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How To: Determine the coordinate system for vector data

Summary

ArcView draws data based on the coordinate values stored in the file. These coordinate values should be in either the Geographic coordinate system (Decimal Degrees) or a projected coordinate system. This document discusses where to find information about your data's coordinate system.

Procedure

  • Examine the data's metadata.
     
    Metadata is information about data. If you download data from a Web site, check additional links on the site that provide projection information about the data.

    Projection information from spatial data providers may be in a metadata file (.txt), or, for shapefiles and coverages, in a projection file (.prj). If the data does not have an accompanying .prj or .txt metadata file, you can create one in a text editor, such as WordPad or Notepad. You can copy the metadata information from the Web site into the text editor.

    ArcInfo coverages and grids sometimes include a .prj file. If not, you can create one in ArcInfo.

    It is important to maintain current projection information by updating the metadata anytime a data set is re-projected.
    Note:
    The ArcScripts site has sample extensions that may assist you in creating metadata files.
    ArcScripts
    Note:
    If you did not receive metadata with your data, you should consult the data provider.
  • You can use ArcView to determine if vector data is stored in the Geographic coordinate system or if the data is stored in a projected coordinate system.
    1. Add the data set to a new view.
    2. Place the cursor on top of the data in the view window.
    3. Observe the coordinates listed in the top right hand corner of the application.

      If the entire extent of the data falls between -180 and 180 for the X component, and -90 to 90 for the Y component, the data is likely to be stored in the Geographic Coordinate System. Otherwise the data most likely has some projection applied to it, or it is in a random (not real world) coordinate system.
      Note:
      This method does not tell you what datum the data uses. You will need to know the datum if you plan to use the Projection Utility.