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Error: Datums do not share a common transformation

Error Message

Projecting between NADCON datums and Three or Seven Parameter datums returns the following error:

"Datums do not share a common transformation."

Cause

PROJECT supports two types of datum transformation: equation-based and grid-based. The best way to convert between NAD27 and NAD83 is to use a grid based method called NADCON. Other datums, like WGS84, use equation-based methods. The PROJECT command cannot fix and match datums that use these two different transformation methods.

Solution or Workaround

  • Projecting from NAD27 to WGS72:

    1. PROJECT the NAD27 coverage to NAD83.

    2. Use PROJECTDEFINE to change the NAD83 coverage DATUM specification to the NADCON-equivalent equation-based datum such as NAR_C.

    PROJECTDEFINE COVER cov_nad83

    PROJECTION <projection_name>
    UNITS <units>
    DATUM NAR_C
    PARAMETERS



    3. PROJECT the coverage with the NADCON-equivalent datum to WGS84.

    PROJECT COVER cov_nar_c cov_wgs84 filename.prj

    Contents of filename.prj:

    INPUT
    PROJECTION <projection_name>
    UNITS <units>
    DATUM NAR_C
    PARAMETERS
    OUTPUT
    PROJECTION <projection_name>
    UNITS <units>
    DATUM WGS84
    PARAMETERS
    END



    4. PROJECT the WGS84 coverage to WGS72.
  • Projecting from WGS72 to NAD27:

    1. PROJECT the WGS72 coverage to WGS84.

    2. PROJECT the WGS84 coverage to the NADCON-equivalent equation-based datum, such as NAR_C.

    PROJECT COVER cov_wgs84 cov_nar_c filename2.prj

    Contents of filename2.prj:

    INPUT
    PROJECTION <projection_name>
    UNITS <units>
    DATUM WGS84
    PARAMETERS
    OUTPUT
    PROJECTION <projection_name>
    UNITS <units>
    DATUM NAR_C
    PARAMETERS
    END



    3. Use PROJECTDEFINE to change the NADCON-equivalent datum name to NAD83.

    PROJECTDEFINE COVER input_cover

    PROJECTION <projection_name>
    UNITS <units>
    DATUM NAD83
    PARAMETERS



    4. PROJECT the NAD83 coverage to NAD27.
    Note:
    The NADCON equivalent datums, such as NAR_C, are approximations of NAD83 or NAD27. To find the NADCON equivalent keywords see the ArcInfo Help topic 'Supported datums in PROJECT'.


  • If the WGS84 data is not very accurate, use PROJECTDEFINE to convert directly to NAD83.

    PROJECTDEFINE COVER input_cover

    PROJECTION <projection_name>
    UNITS <units>
    DATUM NAD83
    PARAMETERS



    Note:
    Although these are the most accurate methods to PROJECT between the two datum types, the distortion of any data set compounds after each projection.