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

Tue Aug 14 13:08:44 PDT 2012

Zoltan,

All of the Moçambican borders were surveyed by British Royal Engineers in the early 1900's and therefore ONLY the borders are on the Cape Datum, Clarke 1880 ellipsoid.  In regards to the interior of the of the country, all native classical triangulation was indeed referenced to the Clarke 1866 ellipsoid for the Tete Datum of 1960, the Madzansua Datum of 1904, and the Observatorio Campo Rodriques Datum of 1907.

The "Arc of the 30th Meridian Survey" did not touch the country.

In 1995, Norway Mapping did a comprehensive readjustment of the country called MOZNET/ITRF94.

As of September 1999 when I wrote my Grids & Datums column on the country, no geoid had been computed for the country.  Your best bet is likely EGM96 or a variant thereof for a guess of the geoid there.

Clifford J. Mugnier, c.p., c.m.s.

Chief of Geodesy

LSU Center for GeoInformatics (ERAD 266)

Dept. of Civil Engineering (P.F. Taylor 3531)

LOUISIANA STATE UNIVERSITY

Baton Rouge, LA  70803

Academic: (225) 578-8536

Research: (225) 578-4578

Cell:             (225) 328-8975

honorary lifetime member, lsps

fellow emeritus, asprs

member, apsg

________________________________________
From: proj-bounces at lists.maptools.org [proj-bounces at lists.maptools.org] on behalf of Zoltan Szecsei [zoltans at geograph.co.za]
Sent: Tuesday, August 14, 2012 11:06 AM
To: Mikael Rittri
Cc: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Gauss conformal Lo33 on Clarke 1880 Cape Datum to WGS84 UTM36S - NTv2 file needed.

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
>> ===========================================
>>
>> _______________________________________________
>> Proj mailing list
>> Proj at lists.maptools.org
>> http://lists.maptools.org/mailman/listinfo/proj

--

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