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<font size="+1">Hi Lutra / Brett,<br>
<br>
Thanks for taking a look at this, Lutra. I got it sorted out on Linux,
but couldn't get Windows to pick up the distortion grids when I tried.<br>
<br>
Firstly, I don't think there is any problem with the GDA94 projections
into any of the zones across Australia. The problem is just with AGD66
& AGD84.<br>
<br>
There is no single definition for the towgs84 parameters for either
AGD66 or AGD84. It varies depending on where you are. Proj4 therefore
doesn't include any towgs84 paramters for either datum. This results in
~100-200m error and the two datums being identical within Proj4 (and
therefore QGIS).<br>
<br>
Geoscience Australia publishes binary distortion tables to solve the
variability problems. We have been trying to include these tables into
the QGIS database CRS.db so that we can accurately display and mix
AGD66, AGD84 and GDA94 data. The distortion tables linked by Brett are
correct (I have been using them) and cover all of Australia. The only
caveat is that I am not sure if the AGD84 table works in Northern
Territory, New South Wales and Victoria as they didn't adopt AGD84. The
test points are in Zone 51 (Western Australia) so should be OK anyway.<br>
<br>
To use the distortion files (on Linux) I applied the following updates
to my CRS.db.<br>
<br>
<i><small>UPDATE tbl_srs set parameters=replace(parameters, '_SA', '_SA
+towgs84=-117.808,-51.536,137.784,-0.303,-0.446,-0.234,-0.290
+nadgrids=agd66.gsb +wktext') where description like 'AGD66 / AMG%'<br>
UPDATE tbl_srs set parameters=replace(parameters, '_SA', '_SA
+towgs84=-117.808,-51.536,137.784,-0.303,-0.446,-0.234,-0.290
+nadgrids=agd66.gsb +wktext') where description = 'AGD66'<br>
UPDATE tbl_srs set parameters=replace(parameters, '_SA', '_SA
+towgs84=-117.763,-51.51,139.061,-0.292,-0.443,-0.277,-0.191
+nadgrids=agd84.gsb +wktext') where description like 'AGD84%'<br>
</small></i><br>
The towgs84 parameters come out of the Geoscience Australia manual. I
included them as a fall back if the distortion grids could not be
found, but it may be better to make them 0,0,0. The +nadgrids and
+wktext parameters are both included. I put the grid files in
/usr/local/share/proj (in my case) and it works well. My research shows
that the towgs84 parameters are just ignored if the distortion grid is
used.<br>
<br>
I then installed QGIS on Windows7 and tried it there. No matter where I
put the distortion files I could not get QGIS on Windows to use them. I
installed from both the OsGeo installed and the standalone version to
try to get it to work, to no avail. The OsGeo install produced an error
message that it could not display the layer and the Standalone install
displayed it with ~200m error.<br>
<br>
The longer term solution is probably to get the AGD66 and AGD84
distortion grids included in Proj4. This is really a work around so
that Australian users can get there AGD66 & AGD84 data correctly
located now (currently ~200m error).<br>
<br>
Roland<br>
</font>
<div class="moz-signature"> <br>
<div class="moz-signature">-- <br>
<strong><font face="Arial" color="#000080">ROLAND HILL</font></strong><br>
<strong><font face="Arial" color="#000080" size="2">Director</font></strong><br>
<strong><font face="Arial" color="#000080" size="2">Four Winds
Technology Pty Ltd</font></strong><br>
<strong><font face="Arial" color="#000080" size="2">Ph/Fax : +61
(0)2 6366 9425</font></strong><br>
<strong><font face="Arial" color="#000080" size="2">Mobile :
+61 (0)41 880 7472</font></strong><br>
<strong><font face="Arial" color="#000080" size="2"><a
href="mailto:Roland.Hill@fourwindstechnology.com.au">Roland.Hill@fourwindstechnology.com.au</a></font></strong>
<br>
<br>
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<br>
On 11/10/10 01:20, Quantum GIS wrote:
<blockquote cite="mid:042.c845cc89a058544b4a4725fa7b2aba19@qgis.org"
type="cite">
<pre wrap="">#3099: Projection Failure
------------------------------------------------+---------------------------
Reporter: badams | Owner: nobody
Type: bug | Status: new
Priority: major: does not work as expected | Milestone: Version 1.6.0
Component: Projection Support | Version:
Keywords: Projection Failure | Platform_version:
Platform: Windows | Must_fix: No
Status_info: 0 |
------------------------------------------------+---------------------------
Comment(by badams):
Lutra,
I'll need some time to digest this but I'll have a go.
The proposed projections are correct. I've been playing with them for the
last 15 years. The link below will take to the Geoscience Australia site
where they are explained fully.
<a class="moz-txt-link-freetext" href="http://www.ga.gov.au/geodesy/datums/cosys.jsp">http://www.ga.gov.au/geodesy/datums/cosys.jsp</a>
Thanks
Brett
</pre>
</blockquote>
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