v.to.rast conversion

[John L Sparco]

This is to summarize the resolution of a problem I recently encountered.  Thanks
to everyone who offered help.

> 
> >I have been attempting to convert a
> >vector area layer to a raster layer using
> >v.to.rast.  The problem I am having is
> >that the vector layer contains a polygon
> >that is wholly encloses by another
> >polygon.  When executing a d.what.vect,
> >the proper attribute value is returned
> >for this lake.  However, after the
> >conversion, the raster layer has "no
> >data" for the lake.  Is this a known
> >problem with v.to.rast?  I have checked
> >for overlapping areas and there are none.
> >I was able to circumvent the problem by
> >importing the lake separately and doing
> >an r.patch.  This seems like a roundabout
> >way of doing it.  Any comments would be
> >appreciated.
> 
> >John Sparco
> 
> 
> OK I solved your problem
> It was actually very interesting
> make v.out.ascii on your map (soiltype)
> and you will notice that the area you mentioned
> (the one with 24 points) appears twice, it consists of
> one area edge, and it's digitized in different order
> but the same coordinates second time.
> 
> Now, v.support when it sees and attribute, it finds the smallest
> area inside which this attribute is located, and attaches this attribute
> to found area. So it attaches the attribute to only one of identical areas.
> And the other area is unlabeled.
> 
> d.what.vect also finds the same area (which happens to have smaller index than
> unlabeled area, so it reports it correctly.
> 
> But when v.to.rast goes through list of areas, it first draws labeled
> area and then draws unlabled copy right on top if it with category 0.
> 
> The solution is to check for identical areas in v.to.rast and warn user
> about them and ignore the unlabeled copy.
> I will see if I can modify diglib to do the same thing.
> 
> As someone on this list informed me few months ago, GRASS doesn't
> distinguish arc_info islands during arcinfo->GRASS import.
> The islands in arcinfo are stored with diff headers, but GRASS
> imports them just like normal areas, thus creating duplicate
> areas in GRASS vector files (exactly the situation described above)
> 
> I could fix v.in.arc but I don't have any documentation on arcinfo format 
> I really need someone to send me some description of how exactly islands are handled in arcinfo.
> Olga