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<div class="moz-cite-prefix">On 2014/09/30 09:36, Dr Wesley Roberts
wrote:<br>
</div>
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<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Hi
All,</span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> </span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">I
am so close now. I have taken both Gavin’s and Zoltan’s
advice and used the custom CRS definition using the
precision conversion factor suggested by Zoltan.</span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> </span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">I
am able to view the coordinate points in QGIS and can see
that they are almost in the correct location, however, there
appears to be some sort of transformation error. The best
way to describe the error is to imagine points as a
rectangle lying at a 45 degree angle. With the current CRS
setup the points which should be at the top are now at the
bottom and the points at the bottom are at the top.
Difficult to explain without a graphic and I doubt that the
mailing list will accept graphics in images. The points need
to be rotated 180 degrees through the vertical, if that
makes any sense.</span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> </span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">I
can mail a grphic off list should this not make sense.</span></p>
</div>
</blockquote>
Please do - Better still: do you have a scan of the original
diagram, or whatever original info you have - maybe send it to me
off-list if it is "sensitive". An erf or farm portion number so I
can get it from SGO?<br>
Regards,<br>
Zoltan<br>
<blockquote
cite="mid:28d6e5bc2e52dae425c685052e02074d@mail.gmail.com"
type="cite">
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<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> </span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Many
thanks,</span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d">Wesley</span></p>
<p class="MsoNormal"><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1f497d"> </span></p>
<div>
<div style="border:none;border-top:solid #e1e1e1
1.0pt;padding:3.0pt 0in 0in 0in">
<p class="MsoNormal"><b><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:windowtext">From:</span></b><span
style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:windowtext">
<a moz-do-not-send="true"
href="mailto:africa-bounces@lists.osgeo.org">africa-bounces@lists.osgeo.org</a>
[mailto:<a moz-do-not-send="true"
href="mailto:africa-bounces@lists.osgeo.org">africa-bounces@lists.osgeo.org</a>]
<b>On Behalf Of </b>Zoltan Szecsei<br>
<b>Sent:</b> Monday, September 29, 2014 6:45 PM<br>
<b>To:</b> <a moz-do-not-send="true"
href="mailto:africa@lists.osgeo.org">africa@lists.osgeo.org</a><br>
<b>Subject:</b> Re: [OSGeo Africa] Surveyor General
Diagram - Mapping property boundary points</span></p>
</div>
</div>
<p class="MsoNormal"> </p>
<div>
<p class="MsoNormal">On 2014/09/29 18:42, Zoltan Szecsei
wrote:</p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal"><span style="color:#009900">Cape or
English feet?<br>
See below:</span><br>
<br>
On 2014/09/29 16:24, Gavin Fleming wrote:</p>
</div>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal">Hi Wesley<br>
<br>
If you load the coordinates with the correct CRS
definition in QGIS you shouldn't need to swap signs or
axes. Any coordinates you get from a survey diagram will
be south-oriented (SO). Just apply an appropriate SO CRS
definition. <br>
<br>
These are pre-defined options available in QGIS relating
to 'LO19':<br>
<br>
LO19 Cape datum (m) south-oriented: EPSG 22279<br>
LO19 Cape datum (m) north-oriented: South African CRS :
CAPE_NO_19<br>
LO19 WGS84 (or Hartebeesthoek94) datum (m)
south-oriented: EPSG 2048<br>
LO19 WGS84 (or Hartebeesthoek94) datum (m)
north-oriented: South African CRS : HBK_NO_19<br>
<br>
If your Y coordinates are indeed positive and in the
region of 9 million then I'd venture they are in Cape
Feet and south-oriented, in which case there's no
pre-defined CRS. </p>
</div>
</blockquote>
<p class="MsoNormal"><span style="color:#009900">I think at
some point Natal was using English feet - the rest of
South Africa: Cape feet<br>
You might want to add far more precision to the conversion
factor, though:<br>
9760000 * .314858 = 3 073 014.08<br>
9760000 * .31485557516 = 3 072 990.4135<br>
<br>
That's a 23.67m difference.<br>
<br>
Regards,<br>
Zoltan</span><br>
<br>
<br>
</p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<div>
<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
So create a custom CRS using this proj4 definition. Call
it something like 'LO19 Cape SO Cape feet'</p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
<p class="MsoNormal"><span
style="font-size:11.5pt;font-family:"Arial","sans-serif"">+proj=tmerc
+lat_0=0 +lon_0=<b>19</b> +k=1 +x_0=0 +y_0=0
+axis=wsu +a=6378249.145 +b=6356514.966398753
+towgs84=-136,-108,-292,0,0,0,0 <b>+to_meter=0.314858</b>
+no_defs</span></p>
</blockquote>
<p class="MsoNormal" style="margin-bottom:12.0pt"><br>
+axis=wsu makes it south-facing<br>
+towgs84=.... specifies the Cape to WGS84 datum
transformation<br>
+to_meter=... specifies the cape feet to metre
conversion. <br>
<br>
btw the example on my (old) website is for data in
north-oriented data on the WGS84 datum, in metres. It
was also written before SA CRS definitions were
available in GIS software. The QGIS CRS to use for the
same CRS now would be "South African CRS : HBK_NO_19". <br>
<br>
Gavin</p>
</div>
</blockquote>
</blockquote>
<p class="MsoNormal"><br>
<br>
</p>
<pre>-- </pre>
<pre> </pre>
<pre>===========================================</pre>
<pre>Zoltan Szecsei PrGISc [PGP0031]</pre>
<pre>Geograph (Pty) Ltd.</pre>
<pre>GIS and Photogrammetric Services</pre>
<pre> </pre>
<pre>P.O. Box 7, Muizenberg 7950, South Africa.</pre>
<pre> </pre>
<pre>Mobile: +27-83-6004028</pre>
<pre>Fax: +27-86-6115323 <a moz-do-not-send="true" href="http://www.geograph.co.za">www.geograph.co.za</a></pre>
<pre>===========================================</pre>
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</blockquote>
<br>
<br>
<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>
===========================================</pre>
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