Projections, Datums, And Transformations In ArcPad 7—Getting It Straight

Carl Beyerhelm—GIS, GPS, and Data Management
Coconino National Forest
25/June/2006

Purpose

This guidance is intended to help users understand ArcPad’s use of projections, datums, and transformations. General ArcPad functionality is not discussed here, as it is well documented in Using ArcPad and in the ArcPad Reference Guide. All references to ArcPad refer specifically to ArcPad 7.

Background

ArcPad automatically converts latitude and longitude from a GPS receiver to coordinates that are expressed in terms of the current map’s projection and datum. In order for this stealthy, behind-the-scenes, on-the-fly conversion to proceed as intended, users must establish three things.

1. The datum of incoming GPS latitude and longitude.
2. The current map’s intended projection and datum.
3. The parameters necessary to transform coordinates from the GPS datum to coordinates of the map’s datum.

These three steps are described in more detail below.

Step 1—Establish The GPS Datum

The datum of incoming GPS coordinates is established by making a selection from the GPS Datum dropdown list. To do so, tap GPS Preferences…Datum, as at left.

ArcPad will "expect" to receive GPS coordinates referenced to this datum, and will make all subsequent actions based on that expectation. D_WGS_1984 is the default.

This GPS datum is not related in any way to the projection or datum of ArcPad’s current map.

Most GPS receivers issue coordinates only in WGS84. However, when Garmin coordinates are sent to ArcPad, they are referenced to the Garmin’s current Map Datum. For instance, if a Garmin’s Map Datum is set as illustrated at right, the GPS coordinates issued to ArcPad will also be referenced to NAD27 CONUS, not WGS84. Therefore, Garmin users must set Map Datum to WGS84 to avoid a datum mismatch in ArcPad.

Summary

Set GPS Datum to D_WGS_1984, and make sure that GPS receivers are issuing WGS84 coordinates to ArcPad.

Step 2—Establish The Map Projection And Datum

The intended projection and datum of ArcPad’s current map are established by a projection (.prj) file. The sequence ArcPad uses to determine a map’s projection is described below.

1. The initial default projection for a new, empty ArcPad map is WGS84 latitude and longitude.
   
   
2. If a projection file named arcpad.prj is found in the My Documents folder, it then replaces WGS84 latitude and longitude as the default map projection.
   
   
3. If a layer or map with a different projection is added to a new, empty ArcPad map, that data’s projection replaces the default projection.

If the projection of the map’s first layer is undefined, a .prj file can be manually assigned by tapping the first icon along the Table of Contents’ right margin, as at right, and making a selection from a folder containing .prj files.

A map’s current projection may be viewed by tapping the second icon along the Table of Contents’ right margin, as above.

Summary

Users should copy a projection file for their area of interest to the My Documents folder and rename it arcpad.prj, or else, make sure that the first layer or map added to a new, empty ArcPad map has a projection file that accurately reflects its feature’s coordinates.

Step 3—Establish The Transformation Parameters

A transformation is required when incoming GPS coordinates are referenced to WGS84 and ArcPad’s map datum is referenced to NAD27 (or to NAD83) because these datums are based on different ellipsoids. An ellipsoid is a stylized model of the earth’s dimensions. Complications arise, however, as there are 21 different transformations that convert WGS84 coordinates to NAD27 coordinates, each one being suitable for a different portion of North America. Unfortunately, ArcPad defaults to a transformation best suited to the Caribbean.

Nevertheless, users can force Arcpad to use the transformation suitable for their particular area by creating a DefaultTransforms.dbf table, and placing it in their …ArcPad/System folder. This table contains only three fields, as described below.

1. Transform—Name of the transformation suitable for a user’s area of interest.
2. Datum_1—Name of the current map’s datum described in Step 2 above.
3. Datum_2—The GPS datum described in Step 1 above.

Follow these steps to create the DefaultTransforms.dbf table.

1. An ESRI document titled Supported Geographic (Datum) Transformations lists all supported transformations, as well as their area of intended use. Use this reference to determine the transformation best suited to an area of interest.
   
2. In ArcPad, tap Utilities—Export Projection Information. This action writes several files to the …ArcPad/System folder. More on that later. Two of these files are named Export_Transforms.csv and DefaultTransforms.dbf.
   
