[Proj] "8-Parameter Transformation"
support at mnspoint.com
support at mnspoint.com
Wed Mar 29 00:48:07 EST 2017
Hello,
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:
towgs84=577.326,90.129,463.919,5.137,1.474,5.297,2.4232
But we could also have:
towgs84=577.326,90.129,463.919,5.137,1.474,5.297,2.4232,How
where "How" is some predefined identifier or number or mechanism like:
towgs84=577.326,90.129,463.919,5.137,1.474,5.297,2.4232,J2017
or
towgs84=577.326,90.129,463.919,5.137,1.474,5.297,2.4232,23
or (maybe even feed some parameters to the method via the 8th parameter)
towgs84=577.326,90.129,463.919,5.137,1.474,5.297,2.4232,XYZ(2017)
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
parameter.
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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