1. Datums and Coordinate Systems
Lecture 3 cont’d
Datums and Coordinate Systems

2. Reference Datum
Is a known and constant surface which can be used to describe the location of unknown points (in x, y and z)

3. Reference Datums 2 components: Horizontal datum:
origin (X,Y) and orientation of a datum
defined by the size and shape of an ellipsoid
Vertical datum:
the reference surface for height
E.g. Global Positioning system is based
on the World Geodetic System 1984 (WGS-84)

4. Example: NORTH AMERICAN DATUM OF 1927 based on Clarke 1980 Ellipsoid
World Geodetic System (WGS84) based on GRS 1980 Ellipsoid

5. Datum shifts

6. Coordinate Systems
provide a spatial referencing system to locate points on the earth surface
spherical coordinate system
coordinates that describe locations on a sphere
planar coordinate system
coordinates that describe locations within a two dimensional Cartesian space

7. Planar Coordinate Systems
Universal Transverse Mercator (UTM)
units in meters
transverse cylindrical projection
60 vertical zones cover globe
northing in meters from equator
easting in meters from false origin 500,000 meters west of zone central meridian

9. Planar Coordinate Systems
United States State Plane Coordinate System
units in feet
projection in Transverse Mercator or Lambert’s conformal conic
tied to U.S. datum
one or more zones for each state with a false origin southwest of the zone

10. Planar Coordinate Systems
United States State Plane Coordinate System

11. Spherical Coordinate Systems
latitude and longitude
parallel: lines parallel to equator
meridian: lines going from pole to pole

12. Planar Coordinate Systems
Prime Meridian 1
● (x,y)
Equator - 1 +

13. Planar Coordinate Systems
rectangular grid with X and Y axes
X: easting
Y: northing
point is defined by X,Y coordinate

14. W E N S

15. How does this look in ArcGIS?
You have the option of choosing either a pre-defined Datum or an Ellipsoid as your reference surface
Recommendation: use a DATUM
Example: how to project a coverage in ArcToolbox