[Proj] Create custum projection definition PROJ4 for rwanda

[Lavver]

Hi Frank, 

Thanks a lot for the tip. I added the +towgs84=-157,-7,-300 for a 3
parameter spatial translation. 

I found the approximate parameters for coordinate transformation  Arc 1960
to WGS 84 in the EPSG database and adapted them slightly. Now I have still
an error of 1m (which is acceptable for our usage)

Kind regards

Lavuun



Frank Warmerdam wrote:
> 
> 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,
> -- 
> ---------------------------------------+--------------------------------------
> I set the clouds in motion - turn up   | Frank Warmerdam,
> warmerdam at pobox.com
> light and sound - activate the windows | http://pobox.com/~warmerdam
> and watch the world go round - Rush    | Geospatial Programmer for Rent
> 
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> 

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