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Hi Gdal-ers,<br>
This is cross-posted on the proj list, but my query also concerns
whether gdalwarp notices the NTv2 grid or not, as my results vary
from the control data (point BM1) I was given.<br>
I'm using gdal/ogr v1.9.1 on ubuntu 12.04.<br>
Please read on, and comment:<br>
<br>
Hi All,<br>
This issue goes on, slower than a snail.<br>
I have finally managed to get some source and target points from the
original surveyor.<br>
<br>
<table class="MsoNormalTable" style="width:90.0%" border="1"
cellpadding="0" width="90%">
<tbody>
<tr>
<td style="padding:.75pt .75pt .75pt .75pt">
<p class="MsoNormal"><font color="black" face="Times New "
size="1"><span
style="font-size:8.0pt;font-family:"Times
New\000D\000A";color:windowtext">BM1<u1:p></u1:p></span></font><o:p></o:p></p>
</td>
<td style="padding:.75pt .75pt .75pt .75pt">
<p class="MsoNormal" style="text-align:center"
align="center"><font color="black" face="Times New Roman"
size="1"><span style="font-size:8.0pt;color:windowtext">2872274.540m<u1:p></u1:p></span></font><o:p></o:p></p>
</td>
<td style="padding:.75pt .75pt .75pt .75pt">
<p class="MsoNormal" style="text-align:center"
align="center"><font color="black" face="Times New Roman"
size="1"><span style="font-size:8.0pt;color:windowtext">49717.410m<u1:p></u1:p></span></font><o:p></o:p></p>
</td>
<td style="padding:.75pt .75pt .75pt .75pt">
<p class="MsoNormal" style="text-align:center"
align="center"><font color="black" face="Times New Roman"
size="1"><span style="font-size:8.0pt;color:windowtext">7128571.659m<u1:p></u1:p></span></font><o:p></o:p></p>
</td>
<td style="padding:.75pt .75pt .75pt .75pt">
<p class="MsoNormal" style="text-align:center"
align="center"><font color="black" face="Times New Roman"
size="1"><span style="font-size:8.0pt;color:windowtext">450273.237m<u1:p></u1:p></span></font><o:p></o:p></p>
</td>
<td style="padding:.75pt .75pt .75pt .75pt">
<p class="MsoNormal" style="text-align:center"
align="center"><font color="black" face="Times New Roman"
size="1"><span style="font-size:8.0pt;color:windowtext">4275103.524m</span></font></p>
</td>
</tr>
</tbody>
</table>
<br>
I load my source image that is on Lo33 Clarke 1880 into QGIS and
hover the mouse over BM1, a nicely painted white target.<br>
I get: -49717.33 -2872272.46 which is good on the eastings, but
my source image is 2m north of this value from the surveyor. For the
moment, so what.<br>
<br>
I then run:<br>
<tt>ogr2ogr -f "DGN" -dsco "3D=YES" -s_srs "+proj=tmerc +lon_0=33
+a=6378249.145 +b=6356514.966398753 +nadgrids=./SA.gsb" -t_srs
"+init=epsg:32736" Cl_Revised_v7_36s.dgn Cl_Revised_v7.dgn<br>
gdalwarp -s_srs "+proj=tmerc +lon_0=33
+a=6378249.145 +b=6356514.966398753 +nadgrids=./SA.gsb" -t_srs
"+init=epsg:32736" images.vrt images_36s.tif<br>
</tt><br>
My UTM 36S WGS84 coordinate for BM1 is now:<br>
450288.88 and 7128578.76 and this puts my transformed image 15m
to the east and 7m to the north of the Surveyors value.<br>
<br>
My question is: <br>
Did I specify the s_srs and t_srs parameters correctly?<br>
Can I be certain that the SA.gsb grid was used?<br>
<br>
I have no idea how the original surveyor got his source and target
values for BM1, as I have no access to this person.<br>
Regards,<br>
Zoltan<br>
<br>
<br>
<br>
<blockquote
cite="mid:0F348F23-3D73-4611-8C34-B731E69BCEF2@gmail.com"
type="cite">
<blockquote type="cite">
<div>
<div style="direction: ltr;font-family: Times New Roman;color:
#000000;font-size: x-small;">
<p> <br>
I have no idea how big, or what extents are in these NTv2
grid files, as<br>
I have never used them before.<br>
<br>
