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

[Zoltan Szecsei]

Hi Wesley,
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

The Lo19 and WG19 coordinates are much the same in that they both are 
"Metres from Longitude 19degs" and "metres from the Equator"
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.
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".

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.
Although I haven't had to do this for many years, I'd be interested in a 
QGIS solution.

HTH,
Zoltan


On 2014/09/29 14:37, Dr Wesley Roberts wrote:
>
> Dear Colleagues,
>
> 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 
> (http://afrispatial.co.za/foss-gis/south-african-projections-in-foss-gis/). 
> 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?
>
> 1.I am thinking that it might be something to do with the fact that 
> the property was surveyed in 1975, any ideas?
>
> 2.I have used the following proj 4 definition -- have I missed something?
>
> +proj=tmerc +lat_0=0 +lon_0=19 +k=1 +x_0=0 +y_0=0 +ellps=WGS84 
> +datum=WGS84 +units=m +no_defs
>
> Any help on this matter would be greatly appreciated.
>
> Regards,
>
> Wesley
>
> Email_Signature
>
> Wesley Roberts, PhD
>
> Geospatial Manager
>
> Biocarbon Partners
>
> wesley at biocarbonpartners.com <mailto:wesley at biocarbonpartners.com>
>
> Cell RSA: +27 83 5355 646
>
>
>
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-- 

===========================================
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     www.geograph.co.za
===========================================

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