[Proj] Create custum projection definition PROJ4 for rwanda

[Frank Warmerdam]

Lavver wrote:
> Hi,
> 
> I'm struggling to create a custom projection definition for QGis for the
> national system used for Rwanda. I am new to the PROJ.4 syntax. 
> 
> The ArcView prj file (which works fine) shows:
> PROJCS["Rwanda1",GEOGCS["GCS_Clarke_1880",DATUM["D_Clarke_1880",
> SPHEROID["Clarke_1880",6378249.138,293.466307656]],
> PRIMEM["Greenwich",0.0],UNIT["Degree",0.0174532925199433]],
> PROJECTION["Gauss_Kruger"],
> PARAMETER["False_Easting",500000.0],PARAMETER["False_Northing",10000000.0],
> PARAMETER["Central_Meridian",30.0],PARAMETER["Scale_Factor",0.9999],
> PARAMETER["Latitude_Of_Origin",0.0],UNIT["Meter",1.0]]
> 
> I translated it to 
> 
> +proj=tmerc +ellps=clrk80 +a=6378249.138 +rf=293.466307656 +lon_0=30
> +k=0.999900 +units=m +x_0=500000 +y_0=10000000
> 
> But I'm still 200m to 300m away from reality
> 
> The following WGS84 N/E coordinates should give the projected Coordinates
> -1.57741 N /29.93690 E   =>    9825897/492907
> -1.57763 N /29.93723 E   =>    9825873/492943
> -1.61141 N /29.92847 E   =>    9822138/491968
> 
> Could someone help me please ? Maybe  I am omitting a parameter...

Lavuun,

It sounds to me like you are running into datum shifting issues.
I think you need to determine acceptable "towgs84" parameters for
your datum (D_Clarke_1880 isn't really a datum description by
the way, it just says it is a generic datum based on the Clarke 1880
ellipsoid).

This material on towgs84 may be somewhat helpful:

http://trac.osgeo.org/proj/wiki/GenParms#towgs84-DatumtransformationtoWGS84

Best regards,
-- 
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I set the clouds in motion - turn up   | Frank Warmerdam, warmerdam at pobox.com
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