[OSGeo Africa] Digitising a 1929 SG diagram
chris at kirchhoffsurveyors.co.za
Sun Jan 4 23:34:46 PST 2015
The units are in Cape roods, however the coordinates on the diagram are in a
Local system not connected to any formal CRS
In 1929 the surveyor who did this would have just made a local system for
farms surveyed in that area.
The way forward would be to see if any of the portions noted on the diagram
are in a formal CRS - for example there is a note that says the remainder
of farm is now included in Mc taggerts camp on diagram 1011/2000 - this
diagram should be in WGs84 coordinates. You can then then use these
coordinates and the distances and directions on the diagram you are trying
to locate to calculate WGS84 coordinates for this diagram.
If you send me a kml of where the farm is approximately I can try get you
SG diagram shape file data for that area
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From: africa-bounces at lists.osgeo.org [mailto:africa-bounces at lists.osgeo.org]
On Behalf Of Hanlie Pretorius
Sent: 05 January 2015 09:05 AM
To: Africa local chapter discussions
Subject: [OSGeo Africa] Digitising a 1929 SG diagram
I have a problem digitising a 1929 SG diagram, which I have attached.
I'm not sure what the units are, but I assumed they were Cape Feet
(conversion factor = 0.314855575). As you can see, the coordinates are not
in the range that one would expect, so added the following constants that I
got off a 1936 diagram of the same area to the Cape Feet values:
Constant for Survey Y 24000
Constant for Survey X 9972000
However, nothing I do can place the points in the correct position. I have
tried the QGIS Cape /Lo21 (EPSG 22281) coordinate system and not swapping X
and Y or multiplying by minus 1.
I have also tried the QGIS South African CRS CAPE_NO_21
(ZANGI:ZANGI:CPNO21) coordinate systems, and swapping X and Y and
multiplying by minus 1
The diagram is not oriented correctly or it lands in the wrong spatial
I also tried a Cape Rds conversion, but the coordinates are still in the
incorrect range and seems to need some kind of constant added to them.
Can anyone perhaps take a look and suggest a work flow that should put it in
the correct location? I want to get the points in the coordinate table in
GIS format so that I can georeference the diagram.
Can someone perhaps take a look and
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