Parent Topic: PROJ

EARTH MODELS

The earth model is a PCI term for a regular model to represent the surface of the earth. This general term is useful to retain as it can apply to either of the two forms that are supported in PCI.)

The earth model can be specified by either one of two forms:

a Datum code (e.g. "D000") for the horizontal datum.
an Ellipsoid code (e.g. "E000") for the ellipsoid used by the horizontal datum.

Before V6.0, PCI expressed the earth model of a georeferenced data set with an ellipsoid code (i.e. ``E000'' for Clark 1866). Some of these ellipsoids have been assumed to be equivalent to true datums (i.e. E000 is equivalent to NAD27), but the datums were not expressed directly. Furthermore, it was not possible to transform a dataset from one ellipsoid to another, though it was possible to transform between different projections as long as they had a common ellipsoid.

As of V6.0.1, it became possible to express a datum (which carries with it an implicit ellipsoid) directly. For instance, D000 is WGS84. Furthermore, it became possible to transform data between different datums, as well as between different projections within a datum using the standard reprojection tools. However, the use of datum codes was not documented and was restricted to experimental use.

As of V6.2, datum codes are now fully documented and supported for use. Furthermore, the default datum assigned to datasets without a datum by PCI software is now D000 (WGS84) rather than E000 (Clark 1866/NAD27). WGS84 is now the most common datum of data distributed around the world; however, it is significantly different than NAD27, so care should be taken to ensure that the correct datum is assigned when importing, or creating data sets within EASI/PACE. Note that NAD27 can now be expressed as D-01 in the USA, 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 automatically treated according to the implied related datum. However, it is still not possible to transform (reproject) points, or images from one ellipsoid to another if it cannot be corresponded to a datum.

(Ellipsoid is used as a short form of the proper term "ellipsoid of revolution". A true ellipsoid could potentially have different radii in the X, Y, or Z dimensions. An "ellipsoid of revolution" has two of the radii equal. For the earth, the two equal radii are on the equatorial plane. So essentially an "ellipsoid of revolution" would describe revolving an ellipse around its polar axis. "Ellipsoid of revolution" is the same as a "spheroid".)

Reprojection to map projections using different ellipsoids was not supported (as that is a separate problem from reprojections) and would fail. (Reprojection to different horizontal datums using the same ellipsoid would not fail, but would introduce an error seeing as the ellipsoid would be assumed to represent the same horizontal datum.)

In order to address the problem of different map projections using different horizontal datums, the software has been updated and a list of datum codes (e.g. "D000") has been derived.

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 U.S.G.S. GCTP software. The positive numbering for the datums follows the ordering of the datum text file provided with the MADTRAN software. The negative numbering for datums was derived to complement the positive datum numbering.

So to answer the question "Should ellipsoid codes or datum codes be used?":

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 should be 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 FileBrowser.