p 321)
PixelBistro (p 87)
The Bigger Picture (pp 110, 199, 210)
VMS Images (pp 28, 29, 52, 173, 208, 219, 222, 223, 227, 234, 261, 268, 278, 279, 312, 332)
e-ISBN: 978-1-4310-1739-3
Contents
Topic 1
Geographical skills and techniques .....................................
1
Topic 2
The atmosphere .......................................................................
87
Topic 3
Geomorphology ......................................................................
159
Topic 4
Population ................................................................................
227
Topic 5
Water resources .......................................................................
297
Dear learner
This year you will use your geographical skills and techniques throughout the
year. The first topic in this book gives you all the theoretical information and
background you need for these skills and techniques but these will be practised
and applied in all the other topics. Some examples are fieldwork in the
Atmosphere topic, contours in the Geomorphology topic, GIS in the Water
Resources topic – just to name a few! If you struggle with any of these skills and
techniques, your teacher will direct you to the relevant section in Topic 1 where
step-by-step guidelines are given for each skill and technique required by the
curriculum for Grade 10.
Good luck and enjoy becoming a skilled geographer!
Note to teacher: Please refer to the CAPS table on pages 6 to 17 of the
Teacher’s Guide to see where and when geographical skills and techniques
need to be applied/practised in the topics for the Grade 10 year.
c
T
Topic 1
opi
1
Geographical skills and
techniques
What you will learn about in this topic
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How to read and interpret maps, and how to use maps to locate places and
recognise spatial patterns
The four elements of maps: plan view, proportion, spatial distribution, map
language
How to use maps to calculate distances, areas and directions
The value and limitations of different map projections
What atlases have to offer and how to make the most of an atlas index
Aerial photographs and orthophoto maps as sources of spatial information
How satellite remote sensing works
What a Geographic Information System (GIS) is and the reasons for its
development
How to represent the real world in a GIS as points, lines and areas.
Let’s talk about this topic
Geographers approach real-world problems from a spatial perspective. This
means that they ask questions such as ‘Where is…?’, ‘What are they?’, ‘How
many are there?’, ‘Why is it here and not there?’. To answer such questions
geographers use measuring devices, specialised technology and different
sources of spatial data.
Which of the measuring devices, specialised technology and sources of spatial
data shown in the collage above, can you identify? With which of them are you
familiar?
Geographical skills and techniques
•
1
Map reading and interpretation skills
What you know already
Practice makes perfect
In lower grades you learned an impressive variety of skills related to the
interpretation of maps, aerial photographs and also satellite images. Among
others things, you learned how to:
• draw elementary maps to show what is where;
• recognise map symbols and relate these symbols to real world
phenomena;
• describe landscapes; and
• extract information such as location, spatial patterns, distances, areas and
directions from maps.
The problem is that skills tend to get rusty rather quickly if you do not apply
them regularly. In this unit, as well as in subsequent units, we will give you
the opportunity to not only cement existing map skills but also to learn more
advanced map skills. Many of the explanations and examples of skills we are
sharing with you in this section are based on an extract from one of the
1:50 000 topographic maps of South Africa. It is the map of Nelspruit shown
on page 4. Ideally we should have provided explanations and examples based
on your local environment. It is of course impossible for us to do so for each
and every school in South Africa. In a later section, The South African map
reference system, you will learn how you can determine the reference number
of the 1:50 000 topographic map showing your school.
k
Chec lf
myse
2
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1. Draw a map to illustrate the layout of the school grounds.
2. Explain how you can use a map, a piece of string and a
ruler to calculate the length of a winding road.
Map reading and interpretation skills
Topic 1
Word bank
Map:
a generalised and reduced representation of a
portion of the curved Earth on a flat surface
Map elements:
the four characteristics that make maps quite unique.
Maps offer a plan view of the Earth that makes it
possible to see the distributions of phenomena in
correct proportion. The fourth element is the symbol
language used to tell the map reader what the
nature of the mapped features is
Topographic maps: a map showing both physical and constructed
features found on the Earth’s surface
What you still need to know
Understanding the plan view of maps:
Map element 1
Figure 1.1: Features look rather different when viewed from above.
Maps show the landscape as if it
were viewed from directly above.
From this view, what we call a plan
view, the landscape and its features
look quite different from the groundlevel view we are so accustomed to.
Figure 1.1 should convince you of
this. Because the view looks different
from the view we are used to, maps
tend to remain ‘uncertain territory’
for many people. It is only through
practice that reading the plan view
will become second nature to you.
The easiest way is to compare the
look of features in your environment
with a map of your local area.
Map reading and interpretation skills
•
3
Mean magnetic
declanation 18º11'
west of True North
(June 2012). Mean
annual change 3'
eastwards (2012-2016)
18º11'
See Figure 1.3 for a key to the symbols used on the 1:50 000 map sheets.
Figure 1.2: Extract from the official 1:50 000 map 1:50 000 2530BD Nelspruit
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Map reading and interpretation skills
Topic 1
Figure 1.3: A legend for the official 1:50 000 maps used in this book
Map reading and interpretation skills
•
5
Classroom activity 1
Do this in your group:
Viewing a landscape in plan view has some very useful advantages.
Figure 1.4 shows a ground-level view and a plan view. Study the views
and evaluate them in terms of whether we can use them to answer
Where?, What?, How many/much? and Why? questions.
(a)
(b)
Figure 1.4: A ground-level view (a) and a plan view (b)
Homework activity 1
Draw the objects listed below in plan view. Look out for the three traps
we have set.
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a soccer field
a tree
an orchard
a forest
a rugby post
a single-storey house
a double-storey house
a railway line
a steep mountain pass
a railway tunnel.
Map reading and interpretation skills
Topic 1
Word bank
Absolute terms:
Denominator:
Numerator:
Relative terms:
True to scale:
in the context of this unit ‘absolute terms’ refers to a
precise and exact statement. Example: the price of a
new car is exactly R195 000. There can be no
misunderstanding about how much you need to pay
for the car
the value (e.g. 1 000) written below the line in a
1
fraction such as 1 000
the value (e.g. 1) written above the line in a fraction
1
such as 1 000
in the context of this unit ‘relative terms’ refers to an
element of vagueness – it is not exact and precise.
Think about expressing the price of a new car as ‘about
20 times my monthly salary’. If we do not know the
monthly salary we cannot determine the price of the
car
a map is true to scale when the sizes of all phenomena
and the distances between all places on it are in the
same proportion to each other as they would be in
reality or on a globe
What you still need to know
Proportion and the importance of scale:
Map element 2
Expressing scales in absolute terms
Although maps are drawn much smaller than the reality they represent, they
should show things in proportion or true to scale. This involves drawing
features so that their relative sizes on the map correspond to their relative
sizes in real life. If the real distance between the school and the spaza shop is
twice the distance between the school and your house, the map distance
between the school and the spaza shop should also be twice the map distance
between the school and your house.
Map reading and interpretation skills
•
7
(a)
Starting line
Finishing line
(b)
0
10
20
40
60
80
100 m
Figure 1.5: A 1:1 000 line scale of an athletics track
The scale of the map tells us how much smaller than reality a map has been
drawn. Map scale simply means the size of a feature on the map as compared to
the size of the same feature on the Earth. Figure 1.5(a) shows a plan view of the
lanes for the 100 m sprint of an athletics track. Note that the 100 m (10 000 cm)
that the athletes have to run is shown as a map distance of 10 cm (0,1 m).
We can express the scale of Figure 1.5(a) as:
1
• a representative fraction: 10000 or 1:1 000. Note that the bigger the value of
the denominator, the smaller the scale of the map – 18 of a cake is a smaller
portion than 12 of the same cake. Also remember that the numerator of the
fraction should always be one (1).
• a word scale: 10 cm on the map represents 100 m in reality or 1 cm on the
map represents 10 m in reality.
• a line scale: the line scale shown in Figure 1.5(b) is nothing but a graphic
expression of a word scale. Remember that a line scale without a reference to
the unit of measurement (metres in this case) is totally meaningless.
Expressing scales in relative terms:
Often, we refer to map scales in relative terms such as world scale, continental
scale, small-scale or large-scale. Small- and large-scale maps refer to the amount
of detail the map shows. The scale of a map is determined by the purpose of the
map. If we wanted to show the distribution of the world’s deserts, a small-scale
map would be more suitable. This would allow the whole world to be shown
on a page. Suppose your family has just bought a plot of land and wants to find
the best position to build a house. A large-scale map would be needed to give
the information required. A large-scale map therefore shows more detail than a
small-scale map. Note that a large-scale map is not necessarily better than a
small-scale map.
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Map reading and interpretation skills
Topic 1
Compare the three maps shown in Figure 1.6. Note that the scales of the three
maps are 1:50 000, 1:250 000 and 1:1 000 000 respectively. It is obvious that the
features shown in Figure 1.6(a) have been reduced much less than those
shown in the other two figures. We can therefore say that Figure 1.6(a) has a
larger scale than Figure 1.6(b) and Figure 1.6(c). Did you notice that the
smaller the map scale, the larger the area that can be mapped and the lower
the level of detail?
(b) 1:250 000
(a) 1:50 000
(c) 1:1 000 000
Figure 1.6: Cape Town harbour at three different scales
Classroom activity 2
Do this with a friend.
1
Draw a 1:250 000 line scale. Do not forget to add the necessary
annotations (labels)!
2
Use an atlas to find maps of the following three areas: the world,
Africa and South Africa. Write down the scales of each of the maps.
(a)
Which map has the largest scale?
(b)
Which map has the smallest scale?
(c)
What is your conclusion regarding the relationship among the
size of an area shown on a map, the scale of the map and the
amount of detail the map shows?
Map reading and interpretation skills
•
9
Homework activity 2
1
The length of the bottom (horizontal) boundary of the map of
Nelspruit (Figure 1.2) on page 4 is 6.75 km. Calculate the area (in
hectares) covered by the map. Remember that the area of a rectangle is
calculated by multiplying the length by the breadth.
2
The representative fraction of the map of Nelspuit is 1:50 000. Express
the representative fraction as a word scale.
3
Draw a 1:500 000 line scale. Do not forget to add the necessary
annotations (labels)!
What you still need to know
Distributions are shown by spatial location:
Map element 3
A very useful advantage of maps is that we can see where something is
located – what its spatial location is. Another advantage is that we can also see
the spatial distribution of phenomena – where other things are located
relative to one another. When geographers put on their ‘spatial distribution
glasses’ they try to spot whether the distribution of the locations is linear,
circular, clustered, regular, or perhaps random.
