[Proj] NZGD2000 to NZGD49

[Simon O'Malley]

Hi Hamish

Thanks for the info.

Haven't used grass in ages but should have another look.

I updated the projection definition in postgis to contain the gsb file and
it all works fine now. One thing crossed off the list.

Simon





On 5/17/06, Hamish <hamish_nospam at yahoo.com> wrote:
>
> > 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!
>       http://earthquake.usgs.gov/eqcenter/recenteqsww/
>     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
> equivalent.
>
>
>
> 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
>
> NZGD2k + NZTM:
> +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
>
>
>
> luck,
> Hamish
>
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