Coordinate System - Custom Datum

Use the Custom Datum dialog to define a new custom datum for the selected projection. This is useful when your custom projection may use a custom major axis and flattening.

Custom Datum dialog options

Datum name	Enter a unique descriptive name for the new datum of the current coordinate system. The field is set by default to "*NewDatum". The name cannot be one of the existing datum names that are installed with Oasis montaj.
Ellipsoid	Enter the ellipsoid name for the new datum. By default, it is set to the current datum ellipsoid.
Major axis radius	Specify the ellipsoid major radius (i.e., one-half of the length of the major axis). If the current datum has a radius specified, it will be displayed by default.
Inverse flattening	Enter the inverse flattening. If the current datum has a flattening specified, it is displayed by default.
Prime meridian	Specify the prime meridian for the new datum. The current datum prime meridian is displayed by default.

Application Notes

After the dialog is closed, you will be prompted to create a local datum transform for the new datum:

• Selecting "Yes" in the dialog will open the Custom Local Datum Transform dialog to define the new local datum transform (LDT).
• Selecting "No" will take you back to the Modify Coordinate System dialog and set the description for the LTD to "None applied". You can later simply delete the custom datum entry and create a new one if you want to define a new local datum transform for your custom datum.

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