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

Zoltan Szecsei zoltans at geograph.co.za
Mon Sep 29 06:05:54 PDT 2014


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 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
>>> You can UNSUBSCRIBE athttp://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:    +27-86-6115323www.geograph.co.za
>> ===========================================
>>
>>
>> _______________________________________________
>> Africa mailing list
>> Africa at lists.osgeo.org
>> You can UNSUBSCRIBE athttp://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:    +27-86-6115323www.geograph.co.za
> ===========================================
>
>
> _______________________________________________
> Africa mailing list
> 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
===========================================

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