Projection

If the datum is not available, use the Add Datum button to add a custom datum.

To add a custom datum in Global Mapper, there needs to be a transformation defined that relates the system to WGS84. For more direct transformations and a larger library of built-in Coordinate Systems, Datums and Transformations, see GeoCalc Mode: Projections and Datums.

When that button is pressed, the Create New Datum dialog (pictured below) is shown. Specify the datum name and abbreviated name for the custom datum. The abbreviation field is what will be displayed for the datum name in the status bar. If the datum uses a prime meridian other than Greenwich, enter the longitude of the meridian used (relative to Greenwich) in the Prime Meridian entry box.

Datum Transformation Method

Select and specify the parameters for either a 3-parameter Molodensky datum transformation or a 7-parameter Bursa-Wolfe datum transformation, or specify a transformation defined by a text file with a list of control points, use the Custom Shift Based on Control Point File setting.

7-parameter Bursa-Wolfe

Specify either a Position Vector Rotation or a Coordinate Frame Rotation method.

Custom Shift Based on Control Point File

With the Custom Shift Based on Control Point File option, the text file that is selected should have a series of lines defining the mapping from a lat/lon coordinate in the new datum and the corresponding lat/lon value in WGS84. This is a very easy way to define a datum if you don't know any transformation parameters, but do know the WGS84 coordinates for at least 2 coordinates in the new datum. The format of each line would be as follows:

Degree Longitude in new Datum, Degrees Latitude in new Datum, Degree Longitude in WGS84, Degrees Latitude in WGS84

Example

-70.8255835618, 45.4002372880, -70.82558356, 45.40023729
-70.8138608478, 45.3573208993, -70.81386085, 45.35732090
-70.9531879330, 45.3391776290, -70.95318793, 45.33917763
-70.8144989978, 45.3080936147, -70.81449900, 45.30809361

The datum information will be stored in the custom_datums.txt file in the Global Mapper Application Data folder (see Configuration > General > Advanced for the location of this folder). To share custom datums copy that file to that folder on another user's computer.

Ellipsoid Selection

If the necessary ellipsoid is not built in to Global Mapper, use the Add Ellipsoid button to add a new one or Edit Ellipsoid to edit an existing custom ellipsoid definition.

Elliposoid Name - Define the name for the custom ellipsoid. To import a projection from a file (prj) that defines a custom ellipsoid, the custom ellipsoid needs to

Semi-Major Axis (meters) - specify the dimensions of the semi-major axis in meters.

To complete the custom ellipsoid definition, specify either the Semi-Minor axis or the Flattening ratio:

Semi-Minor Axis - specify the dimensions of the semi-minor axis in meters.

Flattening - This is a measure of the compression of a sphere to form an ellipsoid. This value must be a number less than 1.

Another common ellipsoid parameter is the Inverse Flattening , which is typically a number around 298. If you have this value, calculate 1/Inverse Flattening, and enter that decimal as the Flattening value, such as: 1/298 = .00335570