[GRASS5] problems with [rsv].proj

[Glynn Clements]

Morten Hulden wrote:

> > [Aside: there are cases where the "border walking" algorithm won't
> > produce the right results (e.g. converting polar stereographic to any
> > cylindrical projection), but I doubt that such cases are relevant to
> > "real world" use.]
> 
> Have you tested this in practice? With bordwalk r.proj should be able to 
> produce maps composed of discontinous areas. I tested it with some other 
> projections when I wrote the code, but not with polar stereographic. One 
> reason for writing bordwalk.c was that in the previous version of r.proj 
> you could only do reliably cylindrical->cylindrical projections.

If I disable the "special cases" code, the obvious UPS->lat/lon test
fails in the expected manner (i.e. I don't get anything north of the
circle inscribing the region). IOW, the actual "border walking" part
of bordwalk() doesn't suffice in this case.

As the "special cases" code only tests four specific points, it won't
always help; although it will help when projecting UPS to the entire
lat/lon range, as one of those four points happens to be the north
pole (minus half a cell).

However, as I said previously, I doubt that these cases are of
real-world importance; such transformations inherently have extreme
distortion, so the projected raster isn't of much use even when it is
complete.

-- 
Glynn Clements <glynn.clements at virgin.net>