[Proj] Gauss conformal Lo33 on Clarke 1880 Cape Datum to WGS84 UTM36S - NTv2 file needed.

[Zoltan Szecsei]

Hi Again,

I am struggling to locate a copy of the NTv2 file for Clarke 1880 Cape 
Datum to WGS84

I have not had any luck with either Google, or the contacts mentioned in 
the article pointed to (below) by Mikael, so please could someone give 
me a link to download from, or perhaps send it to me via 
transferbigfiles.com or similar method.

I have no idea how big, or what extents are in these NTv2 grid files, as 
I have never used them before.

To this end, my area of interest is in Maputo (Mozambique), and 
Mozambique apparently uses Clarke 1866 (not 1880) - but the surveyor 
"swears blind" that the coords are on "1880" and not "1866".

This begs the question:
Do the extents of this Clarke 1880 NTv2 grid stop at the South African 
border (some 50Km west of my area of interest), or do they cover Maputo 
sufficiently?


Thanks & kind regards,
Zoltan




On 2012/07/26 21:42, Mikael Rittri wrote:
> 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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-- 

===========================================
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
===========================================