Geodesy, Map Projections and
Coordinate Systems
•
Geodesy
- the shape of the earth and
definition of earth datums
•
Map Projection
- the transformation of a
curved earth to a flat map
•
Coordinate systems
- (x,y) coordinate
systems for map data

Learning Objectives:
By the end of this class you should know:
•
the role of geodesy as a basis for earth datums
•
how to calculate distances on a spherical earth
•
the basic types of map projection
•
the properties of common map projections
•
the terminology of common coordinate systems
•
how to use ArcGIS to convert between
coordinate systems

Spatial Reference =
Datum +
Projection +
Coordinate system
•
For consistent analysis the
spatial reference
of
data sets should be the
same.
•
ArcGIS does
projection on the fly
so can display
data with different spatial references properly
if
they are properly specified.
•
ArcGIS terminology
–
Define projection.
Specify the projection for some
data without changing the data.
–
Project.
Change the data from one projection to
another.

Types of Coordinate Systems
•
(1) Global Cartesian
coordinates (x,y,z) for
the whole earth
•
(2) Geographic
coordinates (
f, l
, z)
•
(3) Projected
coordinates (x, y, z) on a local
area of the earth’s surface
•
The z-coordinate in (1) and (3) is defined
geometrically
; in (2) the z-coordinate is
defined
gravitationally

N
E
W
S
(N, W)
(S, W)
(N, E)
(S, E)

Global Cartesian
Coordinates (x,y,z)
O
X
Z
Y
Greenwich
Meridian
Equator
•

Geographic Coordinates
(
f, l
, z)
•
Latitude (
f
) and Longitude (
l
) defined
using an
ellipsoid
, an ellipse rotated about
an axis
•
Elevation (z) defined using
geoid
, a surface
of constant gravitational potential
•
Earth
datums
define standard values of the
ellipsoid and geoid

Latitude
and Longitude
Latitude - distance from the equator along the Y axis
Longitude - distance from the prime meridian along the X axis.
.

Equator
Latitude 0
o
Latitude:
(
90
o
N to 90
o
S)
Latitude 23½
o
North
Tropic of Cancer
Latitude 23½
o
South
Tropic of Capricorn
Longitude
30
o
East
Longitude
60
o
East
Longitude
30
o
West
Longitude
60
o
West
Positioning on the Earth
’
s Surface
East is the direction of
rotation of the Earth
North Pole
South Pole
23½
o
23½
o
66½
o
90
o
90
0
21
st
June
22
nd
December
22
nd
Sept
20
th
March
30
o
E
60
o
E
90
90
o
W
30
o
60
o
Longitude 90
o
East
Longitude 90
o
West
Prime Meridian
0
o
Longitude
Longitude:
(180
o
E to 180
o
W)
Latitude and Longitude together enable the fixing of position on the Earth
’
s surface.
Geographical coordination system

Shape
of the Earth
We think of the
earth as a
sphere
It is actually a
spheroid
,
slightly larger in radius at
the equator than at the poles