Working with Coordinate Systems in ArcGIS

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## Working with Coordinate Systems in ArcGIS

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**1. **Working with Coordinate Systems in ArcGIS Amanda Henley
GIS Librarian
Davis Library Reference

**2. **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:

**3. **Geographic Coordinate Systems Locations are defined on a 3-D spherical surface
Made up of graticules rather than grid cells
Units are in degrees

**4. **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

**5. **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
Geographic Coordinate Systems include
A Datum
An angular Unit of Measure (degrees)
A Prime Meridian

**6. **Datums
Link a spheroid to a location on the earth
Define the origin and orientation of the coordinate systems used to map the earth

**7. **Geodetic Datums
There are many datums
Local:
NAD 27 Datum, uses Clarke 1866 spheroid
NAD 83 Datum, uses GRS 1980 spheroid
Global
WGS 84 Datum, uses WGS 1984 spheroid
In addition to being in the same projection, data themes must also be in the same datum.

**9. **Datum Transformations ArcGIS 9.2 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
-for areas in the 48 contiguous states
Name Code Area of Use
NAD_1983_To_WGS_1984_5 1515 United States
There are many transformations to chose from, each is appropriate for different areas.

**10. **Data in a Geographic Coordinate System

**11. **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

**12. **Map projections: Distortion Converting from 3-D globe to flat surface causes distortion
Types of distortion
Shape
Area
Distance
Direction
No projection can preserve all four of these spatial properties
If some properties are maintained,
errors in others may be exaggerated

**13. **Families of Projections

**14. **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.

**15. **U.S. State Plane Zones

**16. **Georgia State Plane Meters East and West

**17. **North Carolina State PlaneMeters and Feet

**18. **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

**19. **UTM Zones in North Carolina

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

**21. **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.

**22. **Albers Equal Area Conic

**23. **Working With Coordinate Systems in ArcGIS On the Fly Projection
Datum Transformation
Defining a Layers Coordinate System vs. Projecting Data