Supported datums

ArcPad supports the Extended Backus-Naur Form (EBNF) definition for a string representing a coordinate system.

The element of this string that represents the datum, starts with the word DATUM, followed by the specific name of the datum enclosed in quotes and square brackets. This element always has an additional child element of spheroid. For example, the datum of D_WGS_1984 appears like this:

DATUM["D_WGS_1984",
            SPHEROID["WGS_1984",6378137.0,298.257223563]
        ],

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 (as an extension that can optionally be enabled)
  • Molodensky
  • Molodensky Abridged
  • Molodensky-Badeka
  • NADCON (as an extension that can optionally be enabled)
  • Position Vector

A common datum not supported is Longitude_Rotation.

NTv2 transformations are supported in ArcPad 10.2.4 and later. For more information see Use NTv2 transformations in ArcPad.

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. This utility creates a number of files in the ArcPad System folder, including the following:

• Export_Datums.csv—Lists all the datums and the corresponding spheroids.
• Export_Spheroid.csv—Lists all the spheroid parameters.
• Export_Transforms.csv—Lists all the datum transformation parameters embedded in ArcPad, as well as the datums included in the DefaultTransforms.dbf file.
• Export_TransformMethods.csv—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.

Note:

Do 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, and 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.

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 overrides the datum transformation information embedded in ArcPad. (This file is similar to the ArcGIS GEOTRAN table.)

To edit the Transforms.dbf file, supply two datum names and all the associated transformation parameters between these two datums. ArcPad attempts to match the two names to the datum names listed in projection (.prj) files and information, and uses the associated parameters if a match is found. No datum transformation takes place if no match is found. You can use the Datum Configuration tool to define datum transformations in this file.

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. You can use the Datum Configuration tool to select default datum transformations in this file.

Check 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 the Datum tab. Tap to view the Datum Transformation Information message box and datum parameters of the currently selected GPS datum, including the parameters that ArcPad uses when transforming coordinates from the GPS datum to the map datum.

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; therefore, no datum transformation is being performed by ArcPad.

Use NTv2 transformations in ArcPad

NTv2 transformations are supported in ArcPad 10.2.4 and later. For more information on what NTv2 is, see National Transformation version 2.

ArcPad for Windows includes the same grid shift binary (GSB) files that are provided with ArcMap. These files can be found in the folder C:\Program Files (x86)\ArcGIS\ArcPad10.2\System\ntv2. This collection of files contains over 200MB of data, which is very large to consider deploying to a Windows Mobile device. To use NTv2 transformations on Windows Mobile, you must first copy the required GSB files to the ArcPad System folder on your mobile device. For example, if you wish to use the Australian NTv2 transformation between AGD 1984 and GDA 1994, copy the file

C:\Program Files (x86)\ArcGIS\ArcPad10.2\System\ntv2\australia\National_84_02_07_01.gsb

to

\\Program Files\ArcPad\System\ntv2\australia\National_84_02_07_01.gsb

on your Windows Mobile device.

To force ArcPad to use a NTv2 transformation by default, you must create an entry in the DefaultTransforms.dbf file that defines your chosen default. To do this, use the Datum Configuration tool on the desktop. On the Select Default Datum Transformation tab, choose the datum and associated dautm and then select the NTv2 transform from the list. Click Save and Exit. This will create or update an existing DefaultTransforms.dbf file and you can then use this same file with either your desktop installation of ArcPad or copy it to your Windows Mobile device for use in ArcPad on the device.

On Windows the file will be saved to C:\Users\Public\Documents\ArcPad\System.

On Windows Mobile you should copy this file to \\Program Files\ArcPad\System.

Note:

To conserve memory, the NTv2 extension is disabled by default on Windows Mobile. Before using this transformation be sure to enable the extension. Go to Advanced Settings > Extensions, and check the box next to the NTv2 extension. When you click OK you will be advised that some preferences may have changed that require a restart to take effect. Restart ArcPad to use NTv2 transformations. On Windows all extensions are enabled by default.