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A map projection is the mathematical process of taking a view of a spherical globe and drawing that information on a flat piece of paper.

Four properties apply to map projections:

SHAPE    DISTANCE    AREA    DIRECTION

To select a map projection, determine which of the properties is the most important for the project, and select the coordinate system or map projection that best preserves that property.

Under Related Information below, click on the link titled 'Projections table' to access Projections_table_2009.zip containing G37891_Projections_table.pdf. The PDF file lists map projections supported in ArcGIS Desktop through version 10, indicating the specific properties and suitable areas of use for each supported projection.

Additional detailed information about these projections is available at the link titled 'Supported Map Projections'.

The project-on-the-fly functionality in ArcMap is very good for quickly viewing data in different coordinate systems. The exercise below will demonstrate the properties of various map projections and their effect on the data displayed in ArcMap.

1. Start ArcMap with a new, empty map.
2. Depending on the version being used, click the 'Add Data' button and navigate to:
   
   ArcGIS 10:
   <drive_letter>\Program Files\ArcGIS\Desktop10.0\Reference Systems
   
   ArcGIS 10 on 64-bit OS:
   <drive_letter>\Program Files(x86)\ArcGIS\Desktop10.0\Reference Systems
   
   ArcGIS 9.x: <drive_letter>\Program Files\ArcGIS\Reference Systems
   
   ArcGIS 8.x: <drive_letter>\arcgis\arcexe8x\Reference Systems
3. Select GEOREF15.SHP. This is a 15 degree grid of the surface of the earth. The shapefile extent is -180 West to +180 East Longitude, and -90 South to +90 North Latitude.
   
   
    In a Geographic Coordinate System such as GCS_WGS_1984, the North Pole and South Pole, which are points, are displayed as lines at the top and bottom of the data, and these lines are as long as the Equator. This graphically demonstrates the distortion that exists using a Geographic Coordinate System for data.
4. Click on View > Data Frame properties > Coordinate System tab. The projection of the Data Frame (GCS_WGS_1984) is displayed in the upper window and is copied from GEOREF15.SHP. In the lower window, labeled 'Select a coordinate system', a different coordinate system can be selected to display data in the Data Frame.
   
5. In the lower window, click Predefined > Projected Coordinate Systems > UTM > WGS 1984 and select UTM Zone 18N as the projection. Click Apply.
   
   The projection of the data frame is changed. Notice the shape of the square boxes from GEOREF15.SHP have also changed. Because of the limitations of the UTM projection, most of the globe disappears from the view.
   
6. Experiment with other projections to display this shapefile:
   
   A. Click Clear on the Coordinate System tab.
   B. Click the Layers folder in the lower window.
   C. Open GEOREF15.SHP, and select GCS_WGS_1984.
   D. Click Apply. GEOREF15.SHP returns to the original shape.
   E. Click Predefined > Projected Coordinate Systems > World. The World folder contains different projections for the surface of the earth. Experiment with the different projections as shown in the steps above.
   
   
    Changing the projection of the ArcMap Data Frame does not affect the coordinate system definition of the data displayed in the map. There is no permanent change made to the data being displayed in the ArcMap document.
   
   This is the meaning of project-on-the-fly in ArcMap.
7. Use the attached PDF file to determine which properties of a projection are most significant for your project. This file lists projections supported in ArcGIS, their properties, suitable extents and shapes of data for which each projection should be used. Also, a custom coordinate system can be created using ArcGIS and applied to the data for your project.
   
   See the link in the Related Information section for more general instructions for creating custom projection files in ArcMap.
   
   For further assistance in creating a custom coordinate system, contact ESRI Technical Support at 888-377-4575 or at support.esri.com.

• Supported Map Projections
• Projections table
• Create a custom projection file with units of feet or meters in ArcMap
  Instructions provided describe how to create a custom projection file for a projected coordinate system, using linear units in feet or meters, with the tools available in ArcMap.

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