[Proj] Transformation pipelines - your opinion?

[NDavid]

Sorry for the (very) late reply, 

I've read the discussion about this pipelines feature at github and your
proj4 page and
I've definitely see such coordinate transformation pipeline as very useful.
BUT I'm more incline to keep only cartographic projection inside proj4 and
to implement 
pipeline and other coordinate transformation outside proj4 in another
library/librairies. 
And perhaps with a more C++ style than ANSI C.  

Some potential coordinate transform that, I think, could fit into a pipeline
are
- cartographic projection (of course)
- datum conversion with planar gridshift or use of geoid.
- unit transform
- cartesian <-> polar <-> cylindric coordinate
- temporal transform (gps time to utc time ? )
- trajectory georeferencing (for point cloud, mobile mapping camera) 
- conversion between sensor coordinate system and platform/IMU
- ..
Some of this coordinate transformation need additional data (trajectory
files, grid files, 
datum/ellispoid dictionnary) and if implement inside proj4 that could lead
to insert 
into proj4 some dependencies or making it a bigger library. 
And so people who are only interrested in cartographic transformation
functions of 
proj4 will have to pay for something they don't want.
I know that pj_transform/cs2cs/pj_apply_gridshift are not only about
cartographic 
projection but also about datum transform. I understand to keep such
function
inside proj4 for historical and compatibility reasons but I don't think this
is
their "right" place. 

 just my 2 cents about this pipeline proposal.
Nicolas



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