[Proj] +towgs84 approximation error

[Jochem]

Hi Noel,

You are right about the corrections between rotations and translations. This
will cause the parameters to look very different if you estimate them on new
data. However, besides that, I don't see what the problem is. You still get
precise transformated coordinates (and you don't always get that with
approximate formulas). Or do you see other problems? 

You know the solution to this correlation problem by using a point in the
centre of the country as rotation pivot point and thus using a 10 parameter
transformation? 
Section 2.4.3.2 of  Guidance Note 7, part 2
<http://www.iogp.org/pubs/373-07-2.pdf>   (these are approximated formulas
but of course a strict equivalent is possible).

Regards, Jochem



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