Shapefiles defined as GCS_Assumed_Geographic_1 display as Unknown and the definitions are gone

Last Published: December 27, 2023


Shapefiles defined as GCS_Assumed_Geographic_1 display as Unknown coordinate system and do not project on the fly.


GCS_Assumed_Geographic_1 is a default geographic coordinate system (GCS) assigned to certain data sources, which was used for data in ArcGIS versions 8.x, 9.0 and 9.1.

It was used only for shapefiles that had coordinates in decimal degrees (DD): the data extents were between -180 and +180 for X coordinates, and -90 and +90 for Y coordinates. These data sources now use Unknown for the coordinate system. ArcMap is not able to project them on the fly to match other layers in the map. Data that used to align correctly in ArcGIS version 9.1 or earlier no longer lines up in version 9.2.
This default coordinate system definition was removed from ArcGIS version 9.2 for the following reasons:

  • The default coordinate system identified the datum as D_North_American_1927. In most cases, this datum was not correct for the data, and can result in alignment errors of the data when displayed in the ArcMap data frame.
  • The datum D_North_American_1927 is defined only for the North American continent, and cannot be used for areas outside North America.
  • GCS_Assumed_Geographic_1 is not a valid coordinate system. ArcMap does not have any geographic (datum) transformations that convert from this coordinate system to any valid GCS.
  • Data having this default coordinate system cannot be projected to a different coordinate system in ArcToolbox.
  • When ArcGIS 8.0 was first released, many shapefiles did not have coordinate system information. GCS_Assumed_Geographic_1 was added to ArcGIS to allow shapefiles that did not have a defined coordinate system to be projected on the fly in ArcMap.

Solution or Workaround

Determine the correct geographic coordinate system for the data to assign a valid coordinate system definition using the Define Projection tool.

The Define Projection tool in ArcMap is located in ArcToolBox > Data Management Tools > Projections and Transformations.

The Define Projection tool can be used to define coordinate systems for both vector and raster datasets.
  1. The most reliable method to find out the coordinate system for the data is to contact the data provider for this information.
    If the data provider is unable to provide this information, go to Step 2.
  2. Data obtained from a Global Positioning System (GPS) is most frequently collected on the WGS 1984, and is defined as GCS_WGS_1984 in ArcGIS Desktop. Find this definition in the Geographic Coordinate Systems > World folder.
  3. Data with coordinates in decimal degrees within the United States is most frequently on the North American Datum 1983 (NAD 1983), although it may be on the North American Datum 1927 (NAD 1927).
    Because the coordinates for data within the U.S. on the NAD 1927 datum will be south-southwest of coordinates for the same point on the NAD 1983 datum by approximately 200 to 450 feet, test the geographic coordinate system to determine which is correct for the data.
    1. Define the coordinate system for the data as 'North American Datum 1983.prj', located in Geographic Coordinate Systems > North America.
    2. Add the data to a new, empty ArcMap session with data which has a correctly defined coordinate system. If the layers line up, the data is on NAD 1983.
    3. If the data does not line up with the known data, redefine the coordinate system as 'North American Datum 1927.prj' and then set the geographic (datum) transformation for the ArcMap Data Frame.
      To set the Geographic (datum) Transformation in ArcMap:
      1. Navigate to View > Data Frame Properties > Coordinate System tab > and click the Transformations button.
      2. Select the correct GCS in the 'Convert from' box, and then select the appropriate geographic transformation in the Using drop-down list.
      3. The Into box is the Geographic Coordinate System to which the ArcMap Data Frame is set. Do not change this entry during this process.
        For a list of supported Geographic (datum) Transformations and their areas of use, refer to the link under Related Information for more information.
  4. If the data still does not align correctly, contact Esri Technical Support.

Article ID:000008816

  • ArcMap

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