Supported datums

ArcPad supports any datum that meets the following criteria:

• The transformation parameters for going to the WGS84 datum are known.
• The transformation uses one of the following transformation methods:
  • Bursa-Wolf
  • Coordinate Frame
  • Geocentric Translation
  • HARN
  • Molodensky
  • Molodensky Abridged
  • Molodensky-Badeka
  • NADCON
  • Position Vector

Developers can also add support for additional datum transformation methods by building a custom datum transformation extension, using ArcPad Studio.

Datum Configuration

You can view the list of supported datums and datum transformation methods by running the Export Projection Information utility. The Export Projection Information Utility creates a number of files in the ArcPad System folder, including the following:

• Export_Datums.csv, which lists all the datums and the corresponding spheroids.
• Export_Spheroid.csv, which lists all the spheroid parameters.
• Export_Transforms.csv, which lists all the datum transformation parameters embedded in ArcPad as well as the datums included in the DefaultTransforms.dbf file.
• Export_TransformMethods.csv, which contains a list of all the supported datum-to-datum transformations built into ArcPad. Additional datum-to-datum transformations can be added to Transforms.dbf. Existing transformations listed in Export_Transforms.csv can also be overridden via Transforms.dbf.

You should not edit these exported files, as they are not used by ArcPad.

ArcPad includes two datum configuration files, Transforms.dbf and DefaultTransforms.dbf, which are located in the ArcPad System folder. These files can be edited to add new datum transformations as well as to override the default datum transformations within ArcPad. The ArcPad Datum Configuration tool can be used to edit the Transforms.dbf and DefaultTransforms.dbf files. More information on the Datum Configuration tool is located in Appendix C of the ArcPad User Guide.

Transforms.dbf: This file allows the user to add datum transformation parameters that are searched before the internal datum transformations. This means that the data in this configuration file will override the datum transformation information embedded in ArcPad. (This file is similar to the ArcGIS GEOTRAN table.)

To edit the Transforms.dbf file, you must supply two datum names and all the associated transformation parameters between these two datums. ArcPad will attempt to match the two names to the datum names listed in projection (.prj) files and information and use the associated parameters if a match is found. No datum transformation will take place if no match is found.

DefaultTransforms.dbf: Sometimes there is more than one set of parameters to transform between two datums. By default, ArcPad uses the first match it finds in the internal tables embedded in ArcPad. The DefaultTransforms.dbf file forces ArcPad to use a specific datum transformation if there is more than one available for a given pair of datum names. For example, there are multiple sets of parameters to perform a datum transformation between the D_WGS_1984 and D_Australian_1966 datums:

• AGD_1966_To_WGS_1984
• AGD_1966_To_WGS_1984_12
• AGD_1966_To_WGS_1984_13
• AGD_1966_To_WGS_1984_14
• AGD_1966_To_WGS_1984_15

By default, ArcPad always uses the first set of datum parameters for a particular datum transformation, which in this case is AGD_1966_To_WGS_1984. However, by adding AGD_1966_To_WGS_1984_14 to the DefaultTransforms.dbf file, you override the ArcPad default and force ArcPad to use this set of datum transformation parameters.

Checking datum transformation used by ArcPad

You can check what datum transformation is being used between the GPS datum (usually WGS_1984) and the datum for the current map projection. Open the GPS Preferences dialog box and tap Datum tab. Tap to view the Datum Transformation Information message and datum parameters of the currently selected GPS datum, including the parameters that ArcPad will use when transforming coordinates from the GPS datum to the datum of map.

The parameters listed correspond to the first set of parameters found, first, in the datum configuration files or, second, in the internal datum tables. Missing information indicates that no matching transformation parameters were found between the GPS datum and the datum of the current map projection so no datum transformation is being performed by ArcPad.