[GRASS-user] Islands and v.to.rast

[Hamish]

Priit Voolaid wrote:
> My goal is to convert a vector map into raster format. I have the data
> in MapInfo format, which comprises of area-objects. v.in.ogr works
> fine. Only thing, at the end of the process i get an warning that
> numerous areas “represent more (overlapping) features” and that “such
> areas are linked to more than 1 row in attribute table”. This produces
> problems in format conversion stage with v.to.rast - small areas
> inside bigger ones (islands) will not be present with their respective
> category numbers in output raster map. The result looks much more
> generalized as the source data. I suggest this is due linking one
> vector object with two rows in attribute table.
..
> All the small islands within a bigger one have in raster output the
> same pixel category, the category of that big area they belong to.
> 
> Is there any way to work this problem around? Thanks!


to understand why the problem happens, you might check out the
"topological vs non-topologioc" slides (9,10) of Markus's Como 2006
seminar:  http://mpa.itc.it/markus/teaching.html


as for solutions:

quick and dirty: try importing with "v.in.ogr -c"
  (to be avoided if possible)


FYI, if you don't mind some generalization you can remove small areas
with "v.clean tool=rmarea" (any cat) or v.dissolve (same cat).



not much of a full solution, but maybe it helps,

Hamish