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

[Zoltan Szecsei]

Hi All,
The "missing step" is the Datum shift.
If you do it manually, then the sequence is "Cape Lo19 to LatLong 
<=Datum shift=>LatLong to Harte94 WG19"
If with QGIS you specify your original CRS n Cape Lo19 and your Tagert 
CRS on Harte94 WG19 the QGIS should handle the Datum shift, and it will 
seem transparent to you.

The "Survey Convention" is South Africa is to use the "Lo" prefix to 
suggest Cape Datum, and the "WG" prefix to show WGS84, Harte94 Datum. 
The numeric value after the "Lo" and the "WG" is the longitude of the 
central meridian.

HTH,
Zoltan


On 2014/09/29 15:20, Dr Wesley Roberts wrote:
>
> Hi Sarel,
>
> Many thanks for the input. I have found the specific EPSG and as 
> before the same location appears to have different coordinates to 
> those specified on the surveyors diagram. I think I am missing a step 
> here. Based on Zoltans response I think I need an intermediate step to 
> get the points into the correct location, is this correct?
>
> So, my workflow is
>
> 1.Generate text file with X and Y coordinates
>
> 2.Open in QGIS using delimited text file import
>
> 3.Assign a suitable projection to the mapped points (LO19 Cape Datum)
>
> .........An intermediate step......... <- Think of the maths cartoon 
> where the prof says "and then a miracle happens"
>
> 4.Re-project to WGS 84\
>
> I am still lost when it comes to the north facing vs south facing 
> projection and how I get the proj to match the coordinates on the SG 
> diagram. Could this not al be a change in the constant?
>
> 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
>
> *From:*africa-bounces at lists.osgeo.org 
> <mailto:africa-bounces at lists.osgeo.org> 
> [mailto:africa-bounces at lists.osgeo.org 
> <mailto:africa-bounces at lists.osgeo.org>] *On Behalf Of *S Coetzer
> *Sent:* Monday, September 29, 2014 3:05 PM
> *To:* Africa local chapter discussions
> *Subject:* Re: [OSGeo Africa] Surveyor General Diagram - Mapping 
> property boundary points
>
> Hi Wesly
>
> Zoltan is correct, use Cape datum for 1975 data, I think the EPSG code 
> is: 22279
>
> In QGIS, use 22279 to filter your CRS and you'lle find it.
>
> Good luck.
>
>
> Sarel Coetzer
>
> On Mon, Sep 29, 2014 at 2:37 PM, Dr Wesley Roberts 
> <wesley at biocarbonpartners.com <mailto:wesley at biocarbonpartners.com>> 
> 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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