[Proj] +towgs84 approximation error

[Jochem]

Dear all,

I'll give a short update.

For my home country, the approximation error is just 0.2 mm. For the country
I visited. I do not have the time to learn how to contribute to Proj.4 with
Git (I am interested to learn this, but I have to give other things
priority), nor can I wait for a new version of Proj.4 to be implemented in
QGIS. Luckily, I found a workaround for the country I visited:

I transformed a point in the centre of the country with the strict
parameters and the approximated formulas. By this I found approximation
errors in XYZ of that centre point. Next I added these approximation errors
to the strict translation parameters. By that I get perfect results for the
centre point with the tweaked parameters and the approximate formulas of
Proj.4. For the extreme points of the country, the error is <0.5 mm with
this tweaked transformation. This probably solves my problem for the moment.
However, this works only in one direction. So, I would have to use different
tweaked +towgs84 parameters for the transformation from the old to the new
coordinate frame than for the transformation from new to old, which is a
recipe for future mistakes. 

Regards, Jochem



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