Hi Hamish<br><br>Thanks for the info.<br><br>Haven't used grass in ages but should have another look.<br><br>I updated the projection definition in postgis to contain the gsb file and it all works fine now. One thing crossed off the list.
<br><br>Simon<br><br><br><br><br><br><div><span class="gmail_quote">On 5/17/06, <b class="gmail_sendername">Hamish</b> <<a href="mailto:hamish_nospam@yahoo.com">hamish_nospam@yahoo.com</a>> wrote:</span><blockquote class="gmail_quote" style="border-left: 1px solid rgb(204, 204, 204); margin: 0pt 0pt 0pt 0.8ex; padding-left: 1ex;">
> I am trying to test the datum shift (using nzgd2kgrid0005.gsb file)<br>> for conversions between NZGD2000 and NZGD49(NZMG).<br><br><br>Greetings from Dunedin,<br><br>I use PROJ transforms of NZGD2k+NZTM, NZGD49+NZMG, and LatLon WGS84
<br>almost every day- have been doing so since we worked this all out for<br>GRASS GIS about 2 years ago. nzgd2k and nzgd49 are defined in GRASS as<br>datums for setting up new locations. If you try GRASS 6.1 I recommend<br>
the "m.proj" module as a simple frontend to cs2cs to automatically set<br>up the +proj parms.<br><br><br>NZGD2k is pretty much just WGS84 with a slightly different flattening<br>parm, only difference will be about 10cm in the vertical, IIRC.
<br>The filename of the grid file reminds me that it is really for NZGD49-><br>NZGD2k, not WGS84, but as above this doesn't really matter.<br><br>NZGD49 will need to use the NTv2 .gsb transform grid to get within about
<br>10-30cm horizontal accuracy*. The 7 term +towgs84 transform will get you<br>within approx 4m. If you use the 3 term +towgs84 it will be even worse.<br>If you fail to define a datum for NZMG, it will be about 150m off.
<br>Mind that some of the terms in the 7-term list may be negative vs. LINZ<br>published values; this is because a different convention is used. Also<br>note the NTv2 .gsb grid doesn't extend very far off-shore.<br><br>[*] That is until earthquakes like today's move the plates about!
<br> <a href="http://earthquake.usgs.gov/eqcenter/recenteqsww/">http://earthquake.usgs.gov/eqcenter/recenteqsww/</a><br> But less than 1m for NZMG is non-meaningful anyway.<br><br>put the grid file in with the other ones in /usr/share/proj/ or local
<br>equivalent.<br><br><br><br>PROJ.4 parms:<br> (from GRASS's `g.proj -jf`, [latlong swapped, mind the x,y input order!])<br><br><br>NZGD2k + lat/lon:<br>+proj=longlat +a=6378137 +rf=298.257222101 +no_defs +towgs84=0.000
,0.000,0.000<br><br>NZGD2k + NZTM:<br>+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
<br><br><br>NZGD49 + NZMG: (NTv2 grid)<br>+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<br><br>NZGD49 + NZMG: (7parm)
<br>+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<br><br>NZGD49 + NZMG: (3parm)<br>+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<br><br><br><br>Proj Datum EPSG Code<br>NZMG NZGD49 27200<br>NZTM NZGD2k 2193
<br>Lat/Lon WGS84 4326<br><br><br><br>luck,<br>Hamish<br></blockquote></div><br>