Projection WGS84 in decimal degree

[zze-SIGALE PORTANERI F ext RD-BIZZ-SOP]

Hi Frank & Ed,

Thanks for your prompt and efficient answers!!!

You're right, the pbm seems to come from the prime meridiam mismatch (and not decimal degree conversion issue as I thought before).
I was able to confim this by manually doing some conversion tests using the Convers useful little tool (http://vtopo.free.fr/convers.htm ). 

As per example, for a WGS84 GPS point in Paris: longitude=2.38388   latitude=48.85562 

In Lambert II Etendu, I have today (wrong value) : X=775063.135566471  Y=2431242.54750943 
which is given using "Paris" as origin in Convers and which corresponds to the postgis transform call : 

  psql> select public.transform(GeometryFromText('POINT (2.38388 48.85562)',  4326), 27582);
   SRID=27582;POINT(775063.135566471 2431242.54750943) 

So I have to find other EPSG values to get the correct X/Y that should be: 
X=603478.33780249   Y=2428587.76696934.  

I didn't find it yet, but I keep investigating in the spatial_ref_sys postgis (0.9.1) table!!!
Regards,
FP

-----Message d'origine-----
De : UMN MapServer Users List [mailto:MAPSERVER-USERS at LISTS.UMN.EDU] De la part de Ed McNierney
Envoyé : mercredi 11 janvier 2006 16:47
À : MAPSERVER-USERS at LISTS.UMN.EDU
Objet : Re: [UMN_MAPSERVER-USERS] Projection WGS84 in decimal degree

Frank & FP -

Good catch, Frank - that's an excellent possibility.

FP, make sure your EPSG file contains an accurate definition of <27582>, including the prime meridian (primem) of 2.337229166666667.

	- Ed

Ed McNierney
President and Chief Mapmaker
TopoZone.com / Maps a la carte, Inc.
73 Princeton Street, Suite 305
North Chelmsford, MA  01863
ed at topozone.com
(978) 251-4242 

-----Original Message-----
From: fwarmerdam at gmail.com [mailto:fwarmerdam at gmail.com] On Behalf Of Frank Warmerdam
Sent: Wednesday, January 11, 2006 10:16 AM
To: Ed McNierney
Cc: MAPSERVER-USERS at lists.umn.edu
Subject: Re: [UMN_MAPSERVER-USERS] Projection WGS84 in decimal degree

On 1/11/06, Ed McNierney <ed at topozone.com> wrote:
>
> FP -
>
> Your first GPS layer definition should be fine.  You don't say what 
> data source you are using for your GPS data, but if it is a shapefile 
> or inline features you will be using decimal degrees and those will 
> work fine with the "init=epsg:4326" definition.
>
> The first thing I would suggest is that you confirm that the
> EPSG:27582 projection is in fact correct. The related Paris / Lambert 
> projections (27581-27584, for example) are all rather similar and can 
> certainly look the same.  Can you check the coordinates of a few 
> points to make sure they're accurate?  An error of 200km is far too 
> large to be a datum error, but could easily be due to the different 
> false easting/northing values used in these similar projections.

FP,

I would add to Ed's comments that the PROJ definitions files for EPSG zones with alternate prime meridians are often messed up, and a horizontal offset of 250km is approximately the distance of the paris meridian from greenwich.

Best regards,
--
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