[Proj] "8-Parameter Transformation"

support at mnspoint.com support at mnspoint.com
Wed Mar 29 00:48:07 EST 2017


it looks like one solution could be to add 8th parameter to the 
"towgs84" statement.

The additional 8th parameter would define the exact method how the 
transformation should be done (and the default would be the same as is 
used nowadays). That would make it then possible in the future to define 
the "towgs84" more accurately .. and have also several versions of it 
available at the same time.

Something like..

Now we have for example:


But we could also have:


where "How" is some predefined identifier or number or mechanism like:




or (maybe even feed some parameters to the method via the 8th parameter)


etc.. etc...

in the Proj.4 library you (and maybe a fresh version also) would then 
try to figure out if there is this 8th parameter given and if so switch 
to some other transformation method according to the program and the 

This would of course require some additional programming. If the 8th 
parameter is omitted then everything would default to the same as it is 
today. (And hopefully if using old version of the Proj.4 library .. the 
8th parameter would just be ignored). I have not tested how the library 
reacts to 8th parameter as it is now when it does not expect it?!

Regards: Janne.


Jochem kirjoitti 2017-03-22 23:41:
> Hello,
> I joined the mailing list because I noticed that the PROJ.4 library 
> uses
> approximate formulas for the 7 parameter coordinate transformation 
> defined
> by +towgs84 (technical details: The elements of the rotation matrix are 
> the
> rotation angles instead of the product of sines and cosines of the 
> angles).
> For really small rotation angles this gives the same result, but not 
> for
> larger rotations.
> For my current application I want to transform a map in QGIS with 1 mm
> accuracy. However, the approximation causes an error of 4 cm. Since 
> PROJ.4
> normally gives results within 0.1 mm accuracy, I did not expect such
> approximation.
> What can I do to help to fix this?
> Regards, Jochem

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