[GRASS5] adding a new datum/projection?

[H Bowman]

Hi again --


I've got some new data in yet another new projection, which uses its own
new datum.


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).

So as far as I understand: NZGD2k (==ITRF96) is the WGS84 datum combined
with the GRS80 ellipsoid.

Except g.setproj won't let me do that, either with a new location or
retroactively. The old 5.0.2 version asks for ellipsoid first, then
datum, but complains when you try to mix wgs84 and grs80 and quits
without writing anything. The latest g.setproj never gives you a chance
to set the ellipsoid, it is chosen from your datum.

I'm not sure if I get the correct result if I just change the ellps:
line to grs80 in the PROJ_INFO file?

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 (??).

see:  (102k)
http://www.linz.govt.nz/rcs/linz/17880/difference_wgs84_nzgd2000.pdf



As for the new projection, it is New Zealand Transverse Mercator (NZTM),
which is in terms of that new NZGD2k datum.
Details:
Datum: NZGD2k
Origin Latitude: 0° South
Origin Longitude: 173° East
False Northing: 10 000 000m N
False Easting: 1 600 000m E
Scale Factor: 0.9996

Setting projection as tmerc and entering those terms goes smoothly.
Should NZTM get its own projection entry or is entering by hand with the
tmerc projection every time the way to go?

for more details see:  (44k)
http://www.linz.govt.nz/rcs/linz/5684/nztransverse_mercator.pdf

That PDF also has a page of useful ellipsoid to grid formulae which
might be useful to someone.


It doesn't help much that half the country is moving 5cm/year in the
opposite direction to the other half..



thanks for any insight,
Hamish