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

Fri Jan 22 06:40:33 PST 2010

Jan, you wrote 

> All my observations about the towgs parameters have been tested with real Belgian coordinates overlaid over Google.

Fine.  But if you did make an error when transcribing the newer transforms into Proj.4 syntax, and 
the error was just in the rotation angles, then the effect may be too small to notice.  

I mean, the angles describe a rotation, so there is presumably a fixed point in Belgium where 
the rotation has no effect.  If your test points happened to be near the fixed point, the 
effect of the reversed angle signs could be very small.  

> As I said before, the EPSG database is not very systematical in the way it represents its formulas, 
> it just takes them from whatever source is available, untested AFAIK. Never use them without testing them with real world coordinates.

I agree completely.  But I am not yet convinced that you are right and EPSG wrong for the newer transforms,
although that is certainly possible.  Do you have some link to the offician Belgian documents? 

Best regards,

--
Mikael Rittri
Carmenta AB
SWEDEN
www.carmenta.com 

________________________________

From: Jan Hartmann [mailto:j.l.h.hartmann at uva.nl] 
Sent: Friday, January 22, 2010 3:27 PM
To: Mikael Rittri
Cc: PROJ.4 and general Projections Discussions; Thibaut Gheysen
Subject: Re: [Proj] Belge 1972 / Belgian Lambert 72 (31370) - towgs84parameters

Mikael,

All my observations about the towgs parameters have been tested with real Belgian coordinates overlaid over Google. As I said before, the EPSG database is not very systematical in the way it represents its formulas, it just takes them from whatever source is available, untested AFAIK. Never use them without testing them with real world coordinates.

Jan

On 22-1-2010 13:31, Mikael Rittri wrote: 

	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.)  

	Regards,

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

	Jan

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