[Proj] NZGD2000 to NZGD49

Hamish hamish_nospam at yahoo.com
Wed May 17 04:41:10 EDT 2006

> I am trying to test the datum shift (using nzgd2kgrid0005.gsb file)
> for conversions between NZGD2000 and NZGD49(NZMG).

Greetings from Dunedin,

I use PROJ transforms of NZGD2k+NZTM, NZGD49+NZMG, and LatLon WGS84
almost every day- have been doing so since we worked this all out for
GRASS GIS about 2 years ago. nzgd2k and nzgd49 are defined in GRASS as
datums for setting up new locations. If you try GRASS 6.1 I recommend
the "m.proj" module as a simple frontend to cs2cs to automatically set
up the +proj parms.

NZGD2k is pretty much just WGS84 with a slightly different flattening
parm, only difference will be about 10cm in the vertical, IIRC.
The filename of the grid file reminds me that it is really for NZGD49->
NZGD2k, not WGS84, but as above this doesn't really matter.

NZGD49 will need to use the NTv2 .gsb transform grid to get within about
10-30cm horizontal accuracy*. The 7 term +towgs84 transform will get you
within approx 4m. If you use the 3 term +towgs84 it will be even worse.
If you fail to define a datum for NZMG, it will be about 150m off.
Mind that some of the terms in the 7-term list may be negative vs. LINZ
published values; this is because a different convention is used. Also
note the NTv2 .gsb grid doesn't extend very far off-shore.

[*] That is until earthquakes like today's move the plates about!
    But less than 1m for NZMG is non-meaningful anyway.

put the grid file in with the other ones in /usr/share/proj/ or local

PROJ.4 parms:
 (from GRASS's `g.proj -jf`, [latlong swapped, mind the x,y input order!])

NZGD2k + lat/lon:
+proj=longlat +a=6378137 +rf=298.257222101 +no_defs +towgs84=0.000,0.000,0.000

+proj=tmerc +lat_0=0.0000000000 +lon_0=173.0000000000 +k_0=0.9996000000 +x_0=1600000.0000000000 +y_0=10000000.0000000000 +a=6378137 +rf=298.257222101 +no_defs +towgs84=0.000,0.000,0.000

NZGD49 + NZMG: (NTv2 grid)
+proj=nzmg +lat_0=-41.0000000000 +lon_0=173.0000000000 +x_0=2510000.0000000000 +y_0=6023150.0000000000 +a=6378388 +rf=297 +no_defs +nadgrids=nzgd2kgrid0005.gsb

NZGD49 + NZMG: (7parm)
+proj=nzmg +lat_0=-41.0000000000 +lon_0=173.0000000000 +x_0=2510000.0000000000 +y_0=6023150.0000000000 +a=6378388 +rf=297 +no_defs +towgs84=59.47,-5.04,187.44,0.47,-0.1,1.024,-4.5993

NZGD49 + NZMG: (3parm)
+proj=nzmg +lat_0=-41.0000000000 +lon_0=173.0000000000 +x_0=2510000.0000000000 +y_0=6023150.0000000000 +a=6378388 +rf=297 +no_defs +towgs84=54.400,-20.100,183.100

Proj 	Datum 	EPSG Code
NZMG 	NZGD49 	27200
NZTM 	NZGD2k 	2193
Lat/Lon WGS84 	4326


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