Coordinate System - Custom Local Datum Transform

Use the Custom Local Datum Transform dialog to define a custom local datum transform (LDT) for the selected projection. This is useful when your measurements (dX, dY, dZ, scale, or rotation) are slightly different than the provided standard ones.

Custom Local Datum Transform dialog options

Datum	Non-editable field that displays the datum name of the current coordinate system.
Area of use	Enter the description for the new local datum transform. The initial input is extracted from the existing local datum transform (if defined), which is normally in one of the following forms: [Datum] Description (this is actually the local datum entered when no transform is defined), or [Datum to TargetDatum] Description.
Translation	Specify the translation parameters in X, Y, and Z. If the current local datum transform has a translation in the X, Y & Z, the corresponding dX, dY, and dZ translations are displayed.
Rotation	Specify the rotational angles in X,Y, and Z. If the current local datum transform has an additional rotation applied, the corresponding Rx, Ry, and Rz rotational angles are displayed.
Scale adjustment	Enter the scale adjustment (in ppm). This adjustment is defined in terms of the difference from unity. That is, a value of 10 ppm (+0.00001) will translate to a scaling factor of 1.00001, and a value of -10 ppm means the actual scale is 0.99999. If the current local datum transform has an additional rotation applied, the scale adjustment along with the rotations in X,Y & Z are displayed.

Application Notes

Once the parameters and the area of use description are defined, the custom local datum transform will be added to the file ldatum.csv and to any other files that need to be aware of it (i.e., datum.csv, and/or ipj_pcs.csv, and/or datumtrf.csv, and/or area_of_use.csv) and saved in the user folder %USERPROFILE%\Documents\Geosoft\Desktop Applications\csv.