To this end, my area of interest is in Maputo
(Mozambique), and<br>
Mozambique apparently uses Clarke 1866 (not 1880) - but
the surveyor<br>
"swears blind" that the coords are on "1880" and not
"1866".<br>
<br>
This begs the question:<br>
Do the extents of this Clarke 1880 NTv2 grid stop at the
South African<br>
border (some 50Km west of my area of interest), or do they
cover Maputo<br>
sufficiently?<br>
</p>
</div>
</div>
</blockquote>
</blockquote>
Yes, they go up to 33E which just covers Maputo, and when I load the
grid plus my data into QGIS, my data is inside the grid extents.<br>
<blockquote
cite="mid:0F348F23-3D73-4611-8C34-B731E69BCEF2@gmail.com"
type="cite">
<blockquote type="cite">
<div>
<div style="direction: ltr;font-family: Times New Roman;color:
#000000;font-size: x-small;">
<p> <br>
<br>
On 2012/07/26 21:42, Mikael Rittri wrote:<br>
> Hello Zoltan.<br>
> Yes, as you say, the traditional transverse mercator
projections for Southern Africa have<br>
> axes positive west and south. This can be handled in
Proj 4.8.0 (see <a moz-do-not-send="true"
href="http://trac.osgeo.org/proj/wiki/TMSO">http://trac.osgeo.org/proj/wiki/TMSO</a>
).<br>
> However, since both your example coordinates are
negative, I think you have data in a less traditional<br>
> but more GIS-friendly variant, where the axes go east
and north and the traditional coordinate values<br>
> have been negated.<br>
><br>
> Another problem is caused by the Cape datum having
distortions that are not handled well<br>
> by a simple Helmert datum shift ( +towgs84 in
Proj.4). Such a datum shift will have errors up to 15
meters,<br>
> perhaps more. You can get much better accuracy from a
grid shift file. So I think your source CRS<br>
> is best constructed as<br>
><br>
> +proj=tmerc +lon_0=33 +a=6378249.145
+b=6356514.966398753 +nadgrids=SA.gsb<br>
><br>
> where SA.gsb is an NTv2 grid shift file from Cape to
Hartebeesthoek94 (or to WGS84),<br>
> covering South Africa. To find out where you can
download it, see<br>
><br>
> <a moz-do-not-send="true"
href="http://eepublishers.co.za/article/datum-transformations-using-the-ntv2-grid.html">http://eepublishers.co.za/article/datum-transformations-using-the-ntv2-grid.html</a><br>
><br>
> About your target CRS, I don't think there is an EPSG
code for the combination of Hartebeesthoek94 and<br>
> a UTM projection. But since H94 is the same as WGS84
except for tectonic motion, I think you<br>
> can use WGS84 / UTM zone 36S, which can be defined as<br>
><br>
>Â Â Â Â +init=epsg:32736<br>
><br>
> Hope this helps,<br>
><br>
> Mikael Rittri<br>
> Carmenta<br>
> Sweden<br>
> <a moz-do-not-send="true"
href="http://www.carmenta.com">http://www.carmenta.com</a><br>
><br>
> 26 jul 2012 kl. 17:14 skrev "Zoltan Szecsei" <<a
moz-do-not-send="true"
href="mailto:zoltans@geograph.co.za">zoltans@geograph.co.za</a>>:<br>
><br>
>> Hi Everyone,<br>
>> I have some orthos and some 3D vector data that I
need to reproject from<br>
>> Clarke 1880 Cape Datum, Lo33 to WGS84
Hartebeeshoek94 datum UTM 36S.<br>
>><br>
>><br>
</p>
</div>
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</blockquote>
<br>
</blockquote>
<br>
<pre class="moz-signature" cols="72">--
===========================================
Zoltan Szecsei PrGISc [PGP0031]
Geograph (Pty) Ltd.
P.O. Box 7, Muizenberg 7950, South Africa.
65 Main Road, Muizenberg 7945
Western Cape, South Africa.
34° 6'16.35"S 18°28'5.62"E
Tel: +27-21-7884897 Mobile: +27-83-6004028
Fax: +27-86-6115323 <a class="moz-txt-link-abbreviated" href="http://www.geograph.co.za">www.geograph.co.za</a>
===========================================</pre>
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