Sunday, October 7, 2007

cs2cs & BeTA2007 - Open Source, Germany & NTv2

Recently in the PROJ.4 Mailing List there was a discussion about working with the DHDN / Gauss Krueger to WGS84 conversion.

One of the posters listed a site and document that covered BeTA2007 (which means Bundeseinheitliche Transformation fur ATKIS) and allowed me to brush up on my German.

In reviewing the document, we find out that the method of Operation for the datum conversion is based on NTv2 and the coordinate reference system is ETRS89/UTM.

Within the document listed above, they even demonstrate how to do a coordinate coversion using cs2cs and how to use the +nadgrids parameter.

An example from the document shows that the +nadgrids does not have to be specific to North America, only that the data file containing the grid has the same format. This format has been specified by the Canadian Government and has been adopted by many countries (such as South Africa and Australia, to name a few).

This is the published example, using the BETA2007.gsb file and a data file containing these two points:

(1) 2490000.00 5652000.00
(2) 2504000.00 5628000.00

>> cs2cs \
+proj=tmerc +lat_0=0 +lon_0=6 +k=1.0000000 \
+x_0=2500000 +y_0=0 +ellps=bessel +units=m \
+to \
+proj=utm +ellps=GRS80 +zone=32

Resulting in:

2490000.00 5652000.00 <-- Input (1)
279488.01 5654871.71 0.00 <-- UTM 32/GRS80 Answer for (1)

2504000.00 5628000.00 <-- Input (2)
292503.36 5630318.18 0.00 <-- UTM 32/GRS80 Answer for (1)

So cs2cs works with other grid systems as long as they are defined the same as the NTv2 standard.

So all the above makes more sense, let me define, from Wikipedia, what ETRF is: "The European Terrestrial Reference System 1989, usually referred to as ETRS89, is a three-dimensional geodetic frame of reference - a mapping coordinate system used as the standard high accuracy system for GPS in Europe. It coincided with the World Geodetic System 1984 in 1989, hence the name, and is based on the same GRS80 ellipsoid. Unlike WGS84 or ITRS it is centred on Europe and diverges from them with the movements of the tectonic plates associated with this landmass."

Please see: for ETRS89 and for WGS84

The parameters for a datum shift (dx, dy, dz, rx, ry, rz, etc.) between ETRS89 and WGS84 are not constant, due to the movement of the Eurasian geophysical plate with respect to WGS84. The differences between both datums can grow by several centimetres a year. Currently they are a couple of decimetres in difference. For many applications, these differences are not relevant. Coordinates or positions in WGS84 have usually been obtained by GPS and this results in an accuracy at the level of several metres. However, satellite positioning techniques continuously improve in accuracy, also without using differential stations. So the differences between the datum's will grow. Note that many nations are using WGS84 to define boundaries nowadays - so WGS84 is relevant.

One poster from France, to the mailing list, replied that they solved the problem of moving plates by adding a date to the location, and they stated their method as follows:

"We have solved this problem with moving plates by adding a simple date with the coordinates of WGS84. That way you can always go back to the original position of that particular date in WGS84 (or any). The WGS84 datum (with date) will stay accurate for ever, since it is always possible to trace back where the plates were at that specific date.


There are always solutions and answers to questions out there. Just feel free to ask, whether it be to a mailing list or your local expert, people are always willing to help and pass on knowledge.