   
3. Copy and paste content for the desired datum transformation from fields TransformName, DatumName1, and DatumName2 in table Export_Transforms.csv to the corresponding fields in table DefaultTransforms.dbf.
   
   
4. Place the DefaultTransforms.dbf table in the …ArcPad/System folder.

For example, the table at right is suitable for transforming WGS84 coordinates to NAD27 coordinates in the western continental United States.

ArcPad interrogates DefaultTransforms.dbf before referring to its internal transformation table, and will use the listed transformation if it finds a record matching the current GPS datum and the current ArcPad map datum. If no match is found, it proceeds to use its internal transformation table.

Once the DefaultTransforms.dbf table has been created and placed in the …ArcPad/System folder, users can tap the icon above the GPS Datum dropdown list (described in Step 1) to discover or confirm the active transformation. For a WGS84 to NAD27 transformation in the western U.S., the result should look like that at left.

Summary

Create a table named DefaultTransforms.dbf that matches an intended combination of GPS datum and ArcPad map datum, and pit in the …ArcPad/System folder.

Additional Information

This section provides additional information on the datums, ellipsoids, transformations, and projections supported by ArcPad. Most of this text is taken verbatim from Chapter 13 of the ArcPad Reference Guide.

In ArcPad, tap Utilities—Export Projection Information. This action writes five files to the …ArcPad/System folder, as described below. These are copies of internal ArcPad files that are provided solely for the user’s reference, and editing them has no effect on ArcPad.

1. Export_Datums.csv—Lists all of the datums, and their corresponding ellipsoids.
2. Export_Spheroid.csv—Lists all of the ellipsoids’ parameters.
3. Export_Transforms.csv—Lists all of the datum transformation parameters embedded in ArcPad. This is the file whose content may be copied and pasted to create a user’s custom DefaultTransforms.dbf table described in Step 3 above.
4. Export_TransformMethods.csv—Lists all of the supported datum-to-datum transformations built into ArcPad.
5. Export_Projections.txt—Lists all of the ArcGIS Projection Engine projections, and notes whether or not each projection is supported by ArcPad.

Two datum configuration files also appear in the …ArcPad/System folder that do have an effect on ArcPad. By default these files are empty, but they include all of the fields required by ArcPad.

1. Transforms.dbf—This file allows users to add customized datum transformation parameters that are searched before internal datum transformations are referenced. This means that the data in this configuration file will override datum transformations embedded within ArcPad. To modify the Transforms.dbf file, users must supply two datum names and all of the associated transformation parameters between these two datums. ArcPad will attempt to match these two datum names to the current GPS datum and ArcPad map datum, and use the associated parameters if a match is found.
   
   The purpose of the Transforms.dbf table is to facilitate creation of a new datum-to-datum transformation that ArcPad does not provide internally.
   
   
2. DefaultTransforms.dbf—Sometimes there is more than one set of parameters to transform between two datums. By default, ArcPad will use the first match it finds in its internal tables. 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.
   
   DefaultTransforms.dbf is empty initially, but users can copy and paste content to it from the Export_Transforms.csv file as described in Step 3 above.

A Suggested Strategy To Facilitate Alternating Use Of Multiple Transformations

Users could create several different versions of the DefaultTransforms.dbf table, one for each situation they periodically encounter.

For instance, those transforming between WGS84 and NAD83 might need two versions of DefaultTransforms.dbf. One version would be for use with an NDGPS differential correction source, and the other version for autonomous use, or with a CORS or WAAS correction source. Here’s a suggested strategy.

• Using information in the table (above) as a guide, create two versions of DefaultTransforms.dbf. One version would be for use with an NDGPS differential correction source, and the other for autonomous use, or with a CORS or WAAS correction source.
  
  
• Name the two files DefaultTransforms_NDGPS.dbf and DefaultTransforms_Auto_CORS_WAAS.dbf respectively.
  
  
• File them somewhere other than the …ArcPad/System folder.

Then, as needed for each GPS mission:

• Delete the DefaultTransforms.dbf table from the …ArcPad/System folder.
  
  
• Copy either DefaultTransforms_NDGPS.dbf or DefaultTransforms_Auto_CORS_WAAS.dbf as appropriate from their folder to the …ArcPad/System folder.
  
  
• Rename the copied file to DefaultTransforms.dbf.