How To: Identify an unknown projected coordinate system using ArcMap


Instructions provided help identify the projected coordinate system for datasets that do not have coordinate system information in the metadata or a projection definition, using the project-on-the-fly capability in ArcMap.

Coordinate system (projection) information should be obtained from the data source or provider, but are not always available, such as with legacy data. The technique described below helps to identify the correct projected coordinate system.

The term 'coordinate system' can refer to data in a Geographic Coordinate System (GCS) with coordinates in decimal degrees; or a Projected Coordinate System (PCS) with linear units in meters or feet. The term projection or PRJ, is an older term that is also used, but it is not as precise.


Before beginning, read FAQ: Projection Basics: What the GIS professional needs to know. This article includes the parameters required for various coordinate systems, as well as useful information about working with coordinate systems in ArcMap.

  1. Start ArcMap with a new, empty map, and add the data with the unknown coordinate system to the map. This data must not be associated with a PRJ file or coordinate system definition.
  2. Right-click the layer name in the Table of Contents, select Properties > Source tab, and examine the extent of the data.

    [O-Image] Layer Properties >Source Tab
    If the coordinates are in decimal degrees, such as between longitude -180 and +180, and latitude -90 and +90, identify the Geographic Coordinate System (datum) used for the data. Prior to version 9.2, ArcMap assigns GCS_Assumed_Geographic_1 to the data, by default. This places the data on the NAD_1927 datum, which is incorrect in most cases. FAQ: Is GCS_Assumed_Geographic_1 a coordinate system?

    In ArcGIS Desktop at version 9.2 and later versions, no default coordinate systems are assigned to any data. Problem: Shapefiles defined as GCS_Assumed_Geographic_1 display as Unknown and the definitions are gone

    If the data is in the United States and shows an extent in which the coordinates to the left of the decimal are 6, 7 or 8 digits, the data is probably projected to the State Plane or UTM coordinate systems.
  3. If the unknown data lies within the United States, add comparison data indicated below to ArcMap:

    ArcGIS 8.x
    Navigate to the folder <drive_letter>\arcgis\arcexe8x\Reference System, and add the file USSTPLN83.SHP to ArcMap.

    ArcGIS 9.x
    Navigate to <drive_letter>\Program Files\ArcGIS\Reference Systems, and add the file USSTPLN83.SHP to ArcMap.

    ArcGIS 10.0
    Navigate to <drive_letter>\Program Files\ArcGIS\Desktop 10.0\Reference Systems, and add the file USSTPLN83.SHP to ArcMap. If the computer has a 64-bit operating system the install folder will be 'Program Files(x86)'.
  4. Click this link to view an image of USSTPLN83.SHP.
The original data and the newly added shapefile are not displayed in the same location.
[O-Image] View of USSTPLN83.SHP
  1. Select and assign the coordinate system.
    The U. S. Survey Foot (foot_us) is defined as exactly 1200/3937 of a meter, or 0.3048006096012192....
    The International Foot (foot) is defined as exactly 0.3048 of a meter.
    1. Click View > Data Frame Properties > Coordinate System tab.
    2. In the lower window labeled Select a coordinate system, click Predefined > Projected > State Plane.
    3. Select and assign the coordinate system for the State Plane FIPS zone in which the data should be located. Select the relevant projection files from the folders as listed below. Note that additional folders have been added at later versions of ArcGIS Desktop:

      ArcGIS 8.x
      NAD 1927
      NAD 1983 with no descriptor, units are in meters
      NAD 1983 (feet)
      NAD 1983 HARN

      ArcGIS 9.0; 9.1
      NAD 1927
      NAD 1983 with no descriptor, units are in meters
      NAD 1983 (feet)
      NAD 1983 (Intl Feet)
      NAD 1983 HARN
      NAD 1983 HARN (Feet, Intl and US)

      ArcGIS 9.2, 9.3, 9.3.1
      NAD 1927
      NAD 1983 with no descriptor, units are in meters
      NAD 1983 (feet)
      NAD 1983 (Intl Feet)
      NAD 1983 HARN (Feet, Intl and US)
      NAD 1983 HARN

