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.