[GRASS-user] Islands and v.to.rast
hamish_nospam at yahoo.com
Sun May 6 02:26:39 EDT 2007
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
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,
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