[GRASSLIST:4582] Datum transform frustrations

[Joel Peter William Pitt]

Hi all,

I've been playing around with datums and projections
in order to get two maps to correspond to one another.
I've tried several things, but I always end up with the
the two maps about 2000m off when they should only
be around 200m off without transformation!
(Without transformation they are completely off)

Specifically I'm trying to convert a New Zealand Mapping
Grid raster file (from LENZ) based on datum nzgd49,
to New Zealand Transverse Mercator, datum nzgd2k.
The way I've tried doing it is by creating two locations, one
for as a source with nzgd49 data and the other as a destination.
Then I use r.proj within the destination to import layers
from the source location.

I would assume that one of the layers had error, but when I loaded
the two up in ArcView they show a 200m diff as expected before
transformation, and when I transform the data they align perfectly.

I've included information about the two layers below and it all
seems in order.

-PROJ_INFO-------------------------------------------------
name       : New Zealand Map Grid
datum      : nzgd49
towgs84    : 59.47,-5.04,187.44,0.47,-0.1,1.024,-4.5993
proj       : nzmg
ellps      : international
a          : 6378388.0000000000
es         : 0.0067226700
f          : 297.0000000000
lat_0      : -41.0000000000
lon_0      : 173.0000000000
x_0        : 2510000.0000000000
y_0        : 6023150.0000000000
-PROJ_UNITS------------------------------------------------
unit       : meter
units      : meters
meters     : 1.0

and for the destination map:
-PROJ_INFO-------------------------------------------------
name       : Transverse Mercator
datum      : nzgd2k
towgs84    : 0.000,0.000,0.000
proj       : tmerc
ellps      : grs80
a          : 6378137.0000000000
es         : 0.0066943800
f          : 298.2572221010
lat_0      : 0.0000000000
lon_0      : 173.0000000000
k_0        : 0.9996000000
x_0        : 1600000.0000000000
y_0        : 10000000.0000000000
-PROJ_UNITS------------------------------------------------
unit       : meter
units      : meters
meters     : 1.0

Any one have any ideas?

Thanks,
Joel