(a) A linear
distribution
(b) A clustered
distribution
(c) A regular
distribution
(d) A random
distribution
Figure 1.7: Different spatial distribution patterns
Being able to identify distribution or arrangement patterns is a very useful
skill. Think of the entrepreneur who mapped the location of all places selling
fast food in a specific area. If he/she was looking for a business opportunity, it
would be important for the entrepreneur to choose a site where no other fast
food franchises were doing business nearby.
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Map reading and interpretation skills
Topic 1
Studying the distribution of two or more phenomena at the same time is even
more useful. A comparison of the distribution and capacity of schools with the
distribution of the population between the ages of 7 and 18 might reveal those
areas that are in desperate need for one or more additional schools.
You will be fascinated by the patterns that atlas maps reveal. Different maps can
show the distribution pattern of deserts, where rainfall occurs, the distribution
of population and where different types of economic activity occur.
The spatial distribution patterns shown on a map can only be accurate if the
spatial location of individual features is correct. In Geography we can describe
the location of features in relative and in absolute terms.
Classroom activity 3
Do this in a group. Consult an atlas to help you.
1
Describe the spatial distribution of the harbours of South Africa.
2
Discuss the spatial variation of the harbours of South Africa. In other
words, discuss how the harbours differ from each other.
3
Describe the spatial distribution of the homes of your best friends.
Word bank
Bearing:
an expression of direction in terms of an angle
measured from a base line such as a line pointing
to true north or magnetic north
Magnetic bearing:
the direction from point A to B in the field
expressed as an angle measured from the magnetic
north baseline
Magnetic declination: the angle by which the magnetic North Pole (as
shown on a magnetic compass) deviates from true
north as shown on a map
Magnetic north line: the line of direction measured in the field by using
a compass. The compass needle points to the
magnetic North Pole and not to the geographic
North Pole indicated by maps
Quadrant bearing:
an expression of direction in terms of an angle
measured from base lines such as the main
compass directions (north, south, east and west)
True bearing:
the direction from point A to B on a map expressed
as an angle measured from the true north baseline
True north line:
a line on a map pointing to the position of the
geographic North Pole
Map reading and interpretation skills
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11
What you still need to know
Describing relative location in terms of bearing
In everyday language we might hear that ‘Bloemfontein is a four-hour drive
south of Johannesburg’ or ‘my school is a twenty-minute walk from my house
along Mandela Road’. You will agree that such a description will not help us
to accurately pinpoint Bloemfontein or our school. All people do not drive at
the same speed or walk at the same pace. Also note that relative location only
makes sense when it is indicated by reference to another location, of which the
location is known.
Compass directions
We can also use compass directions to describe relative location. In lower
grades you learned about eight compass directions – these eight are all
labelled in blue in Figure 1.8. Imagine we discovered an old document in
which the location of a buried treasure has been vaguely described. We
interpreted the description and plotted the probable location of the treasure as
point B in Figure 1.8. By walking in a north-eastern direction from point A to
point B we might pass point B without even seeing it. We can expand the
compass rose by assigning eight additional directions – see the directions
labelled red in Figure 1.8. By walking in a north-northeast direction from point
A, the chance of finding the treasure is much better.
Figure 1.8: The 16 compass directions
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Map reading and interpretation skills
Topic 1
Bearings
A more accurate way of describing the location of the hidden treasure is to
take bearings. In Figure 1.9 we divided the circle representing the globe into
360°. By placing a protractor in such a way that (1) the centre is on point A and
(2) the baseline coincides with the north-south baseline; and reading the
degree measurements clockwise, we
North
will see that angle DAB is exactly 25°.
We can now state that the treasure is
probably buried somewhere at a site
along 25°. This type of reference to
bearing is called full circle bearing.
Because there are so many ways to
describe direction it might be worth
your while to provide additional
information to avoid any confusion.
An alternative is to refer to the
baseline and whether measurement
has been made clockwise or anticlockwise. The reference N 25° E will
therefore also be correct. We refer to
such referencing as quadrant
referencing.
Figure 1.9: Direction can be expressed as compass bearings.
Remember the following when dealing with bearing:
•
Full circle bearing is simply expressed in degrees varying between 0° and
259°. The degrees are measured clockwise from the baseline.
•
The angle of a quadrant bearing may not be greater than 90° – it varies
between 0° and 89°.
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The international convention when describing direction in terms of
quadrants is to always place north and south before east and west.
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Quadrant bearings can be measured clockwise or anticlockwise.
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When describing quadrant bearing we first refer to the baseline from which
the angle is measured. Secondly we refer to the measured angle, and lastly
to the orientation (east or west) relative to the baseline. Our previous
reference to the position of point B as N 25° E serves as an example.
The decision whether we should measure clockwise or anticlockwise
depends on the quadrant in which the feature, on which a bearing is taken,
is situated. Study the four quadrants shown in Figure 1.10 on page 14 –
note that our position is at A, the centre of the circle.
Map reading and interpretation skills
•
13
Measure
anti-clockwise.
Orientation is
west of north.
Measure
clockwise.
Orientation is
east of north.
A
Measure
clockwise.
Orientation is
west of south.
Measure
anti-clockwise.
Orientation is
east of south.
Figure 1.10: Clockwise and anti-clockwise direction readings
True and magnetic bearing
Note that we still do not know exactly where the treasure has been buried.
Only when we also bring distance into the equation can we pinpoint the
location. We did exactly that and guess what! We could not find the treasure.
What is the hitch?
Upon further scrutiny of the treasure map, we discovered that we overlooked
the probability that the bearing that was written down on the treasure map
might refer to magnetic north (MN) and not true north (TN or geographical
north) as we had assumed. True north points in the direction of what is known
to us as the North Pole. Maps indicate true north. A bearing measured from
true north is called a true bearing. However, when a magnetic compass is
used, the needle does not point to the North Pole. It points to magnetic north.
This is a point that is not fixed but in constant movement. This point is called
the magnetic North Pole. The angle by which the magnetic North Pole
deviates from true north is called the magnetic declination. On each of the
official 1:50 000 maps of South Africa the magnetic declination for each map is
indicated by a diagram similar to the one shown in Figure 1.11. This diagram
shows us that in this particular case magnetic north is exactly 8° west of true
north. We then puzzled out that the reference to 25° on the treasure map might
refer to magnetic north and not true north as we initially thought. To change
the magnetic bearing of 25° to a true bearing we need to subtract the magnetic
declination of 8°. We went to the spot (see point Z in Figure 1.11) at TN 17°E
(the equivalent of magnetic north 25°) but to our disappointment we
discovered that we were not there first.
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Map reading and interpretation skills
Topic 1
8°
north
magnetic
true north
mean magnetic
declination 8° west
of true north
Figure 1.11: True north, magnetic
north and magnetic declination
Figure 1.12: Compass bearings
Study Figure 1.12 very carefully. It shows the true and magnetic bearings from
point A onto points B, C and D.
Classroom activity 4
To be able to answer the next questions you will need a protractor to
make some measurements on Figure 1.12. Do this with a friend.
1
How would you describe (referring to full circle bearing as well as
quadrant bearing) the direction of point E from point A when you are
using a printed map?
2
How would you describe the direction (referring to full circle
bearing) of point E from point A when you are using a magnetic
compass in the field?
Word bank
Latitude lines:
imaginary lines shown as parallels north or south of
the equator
Longitude lines: imaginary lines drawn west or east of the Prime
Meridian (0o – also known as the Greenwich Meridian)
Co-ordinate:
an absolute location expressed in terms of a latitude
and longitude position. May also be referred to as a
grid reference, i.e. the point of intersection of a line of
latitude and a line of longitude
Map reading and interpretation skills
•
15
What you still need to know
Describing absolute (exact) location
Sometimes we need to be very precise and accurate
in our description of location. We then make use of
absolute location. In lower grades you learned how
to locate places in the news by making use of the
geographic references system (geographic
co-ordinate system). You should therefore agree
with us that the absolute locations of the places
(shown as points or co-ordinates A, B, C and D) in
Figure 1.13 are:
Co-ordinate A: 30°N; 30°W
Co-ordinate B: 15°N; 30°E
Co-ordinate C: 15°S; 45°E
Co-ordinate D: 7°30’S; 23°30’E
Figure 1.13: The geographic grid or
geographic reference system
Co-ordinate D was not exactly at the intersection of a line of latitude and
longitude but we could see that the place is situated approximately halfway
between 0° and 15° S and approximately halfway between 15° and 30° E. We
could therefore say that the latitude location of point D is somewhere along
the 7½° S line of latitude. Since half a degree is of course the same as 30
minutes (30’), we described the latitude of point D as 7° 30' S.
Point E is even more ‘awkwardly’
situated between the lines of latitude
and longitude shown in Figure 1.13.
Because we need a more detailed
way of measuring we are not going
to bother with Figure 1.13 any longer
– detailed measurements cannot be
done on small-scale maps.
In Figure 1.14 we marked the
entrance to a cave as point A. We
want to calculate the absolute
location of the cave. Our explanation
of the steps to be followed only
covers the calculation of longitude –
you later need to calculate the
latitude as a homework activity.
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Figure 1.14: Steps in calculating longitude location
Map reading and interpretation skills
Topic 1
Steps in calculating the longitude location of the cave
entrance
1. The cave is between 19° and 20° east.
2. On the map the distance between 19° and 20° is 40 mm. One degree (or 60
minutes) is therefore represented by 40 mm.
3. On the map the cave is located 35 mm east of the 19° E line of longitude.
4. Because we know that 1° is equal to 60 minutes and that in this instance 60
minutes are equal to 40 mm, we can now apply the follow reasoning and
arithmetic:
40 mm on map represents 60 minutes.
∴ 35 mm on map represents 60 × (35 ÷ 40)
The answer: 52.5’ or 52.5 minutes
5. The decimal portion, which we calculated in step 4 means 0.5 or ½ of a
minute. Remember there are 60 seconds in a minute. The 0.5 therefore
actually means 0.5 of 60 seconds.
6. ∴ 0.5 × 60 seconds = 30 seconds.
7. The longitude of the cave entrance is therefore: 19 degrees + 52 minutes +
30 seconds East.
8. We write it as 19°52'30"E.
You are now equipped to describe and locate location by referring to not only
degrees and minutes but degrees, minutes and seconds.
