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

Mon Sep 29 06:55:57 PDT 2014

Hi All

Just as a quick recap, the SG diagrams has a system of westings (y) and southings (x), where:
westings (y) is the distance from the closest odd Lo(Central Meridian) and southings (x) is the distance from the equator

In SA the y range from 110 000m to approx 110 000 m and, x ranges from  2 2 00 000 in Limpopo to 3 800 000 (Cape Aghulus).

When there are values with constants such as +9 760 000, I would automatically assume that the unit of measure is Cape Feet and all coordinates and constants needs to be converted to meters.

The easiest way (for a non surveyor) to transform Cape Datum data (modified Clarke 1880) to Hartebeesthoek94 (WGS84) is by using the NTV2 transformation grid that QGIS can also use. If you correctly read data and define it as Cape Datim, you can reproject pretty accurately into Hartebeesthoek94 using this transformation method.

Please see http://www.ee.co.za/article/datum-transformations-using-the-ntv2-grid.html

I hope this helps

>>> Zoltan Szecsei <zoltans at geograph.co.za> 9/29/2014 3:05 PM >>>

AAaargh! WGS84
(I'll go away, now)
On 2014/09/29 15:03, Zoltan Szecsei wrote:

Hi again,
Sorry - forgot to add:
If you don't do the re-projection, your survey diagram will sit approx 296m N/S and 50m E/W away from your WGS95 orthophoto.
If it sits only some 50m or so away from where it should sit, then it is likely that the Datum shift (between Cape & Harte94) wasn't applied.

Wow - that's the best my memory serves me - I last did this stuff in 2003.
Perhaps there is a surveyor on this list that can add to, or correct, what I have suggested?

Wesley: If you're still stuck in a day or so, contact me off list & we can get it sorted better if I have the data set from you.

Regards,
Z

On 2014/09/29 14:47, Zoltan Szecsei wrote:

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

Wesley Roberts, PhD
Geospatial Manager
Biocarbon Partners
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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Africa mailing list
Africa at lists.osgeo.org
You can UNSUBSCRIBE at http://lists.osgeo.org/mailman/listinfo/africa

-- 

===========================================
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
===========================================

_______________________________________________
Africa mailing list
Africa at lists.osgeo.org
You can UNSUBSCRIBE at http://lists.osgeo.org/mailman/listinfo/africa

-- 

===========================================
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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