Download Introduction to Cartography GEOG 2016 E

Introduction to Cartography
GEOG 2016 E
Lecture-4
Sources of Data
Cartographic Data and Surveying
• To make a map we need data.
• The cartographic data is generally obtained by
surveying.
• There are other methods too, such as
– Remote Sensing, Census and Sampling
• However, surveying is the most commonly used
method of gathering data for generating maps.
• Surveying is the method of determining the
relative positions of points on, above or beneath
the earth’s surface.
Categories of Surveying
• Plane Surveying
– Surveying with the reference base for fieldwork
– Surface is assumed to be flat horizontal
– Generally within a radius of about 20 km, the pull
of gravity is nearly parallel – hence horizontal lines
can be considered straight
• Geodetic Surveying
– To determine relative positions of widely spaced
points, lengths and directions
– Takes earth’s curvature into account
Survey Types
• Photogrammetry
– By camera or other sensors in airplanes or
satellites
• Topographic Surveying
– Collecting data specifically for making maps
• Route Surveying
– For civil engineering projects, such as highways,
railroads, pipelines etc.
• Hydrographic Surveying
– To map shorelines and the bottom of water bodies
Measuring Distance
• Linear measurement is the basis of all
surveying
• Methods of measuring horizontal distance
– Rough Measurements: Pacing, Odometer,
Tacheometer
– Accurate Measurements: Taping, EDM, GPS
– EDM and GPS are the most commonly used
methods for surveying
Taping
• Applying the known length of a graduated tape
directly to a line a number of times
• Steps
– Lining in: shortest distance between two points is a
straight line
– Applying tension: rear point of tape is anchored and
tension is applied to the head
– Marking tape lengths: each application of the tape
requires marking using chaining pins
– Reading the tape: the graduated tape must be read
correctly
– Recording distance: total length must be recorded
accurately
Types of Tapes and Chains
• Gunter’s Chain
– 66’ long with 100 links with each link being 7.92
inches long
– Developed by Edmund Gunter in 1600’s in England
• Engineer’s Chain
– Same construction as Gunter’s chain but each link
is 1.0’ long
Types of Tapes and Chains
• Surveyor’s and Engineer’s Tapes
– Made of ¼” to 3/8” wide stell tapes in 100’, 200’, 300’
lengths
– Multiple types of marking and graduation
• Available in feet and metric
• Invar Tapes
– Made of special nickel steel to reduce length
variations due to temperature changes
– Extremely brittle and expensive
– Used mainly for standard comparison of tapes
• Cloth, Fiberglass, PVC Tapes
– Lower accuracy
– Used for measurements of 0.1’ accuracy
Slope Measurements
• Generally, measurements are made horizontally
• Often the distance can be measured directly on
the slope
• However vertical or zenith angle must be obtained
– Horizontal Dist. = sin(zenith angle) x slope distance
– Horizontal Dist. = cos(vertical angle) x slope distance
Zenith
Angle
Vertical
Angle
Horizontal Distance
Taping Error
• Instrumentation Error
– Tape may be defective
• Natural Error
– Tape length changes due to temperature, wind or
weight of the tape
• Personal Error
– Carelessness in setting pins or reading tape
Transit
• Transit is the most widely
used surveying instrument
• Used for measuring
horizontal and vertical
angles
• Can also measure vertical
and horizontal distances
• Components
– Alidade: upper part
– Horizontal limb: middle part
– Levelling-head: Lower part
Total Station
• Modern version of transit
with an electronic distance
meter (EDM)
• Electronic/optical instrument
• Can measure slope distances
from the instrument to a
particular point
• Extensively used in modern
surveying
Levelling
• Levelling is the determination of the elevation of
a point or difference between points referenced
to some datum
• Terminologies
– Datum: Any level surface to which elevations are
referenced
– Mean Seal Level (MSL): The average height of the
surface of the sea for all stages of the tide over a 19
year period at 26 tide stations along Pacific, Atlantic
and Gulf
– National Geodetic Vertical Datum: Nationwide
reference surface for elevations throughout the US
Levelling
• Mostly mean sea level is used
– MSL varies along the coast
– Pacific is almost 2’ higher than Atlantic and Gulf
• The level surface parallels the curvature of the
earth
– Hence a level line is a curved line
• Generally measured with the Automatic or
Self-levelling level
Differential Levelling
• Also called Spirit Levelling
• Most common type of
levelling
• Determines the difference
in elevation using a
horizontal line of sight and
readings on a graduated
rod
Topographic Surveying
• Process of:
– determining the positions of the natural and
artificial features on the earth’s surface
– Determining the configuration of the terrain
• Planimetry
– Location of features
• Topography
– Configuration of the ground
Topographic Surveying
• Scale and accuracy
– Depend on the instrument used and method
• Presentation method
– Contour Lines: Imaginary line on surface of the earth
passing through points that have equal elevation
– Contour Intervals: Vertical distances between lines
– Index Contour: Every 5th contour drawn heavier on
maps
– Slopes can be obtained from contours
Topographic Surveying
• Interpolation
– Can find elevation of any point
– Or find a contour line with known elevation of a point
• Contour Characteristics
– Each contour must close within a map or outside its
borders
– Contours do not cross or meet except in caves, cliffs and
vertical walls
– Contour lines crossing streams form V’s pointing
upstream
– Contour lines crossing a ridge form U’s pointing down
the ridge
Methods of Topographic Surveying
• Factors that influence the choice of method
– Scale of the map
– Contour interval
– Type of terrain
– Available equipment
– Accuracy required
– Extent of area to be mapped
Methods of Topographic Surveying
• Cross section
– Railroad or highway
– Equipment used: transit, tape and level
• Trace contour
– Drainage or impoundments
– Involves finding elevations at different points using transit
• Grid
– Small areas
– The smaller the grid, the better the accuracy
• Controlling point
– Large area, plane table
– Positions and elevations of pre-selected control points
• EDM
– Radial
– Equipment: total station
Errors in Topographic Surveys
• Improper selection of contour interval
• Improper equipment or field method for the
particular survey and terrain conditions
• Insufficient horizontal and vertical control of
suitable precision
• Omission of some important topographic
details
Global Positioning System (GPS)
• Developed in early 1980’s by US Department of
Defence
• Made up of 26 satellites orbiting earth
• 24 are functional and 2 spare
• Each satellite is in a fixed position
• Signals from at least 3 satellites are needed for
accurate positioning
• Receivers with high position accuracy are fairly
expensive
Geographical Information System (GIS)
• GIS are computer programs that allow users to
store, retrieve, manipulate, analyze and display
spatial data
• GIS techniques are now being used in many
fields, such as physics and medicine
• GIS components
–
–
–
–
Hardware (computer)
Software
Data
Operator
GIS Data Structures
• A GIS has two main data structures: vector
and raster
• Vector is made up of points, lines and
polygons
GIS Data Structures
• Raster or grid is made up of pixels on
computer screen