Earth models

An Earth model is a term for a regular model that represents the surface of the Earth. This general term can apply to either of the two forms supported by CATALYST Professional.

The Earth model can be specified by either of the following forms:

Datum code (for example, D000) for the horizontal datum
Ellipsoid code (for example, E000) for the ellipsoid used by the horizontal datum

In CATALYST Professional, it is possible to directly express a datum (which carries with it an implicit ellipsoid). For example, D000 is WGS84. Furthermore, it is possible to transform data between different datums and between different projections within a datum by using the standard reprojection tools.

Datum codes are fully documented and supported. The default datum assigned to data sets without a datum is D000 (WGS84). WGS84, however, is significantly different from NAD27, so care must be taken to ensure that the correct datum is assigned when importing or creating data sets in CATALYST Professional.

Note: NAD27 can now be expressed as D-01 in the US and D-02 in Canada.

Using ellipsoids rather than datums is still valid, and selected ellipsoids (E000/NAD27, E008/NAD83, E012/WGS84, and E005/WGS72) are treated automatically according to the implied related datum. It is still not possible to transform points or images from one ellipsoid to another ellipsoid if the datums (that use the ellipsoids) cannot be determined.

Ellipsoid is used as a short form of the proper term "ellipsoid of revolution". A true ellipsoid can potentially have different radii in the x, y, or z dimensions. An ellipsoid has two equal radii. With the Earth, the two equal radii are on the equatorial plane. Essentially, an ellipsoid describes revolving an ellipse around its polar axis. Ellipsoid is the same as a spheroid.

Reprojecting map projections using different ellipsoids is not supported and fails. Reprojecting different horizontal data using the same ellipsoid does not fail but introduces an error, because the ellipsoid essentially represents the same horizontal data.

To address the difficulty posed by different map projections using different horizontal datums, the software includes a list of derived datum codes (for example, D000).

Note: There is no correlation between the numbering of the ellipsoid codes and the datum codes. The numbering for ellipsoids follows the numbering used by the USGS GCTP software. The positive numbering for the datums follows the ordering of the datum text file provided with the MODTRAN software. The negative numbering for datums (using grid-shift files) was derived to complement the positive-datum numbering.

If you use the same horizontal datum in the georeferencing, it is still valid to use ellipsoid codes.

If reprojection between map projections that use different horizontal datums is required, datum codes are used instead of ellipsoid codes.

The Earth model can be upgraded from an ellipsoid code to a datum code in existing PCIDSK database segments with the DATUM program or with the File Browser.

For more information, see: