[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.
> 
> Addition: proj with fail with an error if you try to project a point that
> is a reference point for the projection in question (resulting in division
> by zero usually). I believe that in polar projections 0,90 and 0,-90 are 
> such points. I also believe [rvs].proj in such cases will abort. Maybe 
> this should be changed to just ignoring the point/cell that could not be 
> projected instead of aborting?

When computing the region of interest, cells which fail to project
should just be ignored. When performing the actual projection, cells
which fail to project should be NULL.

Also, I've remembered another issue with r.proj:

When projecting lat/lon to pseudo-cylindrical projections where the
entire globe fits within the region, the resulting map tends to
"wrap"; i.e. the area outside of the "ellipse" contains non-null
cells.

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