Homework activity 3
To be able to answer the next four questions you will need a protractor to
make some measurements on Figure 1.12.
1
How would you describe the direction of point F (see Figure 1.12)
from point A when you are using a printed map?
2
How would you describe the direction of point F (see Figure 1.12)
from point A when you are using a magnetic compass in the field?
3
How would you describe the June 2012 direction (refer to full circle
bearing) of Sierlik station (see block B2 on the map of Nelspruit
shown on page 4) relative to Mataffin station (see block A2) when
you are using a magnetic compass in the field?
4
What is the absolute location of point A in Figure 1.14?
5
What constructed phenomenon on the map of Nelspruit do you
associate with the location of approximately 25°28'27"S; 30°58'10"E?
6
What is the absolute location of Mataffin station? Express your
answer to the nearest minute.
Map reading and interpretation skills
•
17
Word bank
Legend:
Qualitative map symbols:
Quantitative map symbols:
map key showing what the symbols
used on a map represent
tell us where a feature is located and
what the nature of the feature is
show different degrees of importance
and/or the size or quantity of
phenomena
What you still need to know
Map symbols to show where and what vs.
symbols to measure information
The language which cartographers use does not consist of words. It consists of
signs or symbols that are placed on and around the map. If we do not
understand the map language, we cannot understand the map. A map should
therefore have a legend or key that explains the meaning of all the symbols
used on the map. An example of such a legend is the standard legend used to
explain the symbols on the official 1:50 000 topographic maps of South Africa.
We have included such a legend in Figure 1.3 (see page 5).
Cartographers classify the features they want to map either as points, lines or
areas. If you think carefully about this, it makes a lot of sense. A road is
actually a line, a dam covers an area and, in the context of the globe, a house is
only a tiny point or dot. Study the map legend on page 5 to familiarise
yourself with these symbols. Literally hundreds of different map symbols can
be designed by changing one or more of the following symbol characteristics:
the colour; the shape; the size; and the orientation of the symbol. Figure 1.15
illustrates how size and orientation of symbols can be used to distinguish
between different phenomena.
(a)
By varying symbol size,
relative importance (e.g.
output of mines or
number of learners in a
school) can be illustrated.
(b)
Figure 1.15: Using size and orientation of symbols to distinguish between phenomena
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Map reading and interpretation skills
By varying the
orientation of a symbol
we can distinguish
between different types
of mines or schools.
Topic 1
In Figure 1.16 we have grouped
landscape features into the three
groups of points, lines and areas.
These symbols are examples of
qualitative symbols – they simply tell
us where a feature is located and what
the nature of the feature is. It does not
tell us that the feature associated with
a certain symbol is more important
than a feature associated with another
symbol.
Symbols can also be drawn to show
different degrees of importance.
Symbols that measure the importance
or size or quantity of features are
called quantitative symbols.
Figure 1.16: Map symbols that
locate features
Figure 1.17 shows how we can
classify quantitative symbols into
two types – those measuring
relative importance (small, medium,
large) and those measuring absolute
quantities.
You will be required to frequently
use the thematic maps in atlases to
study the spatial patterns of
phenomena such as population, the
lithosphere, hydrosphere and
atmosphere etc. Atlas maps using
quantitative symbols can become
quite tricky to read. Study the map
legend very carefully before you try
to interpret the maps.
Figure 1.17: Map symbols that measure
quantitative information
Map reading and interpretation skills
•
19
Classroom activity 5
Study the legend (Figure 1.3) on page 5.
1
What colour/s is/are used to show constructed features such as roads,
boundaries, railway lines, power lines and buildings?
2
What colour/s is/are used to show water features such as rivers,
swamps, reservoirs and pipelines?
3
What colour/s is/are used to show landforms?
4
What colour/s is/are used to show vegetation such as woodlands,
cultivated land, vineyards and orchards?
5
Discuss your next homework activity to make sure that you have a
clear understanding of what is expected of you.
Homework activity 4
You work as a cartographer specialising in
tourist maps. The editor of a new tourist guide
has given you the text and has asked you to
draw a 1:50 000 map indicating all the
phenomena which are mentioned in the text.
Being a very systematic cartographer, you have
decided to first create a legend block that
contains the title, scale bar, symbols and symbol
descriptions.
Use the following guidelines to help you:
20
•
•
Read the text attentively.
•
•
Arrange the symbols into logical groups.
•
Write the descriptions that will explain
each symbol.
•
Organise the symbols and their
descriptions in a balanced layout.
•
Do not forget the map title and the scale
bar.
•
Make a list of all the features for which
symbols are required.
Design the symbols making use of the four
basic characteristics of map symbols.
Map reading and interpretation skills
Text for tourist guide
Leave the Metropolitan area by
travelling eastwards along the national
road. At the crossing with the R26 (the
secondary tarred road) you turn left on
the road leading to the picturesque
fishing town called Something Fishy.
The road winds between cultivated
fields, orchards, indigenous forests
and two magnificent freshwater lakes.
Approximately 1.2 km after crossing
the railway line you should be on the
lookout for the 12 km long circular
gravel road leading to several hiking
trails, picnic spots, a bird-watching
hide and a scenic lookout. Please note
that Mthimkhulu (Big Tree) is the only
picnic spot offering braai facilities.
Mthimkhulu and the scenic lookout are
the only sites with toilet facilities. The
scenic lookout is the highest point
(1 780 m above sea level) in the
region and offers a magnificent view of
the mighty Fish River and its
tributaries. From the scenic lookout
you should also be able to see the
rocky coast of Something Fishy with its
sheltered sandy bays, the lighthouse
and the recently developed mussel
farm.
Topic 1
Extra practice activity 1
1
While buying stamps at the Post Office (it is indicated with a ‘P’ in
block C3 of the 1:50 000 map of Nelspruit – see Figure 1.2 on page 4)
you are approached by an anxious tourist whose wife urgently needs
medical attention. Write down the directions you would give the
husband to help him to drive to the nearest hospital via the shortest
route.
2
We are thinking of a place shown on the 1:50 000 map of Nelspruit.
The place is surrounded by orchards. The true bearing on this place
from Sierlik station (situated in block B2) is 66°E. The place is
situated 3.8 km from Sierlik station. Of which place are we thinking?
3
The ‘as the crow flies’ distance between Mataffin station (block A2)
and Sierlik station (block B2) on the 1:50 000 map of Nelspruit is 1.75
km (50 000 × map distance of 3.5 cm). How much further is it to
travel from Mataffin to Sierlik by train?
4
Do the following scale conversions:
(a) a word scale of ‘1 cm on the map measures 4 km in reality’ to a
ratio scale (representative fraction) and to a line scale;
(b) a ratio scale of 1:5 000 to a word scale using the words
‘centimetre’ and ‘metre’.
5
What is the approximate area in square metres (m2) covered by a
1” × 1” square? A hint: use the line scale of the 1:50 000 map of
Nelspruit.
Map reading and interpretation skills
•
21
Showing the relief of the land
on maps
What you know already
The height clues on topographic maps
•
•
The shape of the land on a topographic map is called the relief.
•
The brown lines on a topographic map are contour lines showing height
above sea level.
•
Height is also shown using point symbols called spot heights and
trigonometrical beacons (stations).
Topographic maps show features of relief such as plains, valleys,
mountains and hills of varying steepness.
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Refer to the 1:50 000 map of Nelspruit and provide examples
of the two types of point symbols that are used on
topographic maps to indicate height above sea level.
Draw one generalised contour map showing a river valley
and a mountain spur respectively.
Word bank
Contour line:
a line joining all places which are the same height
above sea level
Contour interval: the difference in height between two adjacent
contours. On the official 1:50 000 topographic maps
of South Africa it is 20 m
Relief:
relief refers to the three dimensional shape of the
landscape and includes elements such as height,
slope (steepness) and aspect. It tells us more about
where and how the high land and low land is found
Trigonometrical
beacons (stations): on topographic maps the location of constructed
trigonometrical stations is indicated by an open
triangle. The identifying number of the station
appears next to the triangle while its height above sea
level is written below the triangle
22
•
Showing the relief of the land on maps
Topic 1
Word bank
Spot height:
a spot height also shows height above sea level. You
can recognise it on maps as a small black dot with a
height value written close to it. Unlike trigonometrical
stations, spot heights are not marked on the ground.
They show the height of easily identifiable landmarks
such as hilltops, fences and road crossings
What you still need to know
Different ways of
showing relief and
landforms on maps
Figure 1.18: A hill-shaded map showing relative relief
Figure 1.19: A layer-coloured map showing relative relief
There are literally hundreds of
different landforms, each having
unique characteristics. Think about
the landforms we associate with wind
(dunes), running water (rivers,
meanders and floodplains), wave
action (beaches and coastal caves),
general erosion and weathering
(valleys, canyons, mesas), glaciation
(fjords) and volcanoes (volcanic
craters and cones). In this section we
will share knowledge about the
different mapping techniques that are
used to show the relief of the land so
that map readers can identify unique
landforms on maps.
Figure 1.18 shows how hill-shading
can be used on small-scale maps to
give us a general impression of the
distribution of high and low land.
However, since it only shows relative
relief we cannot accurately determine
the real heights of the mountains,
ridges, valleys and plains.
Showing the relief of the land on maps
•
23
The technique of showing relief by layer colouring (as illustrated in Figure
1.19) is very common on small-scale regional, continental and world maps.
Different colours are used to depict different height intervals (or categories).
Not all cartographers agree on which colours should be used. However, the
most common sequence used begins with green in the lower elevations and
proceeds through yellow to orange, red, purple, and finally white for the
highest elevations. Layer colouring is used only on the smaller scale maps of
South Africa.
The simplest way to represent relief on maps is, of course, by indicating the
position and measured height of spot heights and trigonometrical beacons
(stations) on a map. The symbol used to indicate trigonometrical beacons on
the 1:50 000 maps of South Africa is shown in Figure 1.3 on page 5. Note that
the symbol for spot heights does not appear in the legend. The dot with the
value of 873 written next to it in block C4 on the map of Nelspruit is an
example of a spot height.
A fourth method of indicating relief
is by contour mapping. As you know
a contour line is an imaginary line
joining all points on a map that are at
the same height above sea level. The
function of, for example, a 600 m
contour line shown on a map would
be to:
• indicate all the places that are
situated 600 m above sea level;
•
separate all places situated lower
than 600 m above sea level from
all places situated higher than
600 m above sea level.
