[Proj] +towgs84 approximation error

[Noel Zinn (cc)]

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

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.

    Martin


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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