      ArcGIS 10.0
      NAD 1927 (US Feet)
      NAD 1983 (CORS96)(Intl Feet)
      NAD 1983 (CORS96)(Meters)
      NAD 1983 (CORS96)(US Feet)
      NAD 1983 (Intl Feet)
      NAD 1983 (Meters)
      NAD 1983 (US Feet)
      NAR 1983 HARN (Intl Feet)
      NAD 1983 HARN (Meters)
      NAD 1983 HARN (US Feet)
      NAD 1983 NSRS2007 (Intl Feet)
      NAD 1983 NSRS2007 (Meters)
      NAD 1983 NSRS2007 (USFeet)
      Other GCS
    4. After selecting a projection file, click Apply, and click OK.
    5. Repeat assigning a State Plane projection file from the available folders until the USSTPLN83.SHP file snaps into place and the data with the unknown coordinate system appears in the correct area in the proper state.
    6. Verify the correct place by zooming into the layer and using the Identify Tool on the state where the data is drawn.

      ArcGIS 9.x
      Additional coordinate systems are available for the following states: Alaska, California, Georgia, Idaho, Michigan, Minnesota, Oregon, Texas and Wisconsin. State-wide projections are added for Florida and Mississippi at 9.3. These projected coordinate systems can be found in Projected Coordinate Systems > County Systems, National Grids, or State Systems folders. If the data is located in one of these states, the specific projections for the state or county should also be considered and tested.

      When the coordinate system is assigned to the Data Frame, a warning box may display:

      ArcMap 8.x, 9.0, 9.1 Warning
      [O-Image] ArcMap 8.x, 9.0, 9.1 Warning dialog
      If the above warning appears for 8.x; 9.0; 9.1: Click View > Data Frame Properties > Coordinate System tab > Transformations.

      ArcMap version 9.2/9.3/9.3.1/10.0 Geographic Coordinate Systems Warning
      [O-Image] ArcMap Geographic Coordinate Systems Warning dialog 9.2
      If the above warning appears, click Transformations.

      All Versions: In the Using box, verify that the appropriate transformation method was applied. To access the list of supported datum transformation methods and the areas for which each should be used, refer to "Select the correct datum transformation method when projecting between datums" in Related Information, below.
      If the data lines up correctly using one of the State Plane options, go to step 6. If not, continue to step 5.
  2. If the data does not line up after testing the State Plane options:
    Coordinates in UTM meters on the NAD 1983 datum and coordinates for the same point on the WGS 1984 datum in the continental United States are within a meter from each other.
    Data in the UTM coordinate system, on the NAD 1983 datum, is approximately 200 meters north of the same data on the NAD 1927 datum. There may be a slight shift either east or west between data on these two datums but an approximate 200-meter difference in the northing is diagnostic.
    The 200-meter difference is comparatively slight; therefore, it is essential that precise comparison data be used to determine whether the correct datum is NAD 1927 or NAD 1983 for data in the UTM projection.
    1. Open the UTM folder.
    2. Perform the same steps as described in steps 4 (a through f). Select the correct UTM zone for the unknown data from the following datums; they are the most commonly used in the United States:

      NAD 1927
      NAD 1983
      WGS 1984
  3. When the correct coordinate system is found:
    1. Write down the path and name.
    2. Refer to the instructions in the following Knowledge Base article for the steps to define the coordinate system for your data.
      FAQ: Projection Basics: What the GIS professional needs to know
    3. When the coordinate system is identified and defined, the data lines up in ArcMap with other data added to the ArcMap session, provided that the correct datum transformation was specified.
  4. If the above steps do not line up the data in ArcMap, the data may be in a custom coordinate system. Custom projection files can be created in ArcMap to align data in an unknown coordinate system.

    Additional information is provided in the book Lining Up Data in ArcGIS: A Guide to Map Projections, published by Esri Press. For further information and ordering, click the link in the Related Links section below.

    For further assistance, contact Esri Support Services at 888-377-4575 or at support.esri.com.

Related Information