[GRASSLIST:4584] Re: Datum transform frustrations

[Hamish]

> 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!

I take it you mean datum transform.

> (Without transformation they are completely off)

I take it you mean reprojection.

 
> 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.

Sounds right.
I've done this extensively without any problems (LINZ vectors, TM->MG).
It should be within a few meters. I've never tried the LENZ data.


> 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.

200m is about right for using the wrong datum. Are you sure the LENZ
grids were not put together using the wrong datum? e.g. try loading the
grid into a NZMG location with the 2000 datum and see what happens.
Are there .prj files or some meta data containing projection info?
grasping at straws here.


> 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

those both look right.

 
> Any one have any ideas?

You might check against a test coordinate:

Eastern entrance to Akaroa Harbour:

NZMG projection w/ NZGD49 datum
  2507512.60416667   5702150

NZTM projection w/ NZGD2k datum
  1597502.78125      5140557.1875

Lat/Lon w/ WGS84 datum
  172:58:08.07E      43:53:17.0925S


(give or take a few meters)



You should definitely get the LINZ vector coastline from NZTopoOnline: 
  http://www.nztopoonline.linz.govt.nz

(Uses Arc/Web so horribly slow and mostly unusable... but you can
extract shape files eventually)

those are provided in NZTM/gd2k and you can figure out what's right and
what's wrong from there.



Hamish