Datum File

Geodetic datums define the size and shape of the earth and the origin and orientation of coordinate systems. They range from flat-earth models used for plane surveying to complex models which describe the size, shape, orientation, gravity field, and angular velocity of the earth.

The datum file contains a list of ellipsoid parameters (semi-axes; X, Y, Z) and coordinate transformation parameters (shifts from and rotations about the geocentre) required to make the axis of the ellipsoid coincide with the WGS 84 ellipsoid.

The file name is datum.dat and it is located in the system folder (C:\Program Files\CARIS\Caris\<version>\system).

On-the-fly reprojections are done using a Coordinate Frame Rotation transformation, which uses the following parameters:

• Shift parameters: Three parameters, one for each axis. These represent the distance in metres that the ellipsoid will be shifted along each axis.

• Rotation parameters: Three parameters, one for each axis, representing the angle in arc-seconds that the ellipsoid will be rotated around each axis. Rotation is clockwise when looking along the axis from the centre of rotation.

• Scale correction factor: A single parameter, representing the adjustment in parts per million that will be made to the size of the ellipsoid. (1.0 - (ppm value/1000000))

Scale factor: The scale factor represents the difference in size between two ellipsoids, normally between the ellipsoid of the non-WGS84 datum and WGS84. The scale factor is unitless. It is commonly referred to in parts per million (ppm) but the scale factor in datum.dat is actually computed from the ppm factor as follows:

scale factor = 1.0 + (ppm value/1,000,000)

For example, see EPSG::1314 which defines the scale difference parameter value as -20.489 parts per million. The scale factor is computed as follows:

scale factor = 1 + (-20.489 / 1,000,000)

= 0.9999795106

Sample entries from the datum file are shown below:

The file is structured in the following manner:

Lines that start with an exclamation mark are comments. It is recommended that you put the complete name of the ellipsoid and any necessary description as one or more comment lines immediately before the entry.

Each datum entry must be on a single line.

Datum fields are separated by one or more spaces or tabs. Each definition line has the following format: