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<div class="moz-cite-prefix">Hi again,<br>
Sorry - forgot to add:<br>
If you don't do the re-projection, your survey diagram will sit
approx 296m N/S and 50m E/W away from your WGS95 orthophoto.<br>
If it sits only some 50m or so away from where it should sit, then
it is likely that the Datum shift (between Cape & Harte94)
wasn't applied.<br>
<br>
Wow - that's the best my memory serves me - I last did this stuff
in 2003.<br>
Perhaps there is a surveyor on this list that can add to, or
correct, what I have suggested?<br>
<br>
Wesley: If you're still stuck in a day or so, contact me off list
& we can get it sorted better if I have the data set from you.<br>
<br>
Regards,<br>
Z<br>
<br>
<br>
On 2014/09/29 14:47, Zoltan Szecsei wrote:<br>
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<div class="moz-cite-prefix">Hi Wesley,<br>
That survey will be on the Cape datum, and not the Harte94
datum, so you will have to use the Cape datum to throw it back
to LatLong, and then use the Harte94 Datum to push it forward to
WG19<br>
<br>
The Lo19 and WG19 coordinates are much the same in that they
both are "Metres from Longitude 19degs" and "metres from the
Equator"<br>
Note that if you use proj type definitions, they are +ve South
(ie South facing) - looking at the coords you are showing below,
yours are North facing. <br>
This 'North facing fix' was done because, especially in the old
days, CAD system could not handle South facing coords, so that
was rectified by simply multiplying each column of numbers by
"-1".<br>
<br>
I have a 'C' program somewhere that I wrote many years back,
that handles this - but maybe someone on this list has done this
with proj & QGIS.<br>
Although I haven't had to do this for many years, I'd be
interested in a QGIS solution.<br>
<br>
HTH,<br>
Zoltan<br>
<br>
<br>
On 2014/09/29 14:37, Dr Wesley Roberts wrote:<br>
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<p class="MsoNormal">Dear Colleagues,</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">A friend has asked me to produce a map
using a title deed SG document. The property in question was
surveyed in 1975 and it appears to make use of the LO19
coordinate system. I am not that familiar with LO19 and am
having some trouble with defining a proj 4 projection in
QGIS. I have produced a custom CRS based on the this post by
Gavin Flemming (<a moz-do-not-send="true"
href="http://afrispatial.co.za/foss-gis/south-african-projections-in-foss-gis/">http://afrispatial.co.za/foss-gis/south-african-projections-in-foss-gis/</a>).
All seems well until I look at the location, I can find it
using ancillary data. The coordinates on the title deed
match only in the X domain (I have swopped both the X and
the Y as well as the sign) while the Y domain appears to be
“way out”. The SG title deed coordinate for one of the Y
points is -9105.30 (Constant +9 760 000,00) while the same
location in the QGIS returns a Y coordinate of around -3 769
758. Can anyone shed light on why there is such a large
difference between the SG Diagram and the actual location?</p>
<p class="MsoNormal"> </p>
<p class="MsoListParagraph" style=""><span style="">1.<span
style="font:7.0pt "Times New Roman""> </span></span>I
am thinking that it might be something to do with the fact
that the property was surveyed in 1975, any ideas?</p>
<p class="MsoListParagraph" style=""><span style="">2.<span
style="font:7.0pt "Times New Roman""> </span></span>I
have used the following proj 4 definition – have I missed
something?</p>
<p class="MsoListParagraph"> </p>
<p class="MsoNormal">+proj=tmerc +lat_0=0 +lon_0=19 +k=1
+x_0=0 +y_0=0 +ellps=WGS84 +datum=WGS84 +units=m +no_defs</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Any help on this matter would be greatly
appreciated.</p>
<p class="MsoNormal"> </p>
<p class="MsoNormal">Regards,</p>
<p class="MsoNormal">Wesley</p>
<p class="MsoNormal"> </p>
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<p class="MsoNormal">Wesley Roberts, PhD</p>
<p class="MsoNormal">Geospatial Manager</p>
<p class="MsoNormal">Biocarbon Partners</p>
<p class="MsoNormal"><a moz-do-not-send="true"
href="mailto:wesley@biocarbonpartners.com">wesley@biocarbonpartners.com</a></p>
<p class="MsoNormal">Cell RSA: +27 83 5355 646</p>
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<pre class="moz-signature" cols="72">--
===========================================
Zoltan Szecsei PrGISc [PGP0031]
Geograph (Pty) Ltd.
GIS and Photogrammetric Services
P.O. Box 7, Muizenberg 7950, South Africa.
Mobile: +27-83-6004028
Fax: +27-86-6115323 <a moz-do-not-send="true" class="moz-txt-link-abbreviated" href="http://www.geograph.co.za">www.geograph.co.za</a>
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<pre class="moz-signature" cols="72">--
===========================================
Zoltan Szecsei PrGISc [PGP0031]
Geograph (Pty) Ltd.
GIS and Photogrammetric Services
P.O. Box 7, Muizenberg 7950, South Africa.
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>
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