Figure 1.20: A map showing hill-shading, layer colouring, contours
and spot heights
Figure 1.20 illustrates a map using all four methods of indicating relief. It is
now easy to see from the map that the source of the river is in the high
mountain areas towards the south and that it flows in a northerly direction
through gaps in the two mountain ranges situated more towards the north.
24
•
Showing the relief of the land on maps
Topic 1
Classroom activity 6
Do this with a friend.
1
What is the highest point on the map of Nelspruit (Figure 1.2 on
page 4)?
2
Describe the relative location of this point and give the number of the
relevant spot height or trigonometrical beacon.
3
Speculate why layer-colouring is not practical on the official 1:50 000
maps of South Africa.
What you still need to know
Three keys to reading contours
The three keys to reading contours are to:
• familiarise yourself with the contour information written on the map
•
•
spot differences in contour spacing and
look for patterns in the arrangement of contours.
Key 1: Read all the information
Before trying to interpret a contour map you need to establish the contour
interval, find the index contours, and note how the contour values have been
written on the map.
The contour interval is the difference in elevation (height) between any two
adjacent contours. On the official 1:50 000 maps of South Africa the contour
interval is 20 m. Index contours occur at regular intervals and are printed in a
darker colour. On our official 1:50 000 maps every fifth contour (thus every
100 m) is printed in a darker brown than ordinary (intermediate) contour
lines. Just by noting how the contour values are written
on a map, you can deduce where the higher-lying and
lower-lying areas are found. Very often the contour
values are written upside down. The reason for this is
that the values should point in the direction towards
which the height increases. Furthermore, when the
values do not appear on the line, they should be placed
on the ‘higher’ side of the contour line. The placing of
contour values is illustrated in Figure 1.21. Note that all
Figure 1.21: Orientation of
contour values on maps
the wrong orientations are shown in red.
Showing the relief of the land on maps
•
25
Key 2: Be sensitive to the distance between
contour lines
Height above
sea level (m)
Contour spacing indicates slopes. To refresh your memory we share the
illustrations shown in Figure 1.22 with you. In the top row are side-views of
five different slopes. In the bottom row the contour spacing of each is shown.
100
80
60
40
20
0
(a) A concave slope (b) A convex slope
100
80
60
100
60
0
(d) A stepped slope (e) A gentle slope
100
80
80
40
20
(c) A steep slope
40
20
0
60
40
20
0
100
80
60
40
20
0
Figure 1.22: Contour spacing reveals slopes.
Key 3: See the bigger picture – different
landforms have different contour patterns
In lower grades you learned about the key contour patterns to recognise hills,
ridges, spurs and valleys. These patterns are:
26
•
hills: Concentric rings of contours with the higher contour values in the
middle.
•
ridges: Parallel spaced contour lines with higher contour values in the
middle.
•
•
spurs: V- or hairpin-shaped contours pointing towards low ground.
valleys: V- or hairpin-shaped contours pointing towards high ground.
•
Showing the relief of the land on maps
20
0
Topic 1
What you still need to know
More landforms and their contour patterns
In this section we will explore the contour patterns of some more landforms
you will come across when studying maps or travelling our country. This new
knowledge will form a good basis when you later study geomorphology and
the types of landforms associated with plate tectonics, folding and faulting.
Depressions
Figure 1.23: Contours associated with depressions
Depression contours are distinguished
from regular contours by short ticks at
right angles to the contour line. As
illustrated in Figure 1.23, the ticks
should, of course, point towards the
lower lying areas – towards the
bottom of the depression.
Cliffs and waterfalls
Both cliffs and waterfalls can be recognised on contour maps by the fact that
two or more successive contour lines touch. If the ridge of a mountain spur
happens to end in a cliff, this will be represented by the closed ends of two
successive Vs which touch one another (see Figure 1.25). Where two or more
contour lines touch in a river valley, we find a waterfall (see Figure 1.24).
ed
ersh
wat
cliff
Figure 1.24: Contour patterns representing
a waterfall
Figure 1.25: Contour patterns representing
a cliff
Showing the relief of the land on maps
•
27
Poort
A poort (or gap) is illustrated by Figures 1.26 and 1.27.
A poort is formed where a river carves itself a course
as it cuts through a mountain or a range of hills. In
most cases the poort lies more or less at the same
height above sea level as the surrounding plain. We
can easily recognise a poort on a contour map by the
fact that no V-shaped contour line is to be found in the
opening between the two spurs. A poort usually offers
convenient passages for roads and railway lines. Wellknown examples in South Africa are Meiringspoort in
the Swartberg, Michell’s Pass at Ceres, and Wyllie’s
Poort in the Soutpansberg.
Figure 1.26: Meiringspoort
Saddle/neck
A saddle also occurs in a range of mountains or hills and
may be described as a low-lying ridge between two
higher-lying peaks or spurs. Unlike a poort, a saddle lies
at a higher elevation than the surrounding countryside
and it forms part of the watershed. On a contour map
(see Figure 1.29) a saddle can easily be distinguished
from a poort by the fact that the V-shaped contour lines
which increase in height usually jut inwards like
tongues between the two spurs. In fact, one can see two
sets of Vs, one either side of the saddle, which point
towards each other.
Figure 1.28: A saddle
28
•
Figure 1.27: Contour patterns representing
a poort
Figure 1.29: Contour patterns representing a saddle
Showing the relief of the land on maps
Topic 1
Escarpments, plateaus and plains
An escarpment (see Figures 1.30 and 1.31) is a large regional feature that
separates a low-lying area from a high-lying area. Viewed from the lower
ground, an escarpment looks like an uninterrupted mountain, and on a contour
map it can be distinguished from an ordinary mountain range because the
closely spaced contour lines appear on one flank only. Behind the escarpment
there is a plateau and there is no decrease in elevation as in the case of a
mountain range. A plateau can therefore be recognised by the lack of contour
lines in high areas. A plain can be recognised by the lack of contours in low areas.
Figure 1.30: The Transvaal Drakensberg
escarpment
Figure 1.31: Contour patterns representing
an escarpment
Classroom activity 7
Answer the following questions based on the 1:50 000 map of Nelspuit
(Figure 1.2 on page 4). A hint: you need to look at contour patterns.
1
Explain why the railway line bends towards the river in block A2
instead of keeping parallel to the N4 national road.
2
Why does the layout of the streets in the central business district (see
block C3) look so different from the layout of the streets in the West
Acres residential area (see block B4)?
3
What evidence can you provide of excavating activity in Nelspruit?
4
What is the highest point on the Nelspruit map?
5
Give the height of the experimental farm in block D1 in the northeastern corner of the map.
6
Why do you think there are no residential areas in block C4 on the
map?
7
Give the height of the trigonometrical beacon 101 in block B1 on
the map.
Showing the relief of the land on maps
•
29
What you still need to know
How to draw cross-sections of the landscape
In this section we are going to equip you with the skill to use contour lines to
draw cross-sections of landscapes. A cross-section of the landscape is nothing
other than a graph showing how the landscape varies over distance and in
height. Some people also refer to these graphs as profiles or side-views of the
landscape.
The contour map shown as Figure 1.32(a) has a scale of 1:50 000. We want to
show how the shape of the land varies between the two points marked as A
and B.
The data we need to draw the graph or cross-section is shown in Table 1.1. The
co-ordinates we need to plot on our graph are the ten points where line AB
(called a section line) crosses the contour lines [see Figure 1.32(a)]. For each
co-ordinate we must know how far the co-ordinat is situated from point A
(our x variable) and how high the co-ordinate is situated above sea level (the y
variable). We have plotted the data values listed in Table 1.1 in Figure 1.32(b).
Figure 1.32(c) shows how the crosssection is drawn by joining the coordinates with a line. This line is the
profile of the landscape between
points A and B. It is also important to
add the necessary labels or
annotations for the x-and y-axes. Note
that without this information
somebody else would not be able to
read our graph.
Table 1.1: The co-ordinates we need for our graph of the profile
of the landscape.
Co-ordinates
Distance from
A in mm:
X
Height in
metres:
Y
1 (point A)
0
1 825
2
3
1 800
3
10
1 750
4
12
1 700
5
20
1 650
6
24
1 600
7
33
1 550
8
38
1 500
9
47
1 450
10 (point B)
48,5
1 425*
* Note that point B is situated halfway between the 1 400 m and
1 450 m contours. The height of point B is therefore 1 425 m.
30
•
Showing the relief of the land on maps
Topic 1
Homework activity 5
(c)
Activities 1 and 2 are based on the
1:50 000 map of Nelspruit (Figure
1.2) shown on page 4.
1
Draw a cross-section (profile)
of the relief of the landscape
between the 698 m spot height
in block A2 and the 661 m
spot height in column D. Note
that the spot height in block
A2 is at the intersection of the
N4 and the centre road (next
to the 8 of 698) leading to
Mataffin. Some guidelines:
Use a scale of 1:50 000 for
your x axis and a scale of 1 cm
for a height of 100 metres on
the y axis. Do not forget to
annotate (label) the profile.
2
What is the difference in
height between the highest
and lowest spot heights on the
1:50 000 map of Nelspruit?
3
Some of the contour values
are not correctly orientated on
Figure 1.20. Identify the
mistakes and explain how
they should be rectified.
(b)
(a)
Figure 1.32: Steps in drawing a crosssection of a landscape
Showing the relief of the land on maps
•
31
Extra practice activity 2
32
1
Draw a layer-coloured map illustrating the relief of the Nelspruit
area (see Figure 1.2 on page 4). Decide on four different height
categories. We recommend that you make the bottom and top
categories ‘open categories’. By this we mean ‘lower than x metres’
and ‘higher than y metres’. Your other two categories should have the
same interval, e.g. 1 600 m to 1 800 m and 1 800 m to 2 000 m. Your
map should also show all the major features making it possible for
the inhabitants of Nelspruit to make contact with the neighbouring
area – you need not show the streets in the residential areas.
Remember that a map without a title and a legend cannot be called a
map. It needs to show ‘what is where’.
2
The section line for which you had to draw a cross-section as a
homework activity runs through different types of land use. Tell us
more about what you would find in the landscape by walking from
spot height 689 to spot height 661.
3
You might be wondering: how does one know exactly where to draw
the contour lines on a map? This question will be dealt with in Grade
11. If you cannot wait until then we suggest that you use the library
or the Internet to satisfy your curiosity. The search phrase ‘contour
interpolation’ should produce a long list of sources.
