[OSGeo Africa] Digitising a 1929 SG diagram

Hanlie Pretorius hanlie.pretorius at gmail.com
Sun Jan 4 23:04:52 PST 2015


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 location.

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

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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