[Proj] Belge 1972 / Belgian Lambert 72 (31370) - towgs84parameters

Mikael Rittri Mikael.Rittri at carmenta.com
Fri Jan 22 07:31:58 EST 2010

Jan Hartmann wrote:
> I have been quoting from PROJ 4.7. The older towgs parameter is not exactly erroneous, it's just a bit less exact.
But Thibaut's image showed an displacement of about 190 meters.  I call that erroneous.
Well, if by "older", you mean the EPSG datum shifts 
        1652 "BD72 to ETRS89 (1)" and 1609 "BD72 to WGS 84 (1)"
you are right that they are just a bit less exact than EPSG datum shifts 
        15928 "BD72 to ETRS89 (1)" and 15929 "BD72 to WGS 84 (3)"
But the towgs84 found in the current gcs.override.csv, as well as in PROJ's nad/epsg file 
(at least in 4.6.1), are just wrong.  I think the wrong sign for DX, DY and DZ causes most 
of the 190 meter error. 
By the way, in your first letter, you gave the older transforms as 
>   +towgs84=-99.059,53.322,-112.486,0.419,-0.830,1.885,-1
which I think is correct, but you gave the newer transforms as 
> +towgs84=-106.8686,52.2978,-103.7329,-0.3366,0.457,-1.8422,-1.2747 
which I think has the wrong sign for the rotation angles: it should be 
> +towgs84=-106.8686,52.2978,-103.7329,0.3366,-0.457,1.8422,-1.2747 
instead.  (I base this correction on the fact that EPSG claims that their datum shifts 15928 and 15929
use the Coordinate Frame Rotation, so the rotation signs have to be reversed for PROJ.4.  If the 
claim of EPSG contradicts the official documents - it happens now and then -  please notify EPSG.)  

Mikael Rittri
Carmenta AB
Box 11354
SE-404 28 Göteborg
Visitors: Sankt Eriksgatan 5
Tel: +46-31-775 57 37
Mob: +46-703-60 34 07
mikael.rittri at carmenta.com



From: Jan Hartmann [mailto:j.l.h.hartmann at uva.nl] 
Sent: Thursday, January 21, 2010 11:51 PM
To: Mikael Rittri
Cc: PROJ.4 and general Projections Discussions
Subject: Re: [Proj] Belge 1972 / Belgian Lambert 72 (31370) - towgs84parameters

On 21-Jan-10 12:52, Mikael Rittri wrote: 

	Jan Hartmann wrote: 
	> No, if QGIS uses PROJ, this is just an error. 
	Okay, you may be right that QGIS does not use the file gcs.override.csv.  
	But I see that the nad/epsg file of PROJ.4 contains the same erroneous 
	+towgs84 parameters for Belge 1972 as the gcs.override.csv.  
	(At least PROJ version 4.6.1). 

I have been quoting from PROJ 4.7. The older towgs parameter is not exactly erroneous, it's just a bit les exact

	> PROJ and EPSG use opposite rotational formulas, and PROJ uses degrees,  EPSG radians. 
	I don't agree in the general case.  PROJ uses the Position Vector 
	Transform, while EPSG is neutral on the rotation sign convention: 
	they use the same sign convention as the original source. 
	And PROJ uses arc seconds for rotations, while EPSG is neutral 
	on the angle unit: they use the same angle unit as the original source
	(usually arc seconds, but sometimes microradians or radians). 
	     For the EPSG transforms you quote, EPSG use arc-seconds
	for the rotations, but either the Position Vector Transform or the
	Coordinate Frame Rotation depending on whether they got the 
	transform from Eurogeographics or directly from Belgium. 

My information was for the Dutch and Belgian cases, as from the official documents. I don't know on what principles EPSG operates, I guess they just take it as they get it. It is not an easy-to-use database.


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