•
Showing the relief of the land on maps
Topic 1
Map projections: from the
globe to a flat surface
What you know already
Maps are flat-surface representations
of the Earth
You did not learn about map projections in lower grades but we are sure that
you know that:
•
•
maps are flat-surface representations of the world, or parts of it
the Earth is not flat but spherical (round) in shape.
Based on the above knowledge, we need to accept that when making a map of
the Earth globe, characteristics such as distance, area, shape and direction will
be distorted.
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The image taken from outer space proves that the Earth is not flat. We cannot
see the entire Earth when looking at it from a position in outer space. To
flatten a spherical object such as a soccer ball you will have to tear it apart, and
it might be difficult to recognise even then the flattened outer skin as
originally being a soccer ball. Try to fold a fairly large piece of paper around a
soccer ball without tearing the paper and leaving no folds or wrinkles.
Map projections: from the globe to a flat surface
•
33
Word bank
Globe:
Graticule:
Map projection:
Projecting:
a three-dimensional scale model of the
Earth – see picture
the network of imaginary lines of
latitude (parallels) and longitude
(meridians)
the process of projecting Earth features
onto a flat surface in the form of a map
think about how a movie is shown on the
screen of a movie theatre. The images are
projected from the back of the theatre onto the
screen by a powerful light
What you still need to know
Why map projections are useful
As geographers we are interested in information such as the distance between
Cape Town and Johannesburg, the area of a country, the shape of the African
continent and the direction from one trigonometrical beacon to another. A globe is
a very accurate scale model of the Earth since it preserves all these important
characteristics of the Earth. However, globes also have some practical limitations:
•
•
Globes are difficult to carry around.
•
It is difficult to make measurements on the curved globe.
Small countries practically ‘disappear’ on a globe. It is not practical to make
globes larger as they then become too awkward to handle. It is difficult to
compare different regions of the world because you cannot see all regions
simultaneously.
Fortunately map projections have been developed to overcome these practical
obstacles.
Classroom activity 8
Let’s work together. Choose one person to demonstrate (or explain) how to
determine the approximate distance between Cape Town and Johannesburg
by using a globe or the distance between two spots on a soccer ball. We
assume you have a globe in the classroom. If not, you need to make a plan to
get one. No Geography classroom should be without a globe.
34
•
Map projections: from the globe to a flat surface
Topic 1
What you still need to know
Types of map projections: The method of
construction
Understanding (and explaining) how the Earth is projected onto a flat surface
can be challenging. Fortunately detailed knowledge is not required because
nowadays we have computer programs that can use mathematical equations
to project geographical co-ordinates to any one of literally thousands of
different map projections.
Based on their method of construction we can distinguish between three basic
types namely cylindrical, azimuthal (planar or zenithal) and conic projections.
Imagine the following:
Transparent
globe showing
the graticule
Paper cylinder
folded around
the globe and
touching along
the equator
A
1.
We have a transparent globe (A in Figure 1.33) that is at the exact scale
desired for a map we want to create. The globe shows the graticule (the
lines of latitude and longitude).
2.
Place a light bulb inside and exactly at the centre of the ‘globe’.
3.
Fold a piece of paper (B in Figure 1.33) representing a cylinder around
the globe touching along the equator.
4.
Take the globe into a dark room and switch on the light bulb that you
have placed inside the globe.
C
The graticule
projected onto the
flattened paper
B
Figure 1.33: A cylindrical projection
What will you see? The outline of the
graticule and continents form shadows
on the paper cylinder. Suppose we
could keep the shadows on the paper
cylinder permanently. By then opening
the paper cylinder and flattening it (C
in Figure 1.33), we have successfully
created a map based on a cylindrical
projection of the Earth. The graticule of
the equatorial Mercator projection
shown in Figure 1.33 (C) is the most
classic example of a cylindrical
projection.
Map projections: from the globe to a flat surface
•
35
The cylinder we referred to above is called a development surface. In Figure
1.34 we illustrate two additional and differently shaped development surfaces.
The cone-shaped surface shown as (a) will produce conic projections. The
planar (flat) surface touching the globe at one point only [see Figure 1.34(b)]
will produce azimuthal (or planar) projections.
(a)
(b)
Figure 1.34: Conic (a) and azimuthal (b) projections
Figure 1.35 provides the
evidence of the distortion
which we referred to earlier. It
is clear that the shape and size
of countries differ from
projection to projection.
However the easiest way to
spot the differences is to look
at the pattern of the graticule.
Remember this hint when
comparing different
projections. By comparing the
graticule of the projection
with the undistorted graticule
on the globe, you can spot
what has been distorted.
Figure 1.35: Evidence of presence of distortion
36
•
Map projections: from the globe to a flat surface
Topic 1
What you still need to know
Different map purposes require different
map projections
Word bank
Conformal projections: a projection which portrays shape and direction
accurately
Equivalent projection: a projection which preserves area
Tangent surface:
the one point or surface touching the mapped
sphere
True direction:
a characteristic preserved by conformal maps
URL:
Uniform Resource Locator – the address of a
web page on the World Wide Web
We explained that one can distinguish between different map projections on
the basis of their method of construction. A second way to distinguish
between different map projections is to look at their properties or the specific
purposes for which they have been developed. The four globe properties that
we would like to preserve on our maps are distance, area, shape and direction.
A map projection can show one or more – but never all – of the abovementioned globe characteristics without some degree of distortion. Of
particular importance is the question whether a projection preserves direction
and relative sizes of areas.
Projections preserving direction
The Mercator cylindrical projection to which we referred earlier is an example
of a conformal projection. Since the meridians and parallels intersect (cross)
at right-angles as they do on the globe, it does not distort angles and can
therefore be used for navigation purposes. It is the only conformal projection
that shows the correct compass bearing everywhere on a map by means of a
straight line. The Mercator projection also preserves shape. The limitation of
the Mercator projection is the exaggeration of the sizes of countries in the mid
and high latitudes. Can you remember how we stressed the point that the
cylinder touches the globe along the Equator? The equator is therefore the line
of tangency, or the standard parallel along which there is absolutely no
distortion whatsoever. As we move away from the equator the exaggeration of
areas becomes progressively larger. The exaggeration we are referring to is
illustrated in Figure 1.36 by comparing a Mercator projection and a Mollweide
equal area projection. The Democratic Republic of Congo (situated close to the
Map projections: from the globe to a flat surface
•
37
Equator) in reality is slightly larger
in area than Greenland (situated in
the high latitudes). It will thus be
wrong to use the Mercator projection
to compare countries in terms of
characteristics where size is
involved, e.g. population density or
areas under irrigation.
An example of the use of conformal
projections by The Chief Directorate:
National Geo-spatial Information
(NGI) in Mowbray (Cape Town) is
the Gauss Conformal projection. The
projection is also cylindrical in
nature but differs from the Mercator
projection in the sense that the line
Figure 1.36: Areas in the high latitudes are extremely exaggerated
of tangency is not along a parallel
by the Mercator projection.
such as the Equator but along a
predetermined line of longitude (see
Figure 1.37). The Gauss Conformal
projection is used for the larger scale official
maps of South Africa such as the 1:10 000
orthophoto maps, the 1:50 000 topographic
maps and the 1:250 000 topo-cadastral maps.
On these maps, the strip along either side of
the chosen standard line of longitude
contains virtually no scale distortion. The
maps can therefore be used to determine
location and to measure distances, areas and
direction. Another advantage is that separate
map sheets of the national series can easily
be joined because features that continue on
Figure 1.37: A Transverse Mercator projection: the line of tangency
adjacent sheets match perfectly.
is along a line of longitude.
A second conformal projection used by NGI is the Lambert Conformal Conic
projection. It is used for smaller scale maps such as the official 1:500 000
topo-admin map series of South Africa as well as the nine provincial maps.
Distances are only true along the standard parallels, but are reasonably
accurate elsewhere on the maps. Directions are reasonably accurate. The
distortion of shapes and areas is minimal at the standard parallels but
increases away from the standard parallels.
38
•
Map projections: from the globe to a flat surface
Topic 1
Projections preserving areas
Equivalent maps preserve the relative size of areas. When all areas on a map
have the same proportional relationship as the corresponding areas on the
ground, the projection is said to be equal area or equivalent. Equal area
projections are widely used on thematic maps where it is essential not to
mislead the reader regarding the size or area of phenomena. Examples are
land use and land cover maps as well as maps showing the density of
features. A disadvantage is that size is maintained at the expense of shape.
Retaining both size and shape is only possible on a globe of the Earth. The
Albers’s Equal Area projection is used by NGI for mapping at smaller scales.
Examples are the 1:1 000 000 wall map of South Africa and the 1:2 500 000 wall
map of Southern Africa.
To conclude, we can share the following generic ‘rules’ regarding the selection
of a map projection:
•
There is no ‘best projection’. The selection of the best map projection
depends upon the purpose for which the map is to be used.
•
•
•
For navigation, correct directions are important.
•
The extent and location of the area to be mapped, also impact on
projection choice. The larger the area being mapped, the more significant
is the curved surface of the Earth and, therefore, the greater the distortion
of the desirable properties. Cylindrical projections are best to show low
latitude locations; conical projections for mid latitude locations and
azimuthal (planar) projections for high latitude polar regions.
On road maps, accurate distances are the major concern.
On thematic maps showing area-related data, preservation of the size and
shape of regions is important.
Classroom activity 9
Figure 1.35 illustrated that different projection types project the graticule
differently. A comparison of the arrangement of lines of latitude and
longitude, on a globe and a map respectively, is a clever strategy to spot
distortions.
1
Discuss the arrangement of lines of latitude and longitude on a globe.
2
List as many characteristics as possible that can be used to evaluate
map projections.
Map projections: from the globe to a flat surface
•
39
Homework activity 6
Use an appropriate atlas map to find and list the three largest and the
three smallest countries in Africa. In both cases you should list the
countries from big to small. Is your list correct? Check yourself by using
a globe.
Extra practice activity 3
1
Provide reasons why the following statements are false:
(a) Cylindrical projections show the entire Earth.
(b) The best projection is that projection which simultaneously
preserves direction, distance, area and shape.
(c) Conformal (also known as equal-area) projections are suitable to
show the extent of world deforestation and desertification.
(d) Distortions are more pronounced on large-scale than on smallscale maps.
