[Proj] Setting up a proj4 specification

[Mikael Rittri]

Hello, Hanlie.

Jean-Claude is right that one needs to know the kind of projection, 
but if we assume it is a Transverse Mercator projection, the projection
would be UTM zone 36N, so it is very likely that it is 

   "Arc 1960 / UTM zone 36N", EPSG:21096,

with area of use "Kenya and Uganda - north of equator and 30°E to 36°E".

As for the datum shift from Arc 1960 to WGS84, the EPSG gives four 
possibilities, valid in Kenya and Tanzania, just Kenya, just Tanzania, 
and Burundi, respectively. (Just go to http://www.epsg-registry.org/ ,
and query by filter with Name: "Arc 1960" and Type: Coordinate Transformation.)

Since Tanzania and Burundi is south of the equator, and since the 
datum shift for Kenya and Tanzania is less accurate than the one 
just for Kenya, you can probably use

   +towgs84=-157,-2,-299

which is the datum shift "Arc 1960 to WGS 84 (2)", EPSG:1284,
Area of use: Kenya - onshore and offshore, accuracy 6 meters (one sigma).
This can probably be used in Uganda, too, but with worse accuracy. 

I am not sure about the ellipsoid. EPSG gives seven(!) variants of
the Clarke 1880 ellipsoid, while the only predefined Clarke 1880 
ellipsoid in Proj.4 is 

   clrk80 a=6378249.145    rf=293.4663      Clarke 1880 mod.

which agrees with your specification. 

(According to EPSG, the ellipsoid for the Arc 1960 datum 
 is "Clarke 1880 (RGS)" with
   
   semi-major axis = 6378249.145 m
   inverse flattening = 293.465 

 The name agrees with your specification, but the inverse flattening
 differs slightly. This might be an error in EPSG, and the difference
 could be insignificant in practice.)

So, let's assume that clrk80 is correct. Then, you can try with 

    +proj=utm +zone=36 +ellps=clrk80 +units=m +towgs84=-157,-2,-299 +no_defs

Better datum shifts may be available from national authorities. 

Best regards,

Mikael Rittri
Carmenta
Sweden 
http://www.carmenta.com
 
-----Original Message-----
From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Jean-Claude Repetto
Sent: Wednesday, October 19, 2011 9:43 AM
To: PROJ.4 and general Projections Discussions
Cc: Hanlie Pretorius
Subject: Re: [Proj] Setting up a proj4 specification

Le 19/10/2011 09:03, Hanlie Pretorius a écrit :
> Hi,
>
> Can someone please tell me or point me to documentation that explains 
> how to put the following information into a proj4 string for use in 
> QGIS?
>
> Datum:
> D_Arc_1960
> Spheroid:
> Clarke_1880_RGS
> Semimajor Axis:
> 6378249.144999999600000000
> Semiminor Axis:
> 6356514.869549775500000000
> Inverse Flattening:
> 293.46630000
> False_Easting
> 500000.000000000000000000
> False_Northing
> 0.000000000000000000
> Central_Meridian
> 33.000000000000000000
> Scale_Factor
> 0.999600000000000040
> Latitude_Of_Origin
> 0.000000000000000000
>
> Thanks
> Hanlie

Hi,

This information is not sufficient. It does not specify the projection. 
It may be UTM, or Lambert Conformal Conic, or other ones.
The parameters of the datum transformation to WGS84 are also missing.

The documentation on the proj4 parameters can be found at http://trac.osgeo.org/proj/wiki/GenParms .

Jean-Claude
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