[Proj] +towgs84 approximation error

[Martin Desruisseaux]

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