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

[Zoltan Szecsei]

On 2014/09/30 09:36, Dr Wesley Roberts wrote:
>
> 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.
>
Please do - Better still:  do you have a scan of the original diagram, 
or whatever original info you have - maybe send it to me off-list if it 
is "sensitive". An erf or farm portion number so I can get it from SGO?
Regards,
Zoltan
>
> Many thanks,
>
> Wesley
>
> *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 *Zoltan Szecsei
> *Sent:* Monday, September 29, 2014 6:45 PM
> *To:* africa at lists.osgeo.org <mailto: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:    +27-86-6115323www.geograph.co.za  <http://www.geograph.co.za>
> ===========================================
>
>
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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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