[PROJ] Performance of proj_create_crs_to_crs()

[Greg Troxel]

Even Rouault <even.rouault at spatialys.com> writes:

> The reason is that the NAD27 <-> NAD83 conversion has several dozains of 
> potential paths (one per US state), and PROJ explore them.

(understood about not using WGS84 as an intermediate being the real
issue)

I am curious about the "one per state" comment.  I realize NAD27 is
complicated, but I would think that if there are NAD27-NAD83 transforms
that are valid for a state (because of a local model, or maybe because
there is some NAD83 HARN that is specific for the state, but I'd think
that's a different CRS codepoint), that there would be some process to
only use them in their region of validity.  But the basic pipeline
request is just SRIDs and not region (which is as I expect it).

Can you elaborate?  Or is it just that there are multiple intermediate
datums, but really these all end being the same?