[Proj] +towgs84 approximation error

[Jochem]

Cliff,

The transformation has been estimated by performing static GPS observations
on about 12 points of the old triangulation frame. Ofcourse these parameters
are not at 1 mm precise but more like a few cm. However, these parameters
have been in use for some time now. Therefore these parameters are
conventional* parameters and I need to keep to that convention within 1 mm.

I do not know the number of points to be transformed, the entire digital map
of a small country (probably a few million).  

*)  Van der Marel, 2016
<http://www.citg.tudelft.nl/fileadmin/Faculteit/CiTG/Over_de_faculteit/Afdelingen/Afdeling_Geoscience_and_Remote_Sensing/Study/CTB3310_TerrestrialReferenceSystems_TRS_2-1a.pdf>  
page 33-34

Regards, Jochem



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