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

[Dr Wesley Roberts]

Hi All,



I am so close now. I have taken both Gavin’s and Zoltan’s advice and used
the custom CRS definition using the precision conversion factor suggested
by Zoltan.



I am able to view the coordinate points in QGIS and can see that they are
almost in the correct location, however, there appears to be some sort of
transformation error. The best way to describe the error is to imagine
points as a rectangle lying at a 45 degree angle. With the current CRS
setup the points which should be at the top are now at the bottom and the
points at the bottom are at the top. Difficult to explain without a graphic
and I doubt that the mailing list will accept graphics in images. The
points need to be rotated 180 degrees through the vertical, if that makes
any sense.



I can mail a grphic off list should this not make sense.



Many thanks,

Wesley



*From:* africa-bounces at lists.osgeo.org [mailto:
africa-bounces at lists.osgeo.org] *On Behalf Of *Zoltan Szecsei
*Sent:* Monday, September 29, 2014 6:45 PM
*To:* africa at lists.osgeo.org
*Subject:* Re: [OSGeo Africa] Surveyor General Diagram - Mapping property
boundary points



On 2014/09/29 18:42, Zoltan Szecsei wrote:

Cape or English feet?
See below:

On 2014/09/29 16:24, Gavin Fleming wrote:

Hi Wesley

If you load the coordinates with the correct CRS definition in QGIS you
shouldn't need to swap signs or axes. Any coordinates you get from a survey
diagram will be south-oriented (SO). Just apply an appropriate SO CRS
definition.

These are pre-defined options available in QGIS relating to 'LO19':

LO19 Cape datum (m) south-oriented: EPSG 22279
LO19 Cape datum (m) north-oriented: South African CRS : CAPE_NO_19
LO19 WGS84 (or Hartebeesthoek94) datum (m) south-oriented: EPSG 2048
LO19 WGS84 (or Hartebeesthoek94) datum (m) north-oriented: South African
CRS : HBK_NO_19

If your Y coordinates are indeed positive and in the region of 9 million
then I'd venture they are in Cape Feet and south-oriented, in which case
there's no pre-defined CRS.

I think at some point Natal was using English feet - the rest of South
Africa: Cape feet
You might want to add far more precision to the conversion factor, though:
9760000 * .314858             =  3 073 014.08
9760000 * .31485557516  = 3 072 990.4135

That's a 23.67m difference.

Regards,
Zoltan



So create a custom CRS using this proj4 definition. Call it something like
'LO19 Cape SO Cape feet'

+proj=tmerc +lat_0=0 +lon_0=*19* +k=1 +x_0=0 +y_0=0 +axis=wsu
+a=6378249.145 +b=6356514.966398753 +towgs84=-136,-108,-292,0,0,0,0
*+to_meter=0.314858* +no_defs


+axis=wsu makes it south-facing
+towgs84=.... specifies the Cape to WGS84 datum transformation
+to_meter=... specifies the cape feet to metre conversion.

btw the example on my (old) website is for data in north-oriented data on
the WGS84 datum, in metres. It was also written before SA CRS definitions
were available in GIS software. The QGIS CRS to use for the same CRS now
would be "South African CRS : HBK_NO_19".

Gavin



-- 



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

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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