[OSGeo Africa] Surveyor General Diagram - Mapping property boundary points
Gavin Fleming
gavin at kartoza.com
Mon Sep 29 07:24:19 PDT 2014
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.
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
On 29/09/2014 16:03, Dr Wesley Roberts wrote:
>
> Many thanks Aslam,
>
>
>
> Will make the change and see where I end up.
>
>
>
> Regards,
>
> Wesley
>
>
>
>
> 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 *Aslam Parker
> *Sent:* Monday, September 29, 2014 3:56 PM
> *To:* africa at lists.osgeo.org <mailto:africa at lists.osgeo.org>
> *Subject:* Re: [OSGeo Africa] Surveyor General Diagram - Mapping
> property boundary points
>
>
>
> Hi All
>
>
>
> Just as a quick recap, the SG diagrams has a system of westings (y)
> and southings (x), where:
>
> westings (y) is the distance from the closest odd Lo(Central Meridian)
> and southings (x) is the distance from the equator
>
>
>
> In SA the y range from 110 000m to approx 110 000 m and, x ranges
> from 2 2 00 000 in Limpopo to 3 800 000 (Cape Aghulus).
>
>
>
> When there are values with constants such as +9 760 000, I would
> automatically assume that the unit of measure is Cape Feet and all
> coordinates and constants needs to be converted to meters.
>
>
>
> The easiest way (for a non surveyor) to transform Cape Datum data
> (modified Clarke 1880) to Hartebeesthoek94 (WGS84) is by using the
> NTV2 transformation grid that QGIS can also use. If you correctly read
> data and define it as Cape Datim, you can reproject pretty accurately
> into Hartebeesthoek94 using this transformation method.
>
>
>
> Please see
> http://www.ee.co.za/article/datum-transformations-using-the-ntv2-grid.html
>
>
>
> I hope this helps
>
>
>
> >>> Zoltan Szecsei <zoltans at geograph.co.za
> <mailto:zoltans at geograph.co.za>> 9/29/2014 3:05 PM >>>
>
> AAaargh! WGS84
> (I'll go away, now)
> On 2014/09/29 15:03, Zoltan Szecsei wrote:
>
> Hi again,
> Sorry - forgot to add:
> If you don't do the re-projection, your survey diagram will sit
> approx 296m N/S and 50m E/W away from your WGS95orthophoto.
> If it sits only some 50m or so away from where it should sit, then
> it is likely that the Datum shift (between Cape & Harte94) wasn't
> applied.
>
> Wow - that's the best my memory serves me - I last did this stuff
> in 2003.
> Perhaps there is a surveyor on this list that can add to, or
> correct, what I have suggested?
>
> Wesley: If you're still stuck in a day or so, contact me off list
> & we can get it sorted better if I have the data set from you.
>
> Regards,
> Z
>
>
> On 2014/09/29 14:47, Zoltan Szecsei wrote:
>
> Hi Wesley,
> That survey will be on the Cape datum, and not the Harte94
> datum, so you will have to use the Cape datum to throw it back
> to LatLong, and then use the Harte94 Datum to push it forward
> to WG19
>
> The Lo19 and WG19 coordinates are much the same in that they
> both are "Metres from Longitude 19degs" and "metres from the
> Equator"
> Note that if you use proj type definitions, they are +ve South
> (ie South facing) - looking at the coords you are showing
> below, yours are North facing.
> This 'North facing fix' was done because, especially in the
> old days, CAD system could not handle South facing coords, so
> that was rectified by simply multiplying each column of
> numbers by "-1".
>
> I have a 'C' program somewhere that I wrote many years back,
> that handles this - but maybe someone on this list has done
> this with proj & QGIS.
> Although I haven't had to do this for many years, I'd be
> interested in a QGIS solution.
>
> HTH,
> Zoltan
>
>
> On 2014/09/29 14:37, Dr Wesley Roberts 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
>
>
>
>
> Wesley Roberts, PhD
>
> Geospatial Manager
>
> Biocarbon Partners
>
> wesley at biocarbonpartners.com
> <mailto:wesley at biocarbonpartners.com>
>
> Cell RSA: +27 83 5355 646
>
>
>
>
>
>
> _______________________________________________
>
> Africa mailing list
>
> Africa at lists.osgeo.org <mailto:Africa at lists.osgeo.org>
>
> You can UNSUBSCRIBE at http://lists.osgeo.org/mailman/listinfo/africa
>
>
>
>
> --
>
>
>
> ===========================================
>
> 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 <http://www.geograph.co.za>
>
> ===========================================
>
>
>
>
> _______________________________________________
>
> Africa mailing list
>
> Africa at lists.osgeo.org <mailto:Africa at lists.osgeo.org>
>
> You can UNSUBSCRIBE at http://lists.osgeo.org/mailman/listinfo/africa
>
>
>
>
> --
>
>
>
> ===========================================
>
> 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 <http://www.geograph.co.za>
>
> ===========================================
>
>
>
>
> _______________________________________________
>
> Africa mailing list
>
> Africa at lists.osgeo.org <mailto:Africa at lists.osgeo.org>
>
> You can UNSUBSCRIBE at http://lists.osgeo.org/mailman/listinfo/africa
>
>
>
>
> --
>
> ===========================================
> 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 <http://www.geograph.co.za>
> ===========================================
>
>
> _______________________________________________
> Africa mailing list
> Africa at lists.osgeo.org
> You can UNSUBSCRIBE at http://lists.osgeo.org/mailman/listinfo/africa
--
------------------------------------------------------------------------------------------
Gavin Fleming
Visit http://kartoza.com <http://kartoza.com/>to find out about open source:
* Desktop GIS programming services
* Geospatial web development
* GIS Training
* Consulting Services
Skype: phlemingo
-------------------------------------------------------------------------------------------
Kartoza = Linfiniti ∪ Afrispatial
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.osgeo.org/pipermail/africa/attachments/20140929/b53fc36a/attachment-0001.html>
-------------- next part --------------
A non-text attachment was scrubbed...
Name: KartozaWebBanner.png
Type: image/png
Size: 22131 bytes
Desc: not available
URL: <http://lists.osgeo.org/pipermail/africa/attachments/20140929/b53fc36a/attachment-0001.png>
More information about the Africa
mailing list