[OSGeo Africa] SOLVED RE: Surveyor General Diagram - Mapping property boundary points

[Wesley Roberts]

Hi All,



I would just like to close this post and mark as complete / solved.



Myself, Gavin and Zoltan discussed the sg diagram off-list and with the
help of both Gavin and Zoltan we managed to find a solution.



1.       The SG diagram in question was not in feet but in meters, I
misread a 3 for a 9 (stupid mistake but it comes with using scanned and
printed documents)

2.       The contstants were added to the XY locations before importing the
data into QGIS

3.       The correct projection to load the data into QGIS was EPSG 22279
Cape / Lo19 (http://epsg.io/22279)



The workflow that worked for me was as follows



1.       Copy the coordinates into a spreadsheet remembering to add the
specified constant to each coordinate (check the constant values carefully
– this can cause major headaches if you don’t get it right)

2.       Import into QGIS using the delimited text file import.

3.       Keep X and Y the same (Don’t swop the axes)

4.       Once the layer has been imported assign the correct projection to
the layer (EPSG 22279)

5.       Assign the project projection to the same EPSG code (22279) – Cape
Lo19

6.       The points should now map to the correct locations

7.       I then re-projected to WGS84



Many thanks to all who offered advice and support, especially Zoltan
Szecsei and Gavin Fleming.



Just to note that there were extensive discussions regarding north and
south orientation of the Lo system and how this is represented / handled by
QGIS. I have avoided mentioning this here as a “ticket” has been opened on
the qgis-dev mailing list.



Regards,

Wesley



[image: Email_Signature]

Wesley Roberts, PhD

Geospatial Manager

Biocarbon Partners

wesley at biocarbonpartners.com

Cell RSA: +27 83 5355 646



*From:* africa-bounces at lists.osgeo.org [mailto:
africa-bounces at lists.osgeo.org] *On Behalf Of *Gavin Fleming
*Sent:* Tuesday, September 30, 2014 9:48 AM
*To:* africa at lists.osgeo.org
*Subject:* Re: [OSGeo Africa] Surveyor General Diagram - Mapping property
boundary points



Wesley, send your point text file (or SG 21 chr code) to the list with some
context of where it should be drawing; the list will also accept images
such as a scan of the SG diagram.

On 30/09/2014 09:36, Dr Wesley Roberts wrote:

Hi All,



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.



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.



I can mail a grphic off list should this not make sense.



Many thanks,

Wesley



Gavin Fleming

Visit http://kartoza.com to find out about open source:
* Desktop GIS programming services
* Geospatial web development

* GIS Training

* Consulting Services

Skype: phlemingo

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Kartoza = Linfiniti ∪ Afrispatial
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