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

[Mikael Rittri]

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