[GRASS5] problems with [rsv].proj

[Morten Hulden]

On Wed, 29 May 2002, Morten Hulden wrote:

> So, if you get ymin > to_hd->north at the end of bordwalk, it means all of 
> the cell centers along the lower border of the source map had (projected)
> y-values greater than 'north' in the destination area. Thus 'input area is 
> outside current region'. 

Small correction: actually it means _none_ of the border cells, regardless 
of the edge, had projected y-values smaller than to_hd->north.

> > [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?

Morten Hulden