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

[Zoltan Szecsei]

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 WGS95 orthophoto.
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
>>
>> Email_Signature
>>
>> 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
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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:    +27-86-6115323www.geograph.co.za
> ===========================================
>
>
> _______________________________________________
> Africa mailing list
> Africa at lists.osgeo.org
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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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