2
40
•
Use the library, or the Internet to find fascinating projections that you
can show to the class. The following URLs contain a wealth of
information:
•
•
•
http://egsc.usgs.gov/isb/pubs/MapProjections/projections.html
•
http://geography.about.com/library/weekly/aa030201a.htm
http://www.kidsgeo.com/geography-for-kids/0030-map-projections.php
http://atlas.nrcan.gc.ca/site/english/learningresources/carto_corner/
map_projections.html/
Map projections: from the globe to a flat surface
Topic 1
The South African map
reference system
What you know already
The geographic reference system: a grid of lines
of latitude and longitude
In lower grades you learned that:
•
We can draw an infinite number of the imaginary lines of latitude and
longitude on a map. These lines form a grid called the graticule.
•
We can describe where a place is by specifying its geographic co-ordinates
– the intersection of the line of latitude and line of longitude that run
through the place.
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The three geographic co-ordinates listed below are supposed to
be co-ordinates of places in South Africa. However, all three
references are wrong. Why are they wrong?
1.
32°S; 16°E
2.
30°23’E; 29°37’S
3.
32°37’S; 20°23’N
Word bank
Degree square:
Quadrant:
an area spanned by one degree of latitude and one
degree of longitude
in the context of this section, quadrant refers to one
quarter of a square
What you still need to know
Using the South African map reference system
Each of the official maps of South Africa has a unique name or reference. By
knowing the absolute location of a place and understanding the South African
map reference system, one can contact the Chief Directorate: National Geospatial Information (NGI), provide the reference number and arrange for the
map to be posted to you.
The South African map reference system
•
41
You need to know that the reference system:
•
is based on degrees square and that sixteen 1:50 000 maps cover one
degree square;
•
each map has a six-digit reference number. The first two digits refer to
latitude and the second two digits to longitude. The last two digits are
alphanumeric letters which you will soon understand.
Suppose we want to know the reference number of a place such as
Pietermaritzburg. Carefully follow the explanation below.
1.
Obtain the co-ordinate of Pietermaritzburg. Our atlas has an entry in the
index telling us that Pietermaritzburg is situated at 29°37’S; 30°23’E.
2.
Each degree square of latitude and longitude is designated by a fourfigure number. This number refers to the values of the latitude and
longitude at the north-west corner of the degree square. Since the
geographic reference of Pietermaritzburg is 29°37’S; 30°23’E, we can work
out that Pietermaritzburg is situated somewhere in the degree square as
shown in Figure 1.38(a).
3.
The next step is to subdivide the degree square into four smaller squares,
each being 30 minutes (30’) × 30 minutes. This is illustrated in Figure
1.38(b). The four 30’ squares are designated as A, B, C and D.
Pietermaritzburg is situated in the quadrant labelled C. The first five
digits of the Pietermaritzburg map sheet will therefore be 2930C.
4.
To find the sixth digit we need to subdivide the 30’ squares into 15 minute
(15’) squares. This is illustrated in Figure 1.38(c). Note that the 15’ squares
are also designated as A, B, C and D. The geographic reference of
Pietermaritzburg is in the shaded quadrant B. We have now worked out
the sixth digit and can refer to the map as 2930CB.
31° E
30° E
29° S
29° S
Pietermaritzburg
is situated in the
degree square of
which the
north-west corner
is 29° S, 30° E
30° E
29° S
30° S
30° E
30° S
30° E
30° S
31° E
31° E
29° S
30’
A
30’
45’
D
30’
30° S
31° E
(b)
Figure 1.38: Working out the unique references of South Africa’s 1:50 000 maps
•
The South African map reference system
B
15’
30’
A
C
31° E
29° S
30’
A
B
30’
(a)
42
30° E
29° S
30’
B
45’ D
C
C
30° S
15’
30° E
D
30’
(c)
30° S
31° E
Topic 1
Classroom activity 10
If you do not have maps of your local area in the classroom you need to
obtain some. The Chief Directorate: National Geo-spatial Information
(NGI), provides maps (free!) to schools on request. You will need to provide
the geographic co-ordinate of your school. Find out what it is (you can
Google it or use a GPS) and ask your teacher to contact NGI.
Homework activity 7
1
The geographic reference of Polokwane in Limpopo is 23°54'S;
29°25'E. What is the six-digit reference number of the 1:50 000 map of
Polokwane?
2
What is the six-digit reference number of the 1:50 000 map sheet that
borders the Nelspruit map to the north?
3
What is the six-digit reference number of the official 1:50 000 map
showing the location of your school?
Extra practice activity 4
1
Describe the relative location of the following geographic
co-ordinates: 28.6°S; 16.5°E. Note that 28.6°S is the same as 28°36’S.
The reference 16.5°E is the same as 16°30’S.
2
What is the six-digit reference number of the 1:50 000 map sheet that
borders the Nelspruit map to the east?
3
How many official 1:50 000 maps cover an area of two degrees
square?
The South African map reference system
•
43
Using atlases
What you know already
An atlas is a collection of maps
In lower grades you learned (and we hope also practised) that:
a wealth of information can be extracted from atlases
•
•
atlases show the world at scales ranging from local to global (the world).
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When last did you use your atlas?
Use an atlas – without consulting the index – to show your
friend the location of international places or events that made
headlines during the previous week.
Word bank
Atlas:
Alpha-numeric:
Global village:
Thematic maps:
a collection of maps. The name is derived from Greek
mythology. It was believed that Atlas was the powerful
Titan who supported the heavens
a system of letters and numbers used in an atlas to find
places on a page
the idea that countries are all dependent on each other
and also getting ‘closer’ to each other because of
modern communication and transport systems
different maps in an atlas showing specific types of
data, e.g. population maps, climate maps and maps
showing types of agriculture in an area
What you still need to know
Why should I use an atlas?
Although we are living on the tip of the African continent, we do not live in
isolation. We are very much part of the global village. To be a citizen of the
global village you need to know what is going on out there, where it is
happening and why. Newspapers and news bulletins on television and radio will
inform you about what is happening. To find out where the events are
happening, we suggest that you use an atlas. Make a deliberate attempt to find
out where places being referred to are situated. You need to be geographically
literate.
44
•
Using atlases
Topic 1
What you still need to know
How to use an atlas index
It is important that you become familiar with the contents page of your atlas
as well as the index. The contents page will give you an overview of both the
areas that are mapped and the type of information shown on the maps.
Atlases differ from one another but a good atlas should tell you how to use
the index. The atlas we are using contains the following information:
1. Notes on how to use the index
2. The list of abbreviations used for features. Example: T for town
3. The list of abbreviations used for locations. Example: UK for United
Kingdom
For the sake of legibility, the alphabetically sorted entries are often arranged
in columns. Our atlas index has seven columns. The first entry in our atlas is
the name Aachen. The first row of the table shows the Aachen entry. In row
two we explain the meaning of the individual column entries.
Table 1.2: Entries in the index of an atlas
Aachen
Aachen
T
Germany
Aachen Aachen is
is a
a town in
town.
Germany.
.
64
The number of
the page on
which you will
find ‘Aachen’.
E3
50°47’N
06°04’E
An alpha-numeric The
The
reference system
latitude longitude
unique to the
location. location.
atlas.
The index notes in our atlas contain a wealth of
additional information. It is senseless to share it with you
because your atlas might not contain the same
information in the same format. What is important is that
you acquaint yourself with the format and organisation
of the index of your atlas.
Figure 1.39: An alpha-numeric
reference system
To conclude, we need to explain a typical alpha-numeric
reference system used on the pages of an atlas. As shown
in Figure 1.39 the rows (bands of latitude) are referenced
with numbers. The columns (bands of longitude) are
referenced with letters of the alphabet. Aachen is located
in block 3E – the intersection of row 3 and column 4. The
reference simply means that you will find Aachen on
page 64 of the atlas somewhere within block 3E. It does
not reveal absolute location.
Using atlases
•
45
Classroom activity 11
Use an atlas to do these activities in your groups.
1
List two international and three local places or regions that recently
made headlines in the news. Use an atlas index to find out exactly
where the five places or regions are located. Describe the relative
location of each place or region.
2
Compare the index of the atlas you are using with the one we
described. List and explain the differences.
3
What range of scales is used to map the world on single and double
pages respectively?
What you still need to know
Information from different maps reveals spatial
relationships
You have learned that a map is a generalised and reduced representation of
the world. This implies that we cannot show everything on a single piece of
paper. We can try but the end result will be so cluttered that we will not even
recognise it as a map. Each and every map has a specific purpose. A very
important range of maps is what we call thematic maps. Such maps focus on
single themes such as countries of the world, geology, rainfall, population,
poverty, wealth, literacy, natural resources and many more. Such a range of
themes cannot be shown on a single map. However, you can find such ranges
of maps in an atlas.
The advantage of having a range of maps of the same area is that it makes it so
much easier to answer the ‘Why are they where they are?’ question. In most
cases spatial distribution (where things are) can only be answered by
understanding the spatial association between two or more phenomena. In
other words, to better understand a certain aspect of our world we need to
look at the bigger picture by comparing different themes of maps of the same
area. It is easy to explain:
46
•
the distribution of sinkholes once you compare it with a geology map – the
sinkholes are mostly confined to dolomite areas;
•
the distribution of diamond mines once one spots that diamond-bearing
mines are associated with the carrot-shaped vertical kimberlite pipes
shown on geology maps.
•
Using atlases
Topic 1
Classroom activity 12
Let’s work together looking for spatial associations by using an atlas.
Study the geography of the region on the African continent that borders
the Tropic of Cancer. You need to look at as many maps as possible.
1
Make notes about the population density, the settlement pattern, the
climate and the agricultural activities of the region. Answer questions
such as: Why are people living here? Are there any climatic factors
causing differences in population density? Does the relief of the land
influence types of agricultural activities?
2
Now analyse your notes and try to make connections between the
physical, land use and population patterns.
3
Share this information in a class discussion.
Homework activity 8
1
Use your atlas to make lists of the five thematic maps and three
diagrams about South Africa that you find most fascinating. Your
teacher will suggest a structure for your two lists.
2
We are interested in the geography of the food and beverages you are
going to enjoy during the coming weekend. You will have to read the
labelling and then make a list of the places where the food were
produced and/or processed. You then have to draw a rough map to
show the location and distribution of the places.
Extra practice activity 5
1
Study the thematic maps of any region in the world and select two or
three maps that can be shown to the class as good examples
illustrating spatial association of phenomena. As an example you
could use the atlas extracts that appear on pages 48 and 49 (Figures
1.40 and 1.41).
