How To: Determine the datum transformation ArcPad uses to transform coordinates between two datums
ArcPad stores an internal list of methods and parameters used for transforming coordinates between two datums called the Projection Engine. For many pairs of datums, more than one transformation method or set of parameters exists to perform the transformation. When this occurs, ArcPad selects the first match for the two datums it encounters.
Determine the default method by following these steps:
- In ArcPad, on a PC, click the Tools menu, click Utilities, then click Export Projection Information. Several projection related files are written to ArcPad's System folder. The default is C:\Program Files\System.
- Navigate to ArcPad's System folder and open the file Export_Transforms.csv.
- Look for the first entry that contains the pair of datums of interest; this is the row with the lowest number. The datums for each method are listed in the Datum_1 and Datum_2 fields.
Datum transformations work in either direction.
For example, if Datum_1 is D_North_American_1927 and Datum_2 is D_North_American_1983, the method will support transforming from NAD 1927 to NAD 1983, as well as from NAD 1983 to NAD 1927.
- Look at the last column: the DEFAULT column. If one of the transformation methods for the datum pair has the value 'Yes' in the DEFAULT column, this is the default method. If not, the first method listed for the datum pair is the default, as discussed in Step 3, above.
To verify that a transformation is being applied in ArcPad:
1. Open a map with a projection based on one of the datums of interest.
2. In the GPS Preferences dialog box, Datum page, set the GPS Datum to the other datum of interest.
3. Click the icon and review the transformation parameters displayed.