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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 Plane Meters 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