2
Use an atlas to locate Alaska (a state of the USA) in North America,
and the most eastern border of the Russian Federation. Describe the
location of Alaska relative to the border. Make sure about your facts –
there is a catch in the question.
3
List all the places being referred to on the front and back pages of two
to three recent newspapers. Use an atlas to determine exactly where
these places are. If the places do not appear in the atlas you need to
provide an explanation for the omission.
Using atlases
•
47
48
•
Using atlases
Figure 1.40: Extract from the Macmillan School Atlas showing climatic regions of the world
Topic 1
Figure 1.41: Extract from the Macmillan School Atlas showing natural vegetation regions of the world
Using atlases
•
49
Aerial photographs
What you know already
Aerial photographs: Our bird’s eye view of
landscapes
From the knowledge gained in lower grades regarding aerial photographs
you:
•
•
Know what oblique and vertical aerial photographs look like.
•
You were also exposed to orthophoto maps (images) of South Africa and
learned that they are made from vertical aerial photographs.
Identified natural and constructed features on photographs and learned
that interpreted photographs is the main source of information when
making our 1:50 000 topographic maps.
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1.
2.
What are the differences between an orthophoto and a
vertical aerial photograph?
What is the scale at which the official orthophoto maps of
South Africa is printed?
Word bank
Oblique aerial photographs:
Orthophoto map:
Photogrammetry:
Stereopairs:
Terrestrial photographs:
Vertical aerial photographs:
50
•
Aerial photographs
photos taken from the air with the camera
at an angle
a vertical aerial photograph showing
features of a topographic map such as
contour lines
the process of obtaining reliable
information about physical objects and the
environment through processes of
recording, measuring and interpreting of
photographic images and patterns
an overlapping pair of vertical aerial
photographs
conventional photographs of features
normally taken from ground level. Since it
lacks the plan view it cannot be used to
make maps
photographs taken from the air with the
camera pointing directly down onto the
Earth
Topic 1
What you still need to know
Practice still makes perfect
For a long time all the information we find on maps had to be surveyed in the
field. Imagine the enormous task of measuring the position and establishing the
nature of each and every feature we see on maps. During and after World War 1
the technologies of aerial photography and photogrammetry developed rapidly.
It takes years of continuous practice to become a master in photo interpretation.
The more realistic views offered by aerial photographs are used to update an
existing map. This topic will help you develop your basic understanding of
different types of photographs and your ability to identify features and patterns
on the photograph. This is much easier when you have a topographic map of the
photographed area. You can then compare the photograph with the map to
check whether the area, which you think is an orchard, is indeed an orchard and
not perhaps a natural forest. You can also visit the photographed area to reveal
the nature of the phenomena that were puzzling.
What you still need to know
Different types of photographs:
their advantages and disadvantages
Figure 1.42: Terrestrial photographs
Aerial photographs
•
51
Figure 1.42: Terrestrial photographs (cont.)
Terrestrial photographs
We are sure you can imagine the power of the photographs above as elements
of reports or letters pointing out extreme water or air pollution, lack of road
maintenance or poverty. These photographs are called terrestrial photographs
and can communicate hard facts and generate strong emotions. Geographers
can definitely use them too. However, since such photographs do not show
location or the plan view of maps we referred to earlier, they cannot be used to
make maps.
Oblique aerial photographs
Photographs such as the images shown in Figures 1.43 and 1.44 taken from an
aircraft show much more spatial information about an area than photographs
taken from ground level. The two photographs below are illustrations of lowangle oblique (not showing the horizon) and high-angle oblique aerial
photographs (showing the horizon).
Figure 1.43: A low-angle oblique view
52
•
Aerial photographs
Figure 1.44: View of FNB stadium in Johannesburg,
extracted from a high-angle oblique photograph
Topic 1
In the case of low oblique photographs the camera on board the
aircraft is pointed at angles of between 15° and 60° to the ground.
In the case of high oblique photographs the camera angles are
between 60° and 89°. The advantages of oblique aerial photographs
are:
Figure 1.45: Camera position for
oblique aerial photographs
•
We are familiar with the views – imagine looking at the
landscape from a hill or a high building
•
We can see relative heights and thus the relief of the landscape
Disadvantages associated with oblique
photographs
• These contain ‘dead ground’ as the foreground often hides the
background.
•
Relative location is distorted making it difficult to observe
spatial patterns.
•
They cannot be used to measure distances and calculate areas
because the foreground is at a much larger scale than the
background.
Figure 1.46: Camera position for
vertical aerial photographs
Vertical aerial photographs
In the case of vertical aerial photographs, a line from the camera to the focus
point on the ground strikes the Earth at an angle of 90°. Such a view has a big
advantage over that of other types of photographs. It comes very close to the
plan view that is used to draw maps. Vertical aerial photographs remain the
main source of data to create new and update existing maps.
Advantages of vertical aerial photographs
• They offer more detail and a more realistic view than a map. A map has
been generalised and simplified by the cartographer. An aerial photograph
is a raw image which still has to be interpreted by the user. To check this
statement you are welcome to compare the vertical aerial photograph of
Nelspruit (Figure 1.47 on page 54) with the 1:50 000 topographic map of
Nelspruit (see Figure 1.2 on page 4).
Aerial photographs
•
53
54
•
Aerial photographs
Figure 1.47: A vertical aerial photograph of the Nelspruit area
Topic 1
•
A single vertical aerial photograph is only one of many photographs taken
during a photo survey job of an area. The survey is planned in such a
manner that adjacent photographs overlap. The implication is that the
problematic scale-distorted areas at the margin of a photograph will be at
or very close to the centre of another photograph. By building a photo
mosaic from the centre areas of several photographs one can create a large
composite photograph that has little or no distortion.
•
One can use overlapping pairs of vertical photographs (stereopairs) to
view the landscape in three dimensions.
Disadvantages of vertical aerial photographs
A vertical aerial photograph is only true to scale at the centre of the
photograph. Near the centre the scale distortion is so minimal that you can
ignore it. However, the further one moves away from the centre, the more the
distortion. Another disadvantage is that the view from above is not a familiar
view. With practice this disadvantage can be overcome.
Orthophoto maps
Orthophoto maps combine all the advantages of maps and aerial
photographs. On orthophoto maps the detailed photographic background has
been rectified to remove scale distortion. Accurate measurements can
therefore be made. Additionally, background such as a co-ordinate grid,
contours (5 m interval), spot heights, place names and road numbers that we
associate with maps have also been added. An example of such an orthophoto
is provided in Figure 1.48 on page 56.
What you still need to know
Interpreting photographs of landscapes
Some general guidelines for interpreting photographs are:
1.
Find out where the photographed area is, i.e. localise the photograph.
2.
Orientate the photograph with regard to the cardinal compass directions.
3.
Use a 1:50 000 topographic map of the photographed area to help you
identify several features on the photograph.
Aerial photographs
•
55
Figure 1.48: Extract from orthophoto 2530BD CITRUS. The area shown is the same as the
small framed area in Figure 1.2 on page 4.
56
4.
Determine the scale of the photograph to get a feeling of how large or
long features are.
5.
Distinguish between natural features (e.g. natural forests) and
constructed features (e.g. cultivated land, towns or railway lines).
6.
Look for broad patterns in the landscape, e.g. whether the area is an
agricultural, residential or industrial area.
7.
Look for details, e.g. if the photo shows an agricultural area. The next
step will be to determine whether it is an orchard or a field crop. The last
step will be to determine whether the area has been planted with citrus,
peaches or maize.
8.
Work from the known to the unknown. If you know that an area is
planted with vineyards, the big building nearby is likely to be a wine
cellar.
•
Aerial photographs
Topic 1
9.
See whether you can identify spatial patterns and spatial relationships.
The layout of the streets or the lack of development in certain areas might
be explained by the relief of the landscape. By looking at sites that have
been cleared for development you might detect that the development
tends to be either linear or concentrated in a certain area. In other words,
do not look at a single feature in isolation – try see the bigger picture.
Classroom activity 13
In this activity you need to compare a vertical aerial photograph with a
topographic map of the same area. You can do it with a friend or in small
groups.
1
Which area on the 1:50 000 map of Nelspruit (Figure 1.1 on page 4) is
shown by the vertical aerial photograph (Figure 1.47 on page 54).
Simply write down the most appropriate alphanumeric block
reference/s from the topographic map.
2
Estimate (guess) the approximate scale of the vertical aerial
photograph of Nelspruit.
3
See if you can identify the following features on the aerial
photograph. Describe what they look like: a railway line, a bridge, a
parking area, agricultural land, a residential area, the central business
district.
Homework activity 9
1
Compare the agricultural areas shown on the orthophoto map with
the same areas as shown on the 1:50 000 topographic map. Briefly
summarise the differences in detail that you have spotted.
2
What evidence from the orthophoto map suggests that not all
agricultural land is privately owned?
Aerial photographs
•
57
Extra practice activity 6
58
1
Use the orthophoto map to draw an annotated map showing fruitbearing citrus orchards. The map should at least have a title, a
legend, a north arrow and a line scale.
2
How many school grounds can you spot on the vertical aerial
photograph?
3
Each of the South African official 1:10 000 orthophoto maps cover an
area of 3’ square. How many orthophotos are required to map the
area covered by one 1:50 000 map sheet?
4
Use the library or the Internet to find information about the
fascinating early history of aerial photography that you can share
with the rest of the class. We recommend the first two of the three
URLs listed below. The third URL contains a wealth of information
about aerial photography in general.
•
•
http://www.remembrancetrails-northernfrance.com/learn-more/weapons/
aerial-photography.html
•
•
•
http://www.brighthub.com/multimedia/photography/articles/10374
http://www.colorado.edu/geography/gcraft/notesremote/ remote.html
http://www.papainternational.org/history.html
Aerial photographs
Topic 1
Satellite remote sensing
What you know already
Remote sensing is used in everyday life
•
Satellite images can be used to gather information about phenomena such
as cloud patterns, water surfaces, vegetation and how the land is used.
•
•
Different types of satellites are used for remote sensing.
METEOSAT, SPOT and LANDSAT are examples of different types of
satellites used to gather information about our Earth.
