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.
These are used when selecting the CRS using the Change Coordinate Reference System command.
The file name is datum.dat and it is located in the system folder (for example, C:\Program Files\CARIS\Caris\4.4\system).
Do not edit default file. Create a copy of this file and use the copy for custom datums. |
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 blanks. These can be spaces or tabs. Each definition line has the following format:
Field | Unit | Description | Sample |
Name | Text | Four-character code name for the ellipsoid. | KR40 |
a | Metres | Semi-major axis a | 6378245.000 |
b | Metres | Semi-minor axis b | 6356863.019 |
X | Metres | Shift from geocentre X | 23.9998 |
Y | Metres | Shift from geocentre Y | -122.9998 |
Z | Metres | Shift from geocentre Z | -94.0 |
rx | arc-seconds | Rotation X | 0.1299 |
ry | arc-seconds | Rotation Y | 0.25 |
rz | arc-seconds | Rotation Z | -0.01999 |
sf | Real | Scaling factor | 1.0000010999 |
Sample
Sample entries from the datum file:
! Bessel Ellipsoid (Hafen Hamburg) |
BEHH 6377397.155 6356078.963 503.528 130.167 484.125 -0.7494 -3.6362 4.7644 1.0000062466 |
! |
! Bintulu Local Harbour Grid |
BLHG 6377298.562 6356097.556 -689.00 691.00 -46.00 0.0 0.0 0.0 1.0000 |