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

[Zoltan Szecsei]

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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