[GRASS5] adding a new datum/projection?

[Paul Kelly]

On Wed, 23 Jul 2003, H Bowman wrote:

...
> The datum is New Zealand Geodetic Datum 2000 (NZGD2000), which is based
> on the International Terrestrial Reference Frame 1996 (ITRF96). This is
> pretty much the same as WGS84 but uses the GRS80 ellipsoid (slightly
> different flattening parameter).
...
> Is anything else based on ITRF96? Would it be worth adding that as a
> common denominator datum? I think the Map Grid of Australia might be in
> a similar situation (??).

For consistency with other GIS and because the International Terrestrial
Reference System datum does not exist in the EPSG datum list, I added
nzgd2k to the GRASS list. We might add ITRS in the future.

For now the transformations nzgd49 --> nzgd2k are still very accurate
because we assume the gridshift file gives a transformation to wgs84, and
then specify nzgd2k as having a zero shift relative to wgs84 (dx=dy=dz=0).
It looks like transforming wgs84-->nzgd2k at high accuracy levels cannot
really be done in the current GRASS/PROJ system because the relationship
between nzgd2k and wgs84 is dynamic; assuming no difference between them
should give acceptable results at small scales.

Paul