[Proj] Gauss conformal Lo33 on Clarke 1880 Cape Datum to WGS84 UTM36S

[Mikael Rittri]

Hello Zoltan. 
Yes, as you say, the traditional transverse mercator projections for Southern Africa have
axes positive west and south. This can be handled in Proj 4.8.0 (see http://trac.osgeo.org/proj/wiki/TMSO ).
However, since both your example coordinates are negative, I think you have data in a less traditional
but more GIS-friendly variant, where the axes go east and north and the traditional coordinate values
have been negated. 

Another problem is caused by the Cape datum having distortions that are not handled well
by a simple Helmert datum shift ( +towgs84 in Proj.4). Such a datum shift will have errors up to 15 meters, 
perhaps more. You can get much better accuracy from a grid shift file. So I think your source CRS 
is best constructed as

+proj=tmerc +lon_0=33 +a=6378249.145 +b=6356514.966398753 +nadgrids=SA.gsb

where SA.gsb is an NTv2 grid shift file from Cape to Hartebeesthoek94 (or to WGS84),
covering South Africa. To find out where you can download it, see

http://eepublishers.co.za/article/datum-transformations-using-the-ntv2-grid.html

About your target CRS, I don't think there is an EPSG code for the combination of Hartebeesthoek94 and
a UTM projection. But since H94 is the same as WGS84 except for tectonic motion, I think you
can use WGS84 / UTM zone 36S, which can be defined as

   +init=epsg:32736

Hope this helps,

Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

26 jul 2012 kl. 17:14 skrev "Zoltan Szecsei" <zoltans at geograph.co.za>:

> Hi Everyone,
> I have some orthos and some 3D vector data that I need to reproject from 
> Clarke 1880 Cape Datum, Lo33 to WGS84 Hartebeeshoek94 datum UTM 36S.
> 
> I'm hoping to use gdalwarp (with VRT) and ogr2ogr, but I cannot find the 
> input projection codes.
> 
> Please can someone provide some guidance on this?
> Maybe I have to define them? How?
> 
> The jpw files for the orthos are typically:
> 0.0500000000000000030
> 0.0
> 0.0
> -0.0500000000000000030
> -50999.974999999999
> -2872000.0249999999
> and you'll probably notice that it is not the usual Gauss Conformal 
> projection, as that would be +ve South and West for this area.
> 
> Once the orthos are done I will be needing to transform (as above) a 3D 
> DGN file and an ASCII DEM file as well. Typical DEM values are:
> -50400.000,-2872481.000,-2.74
> -50399.000,-2872481.000,-2.84
> 
> I'd be grateful for some help specifying the input and output projection 
> details (epsg codes??)
> 
> Thanks & regards,
> Zoltan
> 
> 
> 
> -- 
> 
> ===========================================
> Zoltan Szecsei PrGISc [PGP0031]
> Geograph (Pty) Ltd.
> P.O. Box 7, Muizenberg 7950, South Africa.
> 
> 65 Main Road, Muizenberg 7945
> Western Cape, South Africa.
> 
> 34° 6'16.35"S 18°28'5.62"E
> 
> Tel: +27-21-7884897  Mobile: +27-83-6004028
> Fax: +27-86-6115323     www.geograph.co.za
> ===========================================
> 
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