[Qgis-developer] Projection parameters for french NTF

[Tim Sutton]

Hi

Magnus Homann just raised a similar issue for Swedish projections.  
When we created the srs.db the idea was we should not change  
projections with official EPSG/PGIS ID's. The idea being that we  
would update the database before a release with any upstream  
corrections  (coming from spatial_ref_sys table in postgis unless  
there is a more official source). So this means three things:

1) we need to decide what will be the cananical source for SRS  
descriptions (Im leaning towoards postgis / spatial ref sys table
2) we find a way to get suggested changes into the upstream srs  
source if they originate from our users
3) we need to come up with a manageable way to synchronise the srs.db  
that is distributed with qgis with the canonical srs source.

If anyone is interested to take on some or all of these aspects  it  
would be great. It doesnt require any coding knowledge as such and  
would be a very useful contribution to QGIS. In the mean time the  
best way to  introduce changes to the distributed srs.db is in the  
form of a simple sql update command patch that will update the  
records in the sqlite database correctly (but running the risk of  
later undoing if these changes have not made it to the upstream  
source of srs defs too.

Regards

Tim


On 27/09/2006, at 11:22, Laurent C. wrote:

> Hello,
>
> It seem that the projections parameters for the french lcc  
> projection are wrong.
> Today, all the epsg's 275xx have the following parameter :
> +lon_0=-2.33722917
>
> With the current parameters, when I import some gps points(lat-lon)
> and reproj them to french's Lambert zone II, the points are shifted by
> hundreds of kilometers to the east.
>
> Maybe the param should be :
> +lon_0=2.33722917
> or maybe :
> +lon_0=0, because +pm=paris
> Well, I don't know exactly.
>
> Here are my grass PROJ_INFO file (which is working) :
> name: Lambert Conformal Conic
> proj: lcc
> ellps: clark80IGN
> a: 6378249.2000000002
> es: 0.0068034876
> f: 293.4660212940
> lat_0: 46.8000000000
> lat_1: 45.8989188889
> lat_2: 47.6960144444
> lon_0: 2.3372291667
> x_0: 600000.0000000000
> y_0: 2200000.0000000000
>
> Maybe is it a proj.4 problem. Here is the epsg file from proj 4.4.9 :
> <27572> +proj=lcc +lat_1=46.8 +lat_0=46.8 +lon_0=-2.33722917
> +k_0=0.99987742 +x_0=600000 +y_0=2200000 +a=6378249.2 +b=6356515
> +towgs84=-168,-60,320,0,0,0,0 +pm=paris +units=m +no_defs  <>
>
> The epsg file from proj 4.5.0b2 :
> <27572> +proj=lcc +lat_1=46.8 +lat_0=46.8 +lon_0=0 +k_0=0.99987742
> +x_0=600000 +y_0=2200000 +a=6378249.2 +b=6356515
> +towgs84=-168,-60,320,0,0,0,0 +pm=paris +units=m +no_defs  <>
>
> The proj esri file :
> <27572> +proj=lcc +lat_1=46.8 +lat_0=46.8 +lon_0=2.33722917
> +k_0=0.99987742 +x_0=600000 +y_0=2200000 +a=6378249.2 +b=6356515
> +towgs84=-168,-60,320,0,0,0,0 +pm=paris +units=m +no_defs  no_defs <>
>
> I'm not a gis guru, and I don't know exactly what to think. It could
> be great if some French users could give a clue.
>
> Thanks,
>
> Laurent
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> Qgis-developer at lists.qgis.org
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-- 
Tim Sutton

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