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

[Dr Wesley Roberts]

Thanks Gavin and Zoltan,



This helps a bunch. Am going to have a look right now and get back to you.
I was wondering about the feet conversion. Will try both and assess the
difference.



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 *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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