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

Mon Sep 29 06:47:49 PDT 2014

Thanks Zoltan,

I understand the process a little better now. Will have a look tonight and
see if I can get the points into the correct location.

Many thanks for your help,

Wesley

[image: Email_Signature]

Wesley Roberts, PhD

Geospatial Manager

Biocarbon Partners

wesley at biocarbonpartners.com

Cell RSA: +27 83 5355 646

*From:* africa-bounces at lists.osgeo.org [mailto:
africa-bounces at lists.osgeo.org] *On Behalf Of *Zoltan Szecsei
*Sent:* Monday, September 29, 2014 3:33 PM
*To:* africa at lists.osgeo.org
*Subject:* Re: [OSGeo Africa] Surveyor General Diagram - Mapping property
boundary points

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

[image: Email_Signature]

Wesley Roberts, PhD

Geospatial Manager

Biocarbon Partners

wesley at biocarbonpartners.com

Cell RSA: +27 83 5355 646

*From:* 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> 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

[image: Email_Signature]

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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