How To: Add a custom datum to ArcPad


ArcPad automatically transforms incoming GPS coordinates from the datum specified in the GPS Datum combo box (Protocol page of the ArcPad Options dialog box) to the datum of the current map if necessary. During this process, the GPS coordinates are first transformed from the GPS datum to WGS 1984, and then to the map's datum.

All of the datum transformations to WGS 1984 that ArcPad supports out of the box are stored in the file apDatums.dbf. This file is located in the System directory under the ArcPad installation directory.

Additional datum transformation can be added to apDatums.dbf, provided they meet the following criteria:

1. The transformation parameters between the datum and WGS 1984 are known.

2. The transformation method is either Geocentric Translation, Position Vector, Bursa-Wolf which is treated the same as Coordinate Frame, or Coordinate Frame.


Open apDatums.dbf in a DBF editor (for example, MS Excel or MS FoxPro) and fill in each column for the new datum as follows:

  • Column A (DEFAULT): Specifies if this entry is the default when more than one entry exists with the same DATUMID/DATUM values. If set to TRUE, all other entries with the same DATUMID/DATUM values must have this column set to FALSE.
  • Column B (DATUMID): The datum code of the datum from the EPSG database.
  • Column C (DATUM): The name of the datum.
  • Column D (SPHEROIDID): The spheriod code of the spheroid from the EPSG database.
  • Column E (SPHEROID): The name of the spheroid.
  • Column F (A): The semimajor axis length (meters) of the spheroid.
  • Column G (INVERSE_F): The inverse flattening of the spheroid.
  • Column H (GEOGTRANID): The transformation code from the EPSG database. Predefined values are in the 1xxx, 8xxx, and 108xxx ranges. User-defined values should be in the 208xxx range.
  • Column I (METHODID): Transformation method ID from the EPSG database. The ID must match the method specified in column J (METHOD). Valid inputs are:
    9603 --> Geocentric_Translation
    9606 --> Position_Vector
    9607 --> Coordinate_Frame
  • Column J (METHOD): Transformation method name. Valid inputs are: Geocentric_Translation, Position_Vector, or Coordinate_Frame.
  • Column K (DX): The X translation (meters).
  • Column L (DY): The Y translation (meters).
  • Column M (DZ): The Z translation (meters).
  • Column N (RX): The X rotation (arc-seconds).
  • Column O (RY): The Y rotation (arc-seconds).
  • Column P (RZ): The Z rotation (arc-seconds).
  • Column Q (DS): The scale factor (ppm).
  • Column R (DATASET): Not currently used.

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