Download 4Location

Where in the world
are we? …
• A system for describing
location on the earth
• What is the shape of the
earth …
• and how earth’s shape
affects the process of
determining location
Location in 2 dimensions
+
The
Cartesian
coordinate
system
3
(3,2)
2
1
+
(0,0)
1
Y axis
-
X axis
-
2
3
Location on a rotating sphere
Starting
points:
• Origin
(center of
the globe)
• Axis of
rotation
Great Circle
Meridians: the “y” grid lines
 Meridian:
a great
circle that passes
through the earth’s
poles
 Prime
meridian:
Arbitrary starting
point
Longitude
 Longitude:
angular location,
on the earth’s
surface, from the
Prime Meridian
 Circle has 360
degrees total:
Meridians range
from 180° east to
Prime
180° west
Meridian
60°
Location on a rotating sphere: “x”
axis
Great Circle that
is perpendicular
to the axis of
rotation
the equator
Parallels: the “x” grid lines
Small Circle
Parallel:
• A small circle that is
parallel to the equator
(and at right angles to the
axis of rotation)
Latitude
North
Pole
90°
 Latitude:
angular
location on the
earth’s
surface,
between the
equator and
one of the
poles
60°
Equator
0°
90°
South
Pole
Longitude and latitude:
Measures of angle
Longitude
Latitude
Meridians & parallels:
Lines of a constant lat. or long.
Parallel
Meridian
Geographic Coordinate
System:
• Parallels & meridians (the graticule)
• Latitude and longitude
A
system for defining location on
a sphere!
 But
is the earth really a sphere?
The earth’s true shape: an
ellipsoid?
N
 Rotation
of the earth
generates centrifugal
force
 Polar
flattening
S
So …
 Ellipsoidal
shape of earth does not affect
latitude and longitude, but it does affect the
spacing of parallels & meridians!
North Pole
0°

Equator
Length of 1°:
from Equator:
68.7 miles
from N or S Pole: 69.4 miles
Extremely accurate maps must base measurement
of distances on an ellipsoid rather than a
spherical shape.
Don’t need to know the math but
need to know the concept
The earth’s true shape: A geoid?

When doing precise mapping, it turns out that we
need to consider a “bumpiness” to the earth’s
surface.
Not mountains!


This bumpy shape is caused not by mountains, but by
variations in the density of the earth’s crust. These
variations cause variations in the effect of gravity.
Thus, a geoid is a model of the earth’s surface taking
into account variations in gravity.
Variations in gravitational pull
 Geoid:
A model of the earth’s surface
showing variations in gravity.
Given that the shape of the earth is
so complex ...
How do we figure out where,
precisely, we are?