[Proj] Understanding 2D Helmert

[Kristian Evers]

Even, 

I wasn't suggesting that this was something you should take on. Your
comment just peaked my interest as I have been pondering this problem
before.

A gridded solution would definitely be the easiest to implement, but not
necessarily the most satisfactory. I for one would always prefer a solution
as close to the original transformation definition as possible. 

It would probably also be possible to invent a simple text based file format
for storing the triangles and their related parameters. This would be sort
of equivalent to a grid file that can be linked to in the database. Is the
current setup geared for something like that, or is it strictly reserved for
grid files?

/Kristian

-----Oprindelig meddelelse-----
Fra: Even Rouault <even.rouault at spatialys.com> 
Sendt: 28. oktober 2018 17:46
Til: proj at lists.maptools.org
Cc: Kristian Evers <kreve at sdfe.dk>; Andre Joost <andre+joost at nurfuerspam.de>
Emne: Re: [Proj] Understanding 2D Helmert

On dimanche 28 octobre 2018 15:02:05 CET Kristian Evers wrote:
> Why is that? To me it seems somewhat similar to polynomial mappings
> (Horner), in the way that many coefficients are likely to be stored for a
> given transformation. Given a clever way of defining the triangles and
> their corresponding Helmet parameters I think it should be possible to make
> these transformations work. At least as PROJ strings.

You would need to store hundreds of triangles and their parameters, and have 
PROJ figure out in which triangle a point it. Certainly doable, but not 
directly in the scope of my current work.  And avoid the database to grow too 
big with very particular transforms. That said, 600 triangles with 6 double 
parameters each + the x,y coordinates is just 38.4 KB if you store that as a 
compact binary blob. Some balance to know if it is something acceptable in-db 
(EPSG style transformations have at most 18 parameters each) or out-db 
(grids). The suggestion for the grid approach was to make it more immediatly 
usable.

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