[Proj] +towgs84 approximation error

Noel Zinn (cc) ndzinn at comcast.net
Thu Mar 23 18:57:23 EST 2017

Hi Martin,

I'm sure you're right that the majority are 3-parameter, translation-only 
transformations, but EPGS are pretty good about publishing anything in 
common use, even bad 7-parameter transformations.  My favorite is the one 
derived by DMA/NIMA/NGA for Cyprus.  Be wary of those.


Noel Zinn, Principal, Hydrometronics LLC
+1-832-539-1472 (office), +1-281-221-0051 (cell)
noel.zinn at hydrometronics.com (email)
http://www.hydrometronics.com (website)

-----Original Message----- 
From: Martin Desruisseaux
Sent: Thursday, March 23, 2017 6:15 PM
To: proj at lists.maptools.org
Subject: Re: [Proj] +towgs84 approximation error

Thanks Noel for the explanation. Maybe it explains why in the EPSG
database, the majority of datum shift operations are specified only by
translation terms.


Le 23/03/2017 à 22:27, Noel Zinn (cc) a écrit :

> This has been a very interesting thread for me and thanks for all the
> insightful geodetic contributions.  The computer science implementation
> details (git, et al) are beyond my experience.  But one important detail 
> has
> been missed so far.  That is that derivation of a 7-parameter 
> transformation
> for any European country is badly ill-defined.  Even for a country the 
> size
> of Germany, the adjustment correlations between the rotations and the
> translations are extremely high.  That's because, for a "small" area, a
> positional difference (error) at a survey monument can be explained by
> EITHER a combination of rotations and scale OR just a combination of
> translations.  Those effects are only separated out over a very large area
> (continental or larger).  Unavoidable surface errors result in 
> statistically
> meaningless and egregious translations compared with, say, a simple
> 3-parameter transformation.  So, while we're gilding 7-parameter
> transformations here with rotation matrices fully populated with sines and
> cosines, we ought to consider how those transformations were derived. 
> There
> may be acceptance behind them, but little science from a survey-adjustment
> perspective.  Better to keep it simple in Europe.
> Noel

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