[Proj] [proj4] epsg:31370 possible wrong parameters

[Mikael Rittri]

WBL> Has it been fixed in the latest version of proj4 (the epsg file)? 

Not quite fixed. In the trunk, http://svn.osgeo.org/metacrs/proj/trunk/proj/nad/epsg , 
I found the entry

# Belge 1972
<4313> +proj=longlat +ellps=intl +towgs84=-106.869,52.2978,-103.724,0.3366,-0.457,1.8422,1.2747 +no_defs  <> 

where the last number has the wrong sign (should be -1.2747).  

There is a geotiff ticket on this error, http://trac.osgeo.org/geotiff/ticket/32 .
(I believe the datum shifts in the Proj.4 epsg file are generated 
 from the libgeotiff csv files.)

Regards,
Mikael Rittri
Carmenta
Sweden
http://www.carmenta.com

________________________________

From: Willy-Bas Loos [mailto:willybas at gmail.com] 
Sent: den 21 februari 2011 16:50
To: Mikael Rittri
Cc: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] [proj4] epsg:31370 possible wrong parameters


That explains, thanks.
Has it been fixed in the latest version of proj4 (the epsg file)? 

WBL


On Mon, Feb 21, 2011 at 11:43 AM, Mikael Rittri <Mikael.Rittri at carmenta.com> wrote:


	Hello,
	I think your parameters are nearly right, but not quite:
	the last number you have is -1, but it should be -1.2747.
	(Maybe this doesn't make a great difference in the results.)
	
	Apart from the this, your +towgs84 parameters agree with
	the datum shift "BD72 to WGS 84 (3)", EPSG:15929 (except
	that yours have more decimals).
	
	Background: I think the Belgian authorities published this
	datum shift in the opposite direction (from WGS 84 to BD72),
	which somehow made it into the proj/nad/epsg file.  That's
	why you had to reverse so many signs.
	
	Also, the Belgians published the last number, not as a
	scale difference in ppm, but as the scale factor 1.0000012747.
	It appears that this factor has been rounded to 1, and interpreted
	as a scale difference in ppm by proj/nad/epsg (and by postgis too),
	but it really corresponds to the scale difference 1.2747 ppm,
	or -1.2747 ppm if you go from BD72 to WGS 84.
	
	A further problem is that EPSG has published an erroneous
	version of this datum shift, EPSG:15749, where the scale
	difference had the wrong sign. That version was deprecated
	by EPSG in March 2007 and replaced by EPSG:15929.
	
	Regards,
	Mikael Rittri
	Carmenta
	Sweden
	http://www.carmenta.com
	
	________________________________
	
	From: proj-bounces at lists.maptools.org [mailto:proj-bounces at lists.maptools.org] On Behalf Of Willy-Bas Loos
	Sent: den 17 februari 2011 19:47
	To: proj at lists.maptools.org
	Subject: [Proj] [proj4] epsg:31370 possible wrong parameters
	


	Hi, we had issues displaying data from epsg:31370
	We changed some "-" signs, according to a definition that i found in postgis, and that solved the issue.
	We use Rel. 4.7.1, 23 September 2009.
	
	The parameters that  showed us the expected result were:
	<31370> +proj=lcc +lat_1=51.16666723333333 +lat_2=49.80333339 +lat_0=90 +lon_0=4.367486666666666 +x_0=150000.013 +y_0=5400088.438 +ellps=intl +units=m +towgs84=-106.868628,52.297783,-103.723893,0.336570,-0.456955,1.842183,-1 +no_defs <>
	
	The minus signs and the versions in the several packages in which proj4 is included is getting me a bit dazzled, so i might be mistaken.
	
	Hope this is constructive.
	Cheers,
	
	WBL
	
	--
	"Patriotism is the conviction that your country is superior to all others because you were born in it." -- George Bernard Shaw
	
	




-- 
"Patriotism is the conviction that your country is superior to all others because you were born in it." -- George Bernard Shaw