1. Amanda Henley GIS Librarian Davis Library Reference September 2006
Introduction to Coordinate Systems and Working with Coordinate Systems in ArcGIS
Amanda Henley
GIS Librarian
Davis Library Reference
September 2006

2. …what to do?
Orange County, NC

3. Today’s Presentation
Introduction to Map Projections and Coordinate Systems
Modeling the earth
Geographic Coordinate Systems
Datums
Projected Coordinate Systems
Examples of common coordinate systems
Working with Coordinate Systems in ArcGIS
On the fly projections
“Geographic Coordinate System: GCS_Assumed_Geographic_1”
Additional resources

4. Modeling the Earth The Earth is not a perfect sphere
It is an Oblate Spheroid
Different Spheroids have been devised to model the earth- they are distinguished by the length of their axes:
Semi-
Major
Axis
Minor

5. Geographic Coordinate Systems
Locations are defined on a 3-D spherical surface
Made up of graticules rather than grid cells
Units are in degrees
Image Source: Understanding Coordinate Systems, ESRI 2000:

6. Geographic Coordinate Systems
Not uniform:
Distances and measures are not accurate
Meridians Converge Near Poles
1° longitude:
@ Equator= 111 km
@ 60° lat. = 55.8 km
@ 90° lat. = 0km
Distance of 60° long at equator
vs.
Distance of 60° long at 40° latitude
Original Image Source:

7. Geographic Coordinate Systems
Use Decimal Degrees (angles), 3 digits or less
North America:
West of the Prime Meridian, so Longitude (X) is negative
North of the Equator, so Latitude (Y) is positive
Image Source: [

8. Converting between degrees, minutes seconds and decimal degrees
GIS Software takes Geographic Coordinates in Decimal Degrees, not degrees, minutes, seconds
Converting is easy
Divide each value by the number of minutes or seconds in a degree.
Example
37 degrees 36 ' 30"
Divide 36 minutes by 60:
36/60=.60
Divide 30 seconds by 3600
30/3600=.00833
Add up the degrees to get the answer
37 degrees = DD

9. Components of a Geographic Coordinate System
Geographic Coordinate Systems include
A Datum
An angular Unit of Measure (degrees)
A Prime Meridian

10. Datums Described Links a spheroid to a location on the earth
Define the origin and orientation of the coordinate systems used to map the earth
A datum is “a fixed, three-dimensional surface, an oblate spheriod, that is approximately the size and shape of the Earth. From this surface, Latitude, Longitude, and Elevation are computed”(Source Below).
Source:

11. Datums Described, Cont.
A spheroid is an earth model, a datum is a practical application of the model (souce below).
A spheroid model of the earth is fixed to a base point
Example:
The USGS decided that the Clarke 1866 spheroid was a good approximation for the shape of the earth within the US, so they linked it to Meade’s Ranch Kansas.
Thus we have NAD27
It is wrong- as you move away from the fixed point errors increase
We have many datums because we keep re-measuring the earth, hopefully getting closer each time
Source:

12. Geodetic Datums There are many datums
Local:
NAD 27 Datum, uses Clarke 1866 spheroid
NAD 83 Datum, uses GRS 1980 spheroid
Used in US and Canada Only
Global
WGS 84 Datum, uses WGS 1984 spheroid
*Very* similar to NAD 83
In addition to being in the same projection, data themes must also be in the same datum.
Source: Peter H. Dana, The Geographer's Craft Project,
Department of Geography, The University of Colorado at Boulder [
Accessed: 01/25/06

13. Errors up to 1 km can result from confusing one datum for another
Datum Differences May Be Difficult to See
In this case, the boundaries are roughly 32 meters off: datum shifts are not uniform
Errors up to 1 km can result from confusing one datum for another

14. Datum Differences Continued
Latitude and Longitude are measurements based on a datum
They are not absolute locations
Latitude Longitude coordinates alone are not enough
Example Davis Library:
NAD83: ,
NAD27: ,
(almost 29 meters away)

15. Datum Transformations ArcGIS 9.1
NAD 1927 to NAD 1983
-for areas in the 48 contiguous states
Name Code Area of Use
NAD_1927_To_NAD_1983_NADCON 1241 United States (contiguous 48 states - CONUS)
WGS 1984 to NAD 1983
Name Code Area of Use
NAD_1983_To_WGS_1984_ United States
There are five transformations to chose from, each is appropriate for different areas. 5 Supercedes 4.

16. Data in a Geographic Coordinate System: United States

17. Data in a Geographic Coordinate System: North Carolina

18. Projected Coordinate Systems
Projected Coordinate Systems mathematically transform the 3 dimensional earth so that it can be modeled in 2 dimensions.
This results in distortion
Different projections are used for different areas and purposes

19. Different Projections Preserve Different Properties of the Earth
Directions
Distances
Shapes
Areas
If some properties are maintained,
errors in others may be exaggerated

20. Commonly Used Projected Coordinate Systems
State Plane- a coordinate system that divides the United States, Puerto Rico and U.S. Virgin Islands into >120 zones.
North Carolina State Plane Meters, NAD83 is used by the North Carolina CGIA
NC State Plane Feet, NAD83 is used by most local data providers.

21. U.S. State Plane Zones
Image Source: Accessed January 27, 2006.

22. Georgia State Plane Meters East and West

23. North Carolina State Plane Meters and Feet

24. Commonly Used Projected Coordinate Systems Cont.
UTM- Universal Transverse Mercator divides the globe into 60 zones, each 6° longitude.
Transverse Mercator is accurate for narrow zones
Often used for large scale scientific mapping
Units are in meters

25. UTM Zones in North Carolina

26. Spatial Data for Orange County, NC in two different UTM Zones

27. Commonly Used Projected Coordinate Systems Cont.
Albers Equal Area Conic:
“Used by USGS for maps showing the conterminous United States (48 states) or large areas of the United States. Well suited for large countries or other areas that are mainly east-west in extent and that require equal-area representation. Used for many thematic maps.”
Source: USGS Accessed, 1/27/06.

28. Albers Equal Area Conic

29. Data in Albers Equal Area Conic and Geographic Coordinate System

30. Working With Coordinate Systems in ArcGIS
On the Fly Projection
“Geographic Coordinate System: GCS_Assumed_Geographic_1”
Demonstrations
Defining a Layer’s Coordinate System Using ArcCatalog
Defining a Layer’s Coordinate System Using ArcToolbox
Projecting a layer in ArcToolbox
Projecting a layer in ArcMap