Custom Datum Definition

If you have selected the User Defined datum in the Map Projections dialog, you can specify the exact datum parameters and ellipsoid definition by clicking the Custom button. Clicking the Custom button opens the Custom Datum Definition dialog.

Datum Parameters

The Datum parameters group controls the method of datum conversion and the conversion parameters. The conversion methods include Molodensky, Bursa-Wolfe, and Not Applicable.

The Molodensky method is the most widely used method of datum conversion. It adjusts latitude and longitude coordinates by taking into account the displacement between two datum's ellipsoids on all three axes. It does not take into account any rotational differences between the two ellipsoids.
The Bursa-Wolfe method is similar to the Molodensky method, but in some instances it produces more accurate results because it takes into account both displacement and rotational differences between two ellipsoids. MapViewer supports the Bursa-Wolfe method for conversions between the WGS84 datum and the following datums: World Geodetic System 1972, DHDN-1, DHDN, Australian Geodetic 1984, ANS84, MRT - Everest Modified, Switzerland - CH1903, NTF France - Paris Meridian, and Pulkovo 1942 - Hungary.
Choose Not Applicable if the predefined methods do not suite your purpose. If you select User Defined from the Ellipsoid list, you can specify your own ellipsoid model parameters.

You can edit the WGS84 conversion parameters directly by selecting the value you want to edit and typing a new value over it. Parameters vary depending on the selected conversion method. The possible WGS84 conversion parameters include:

Delta X, Delta Y, and Delta Z are the axis displacements in meters.
X Rotation, Y Rotation, and Z Rotation are the axis rotations in arc seconds.
The Scale Factor is the scale factor for the datum.
The PM Shift is the shift from the prime meridian, typically 0 degrees, in decimal degrees.

Ellipsoid Definition

The Ellipsoid definition group contains options for defining the ellipsoid. Use these settings to define the ellipsoid model that best approximates the curvature of the Earth's shape in the map region.

The Ellipsoid list contains a collection of ellipsoid models that cartographers have historically used in attempts to produce more accurate maps of different regions of the Earth.
If you select User Defined as the ellipsoid type, the Major axis (meters) definition is available. Once you define the major axis, you can either define the Minor axis (meters) or use a Flattening ratio to define the minor axis.