There are many different forms of remote sensing. Your eye-brain system is a
wonderful example of remote sensing. You can gather information about the
vegetable garden in the backyard at home by simply looking at it. You might
notice that the leaves of the spinach are wilted and yellowish in colour. You
can act on your observation by deciding to water the spinach and add organic
fertiliser because there is a nutrient deficiency in the soil. Another example is a
medical doctor taking an x-ray image of your knee that got in the way of a
cricket ball that had been fiercely driven to the boundary.
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Can you still remember the different types of satellite images?
Meteorological (weather) satellites give us images of very
large areas so that we can see and predict weather conditions.
Earth resource satellites such as SPOT and LANDSAT provide
detailed images of much smaller areas so that we can monitor
our use (and often abuse!) of natural resources.
‘Google Earth’ is the web-based software one can use to view
a satellite image of the school or your home or the devastation
caused by the earthquake that hit the Japanese city of Sendai
(see Figure 1.52(c) and (d) on page 66) on Friday 11 March
2011. Suggest how images from Google Earth could have been
of assistance in estimating the extent of the flooding after the
earthquake triggered a tsunami.
Satellite remote sensing
•
59
Word bank
μm:
Geostationary satellite:
Pixel:
Sun-synchronous satellite:
the symbol for micron or micrometre. A
micrometre is one-millionth of a metre or
one-thousanth of a millimetre (1/1 000 of
a millimetre or 0.001 mm)
these satellites orbit the Earth at the
same speed as the rotation of the Earth
on its own axis. The result is that the
satellite appears to be stationary above a
specific area on the Earth. These satellites
(e.g. meteorological satellites) provide a
constant flow of images of the same area
as a digital photograph, a satellite image
consists of a matrix or grid of cells in
which data are stored. We refer to such a
format as a raster format. The individual
cells of the matrix are called picture
elements or pixels. The pixels are clearly
seen in Figure 1.51
these satellites are programmed to
encircle the globe along fixed tracks.
Images are still continuously captured
but the images are of different areas. In
the case of LANDSAT satellites it takes
approximately 16 days for the satellite to
revisit a specific area on the ground to
capture a new image
What you still need to know
How satellite remote sensing works
The purpose of remote sensing is to gather information about the Earth so that
we can act upon it and use it to make informed decisions. In the case of aerial
photography the sensor is a camera on board an aeroplane. In the case of
satellite remote sensing the sensor is carried by geostationary or sunsynchronous satellites which are hundreds of kilometres above the surface of
the Earth. Photography as a form of remote sensing produces photographs.
Satellite remote sensing produces images. Figure 1.49 illustrates a satellite
remote sensing system consisting of eight elements. Elements 1 to 6 have to do
with data acquisition. Element 7 focuses on data analysis while element 8
implies application of the results of the analysis.
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Topic 1
Data acquisition
1.
Data analysis
Application
During analysis computers
are used to enhance the
image and to extract the
required information
A major application of
satellite images and
data is the integration
with maps and other
spatial information in
a GIS
5.
2A.
transmission
2B.
reflected energy
4.
6.
7.
8.
3. Interaction at the
Earth’s surface
Figure 1.49: A satellite remote sensing system
1
3000 000 mm
3 × 10–6 µm
gamma
rays
0,28 µm
0,40 µm
long-wave
ultraviolet
1
300 000 mm
3 × 10–5 µm
X rays
0,45 µm
0,01 µm
ultraviolet
0,50 µm
0,28 µm
visible
near
infrared
short-wave
infrared
middle
infrared
0,58 µm
0,59 µm
0,62 µm
0,40 µm
0,70 µm
1,50 µm
3,00 µm
5,50 µm
thermal
infrared
1 mm
1 × 103 µm
microwave
(incl. radar)
0,70 µm
reflected
(near)
infrared
1m
1 × 106 µm
1,50 µm
radio
waves
3 000 km
The eight elements of a
satellite remote sensing
system
1. A source of energy – element 1 in Figure
1.49. To be able to see things we need light.
To produce light we need energy. The sun is
our primary source of energy. The sun
radiates electromagnetic energy that travels
through the atmosphere in the form of
electromagnetic waves. The total range
(spectrum) of waves is known as the
electromagnetic spectrum (see Figure 1.50).
We can distinguish between different waves
on the basis of their wavelength. Some are
long low energy waves (e.g. radio waves
measured in kilometres) whereas others are
very short high energy waves (e.g. x-rays
measured in μm). Only a very tiny portion
of the waves in the electromagnetic
spectrum can be seen with our naked eyes.
That part is known as the visible
wavelengths or visible spectrum and
consists of the colours of the rainbow.
1 × 1012 µm
Figure 1.50: The electromagnetic spectrum
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2. Transmission through the atmosphere – element 2 in Figure 1.49. Not all
electromagnetic waves from the sun reach the Earth’s surface. Dust
particles and gases in the atmosphere cause scattering and absorption of
some of the light and radiation. The regions of the electromagnetic
spectrum which are not seriously affected by scattering and absorption
and thus reach the Earth are called atmospheric windows. Satellite sensors
are designed to be sensitive to the wavelengths that are able to pass
through these windows to the Earth’s surface.
3. Interaction at the Earth’s surface – element 3 in Figure 1.49. At the Earth’s
surface the energy can be absorbed, transmitted or reflected. The amounts
of energy that will be reflected, absorbed or transmitted is unique for
different Earth features. In remote sensing terminology we say that unique
features have unique spectral signatures. This is very important since it
allows us to distinguish between Earth features by measuring the nature of
their interaction with electromagnetic energy. As our signatures and
fingerprints are unique, different phenomena respond in a unique manner
to different waves within the electromagnetic spectrum. Think about a
well-groomed soccer field. The grass reacts in a unique manner to the
waves in the visible spectrum. Most of the light associated with the green
waveband is reflected to our eyes while the light associated with other
colours is absorbed by the grass and does not reach our eyes. In our brain
the incoming reflection is compared with thousands of images already
stored which we use as references (signatures) to recognise features.
Within a fraction of a second our brain reports back that the new image
best matches the image of ‘grass’.
4. Of particular importance is reflected energy – element 4 in Figure 1.49.
Most satellite sensors are designed to measure the amount of Earth
reflection in those regions of the spectrum which we referred to as
atmospheric windows. Note the line numbered 2B. Radar sensors do not
detect reflected solar radiation. They are active sensors – they emit their
own energy and then measure the radiation that is reflected or scattered
back to the sensor.
5. Recording of reflected energy by a sensor system – element 5 in Figure
1.49. A conventional digital camera records all wavelengths within the
visible spectrum as a single image. Satellite sensors are more sophisticated.
They can measure and record reflection within the individual wavelengths
of the visible spectrum as separate numeric images. They can even record
wavelengths which we cannot see with our naked eyes – an example is
detection of infrared radiation or the x-ray image we referred to earlier.
Imagine it would be possible to cover a large area on the Earth with a grid
of which the grid cells all measure 30 m by 30 m. The sensor measures the
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reflection within each grid cell (referred to as pixels or picture elements)
and the measured values are written in data files – one file for each of the
spectral bands for which the sensor is sensitive.
The concept of a data file is shown in Figure 1.51. We have zoomed into a
satellite image and extracted data from it. Figures 1.51(a), (b) and (c) show
the reflectance values as measured in three spectral bands. Low values
represent little reflection whereas larger values represent high reflection.
(a) Band 3 (red
reflectance)
(b) Band 4 (nearinfrared reflectance)
(d) Band 3
(e) Band 4
(c) Band 5 (short-wave
infrared reflectance)
(f) Band 5
(g) A false colour image
using bands 3, 4 and 5
Figure 1.51: Satellite images are matrixes of numerical values representing reflection
measured in different bands of the electromagnetic spectrum.
6. Transmission, reception, and processing at Earth station – element 6 in
Figure 1.49. The numerical images (data files) now have to be transmitted
back to Earth. Line 6 in Figure 1.49 illustrates a scenario where the data are
transmitted directly to a processing station such as the Satellite
Application Centre (SAC) that is located at Hartebeeshoek, 75 km west of
Pretoria. Here the image is processed into a format in which it can be used
for analysis by the South African user community of researchers and local
and national government departments.
7. Interpretation and analysis. We can of course attempt the impossible by
trying to make sense of the individual numbers written into the cells or
pixels of the image. Since a numerical image such as that of LANDSAT TM
consists of more than 38 million pixels for each of the spectral bands, this is
not really an option. Computers and image processing software are
therefore used to create an image that resembles a picture that we can view
and interpret manually or to ‘automatically’ interpret the image data
according to our instructions.
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Visual interpretation. One way to interpret the satellite data is to make an
image or images that we can interpret manually. In Figure 1.51 (d), (e) and (f)
you can see what the numerical matrixes shown in (a), (b) and (c) look like
when shown in shades of grey. A reflection value of zero will appear pitch
black while a reflection value of 255 (the maximum possible value) will appear
pure white.
A popular type of image is a false colour image. It consists of three spectral
bands that are shown in the three primary colours, namely red, green and blue.
Conventionally a spectral band representing reflection measured in a band
outside the visible spectrum (e.g. infrared reflection) is shown in red. Figure
1.51 (g) represents such an image. The near infrared reflectance values have
been shown in red, the short wave infrared reflectance in green and the visible
red reflectance in blue. A false colour image therefore shows varying shades of
red, green and blue which have been superimposed. It is called false colour
because the features are not shown in their natural colours we are familiar
with, e.g. healthy vegetation appears red because it strongly reflects infrared
light. It is common practice today to monitor crop growth by analysing the
crop’s infrared reflection. By doing this one can detect if a crop is drought
stressed or needs nutrient supplements.
Computer analysis is a second method of analysing the images. A computer
has the ability to quickly analyse the millions of numerical values and produce
meaningful information. Just as your personal signature is supposed to be
unique, so the manner in which different features (e.g. dams, plantations and
urban areas) reflect energy associated with different spectral bands is also
unique. We say that each feature has a unique spectral signature. In other
words, it reacts in a unique way to electromagnetic radiation. If we can tell an
image processing system what the spectral signature of, say, a dam, a
plantation or an urban area looks like, the computer simply compares the
reflectance values of each pixel in the database with the three spectral
signatures and reports back by assigning different codes (numbers) to the
pixels which best resemble the spectral signatures of dams, plantations and
urban areas respectively. We can then assign colours to each of the codes and
display the result as a thematic map showing dams, plantations and urban
areas. Because the map is digital in nature it is quite easy to integrate it with
other layers of information in a Geographic Information System (GIS). You will
learn more about this topic in the next unit.
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