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.