[GRASS-user] Re: [GRASS-dev] Should v.in.ogr clean topology by default ?

Hamish hamish_b at yahoo.com
Thu Dec 1 16:01:38 EST 2011

[cc'd from grass-dev ML]


a couple of points--

* Radim said many times: v.in.ogr's cleaning and v.clean are not
the same thing. (with a number of !! added) He explained it and
understood it always better than I ever could, so check in the
mailing list archives for details.

* GRASS is a topological GIS. so when data is imported into GRASS
it is natural and desirable that it gets converted into the
family way at that time.

* In defense of the option to allow non topological imports:
in the past I have imported a shapefile created by leading
proprietary non-topological GIS. this file was a string of
buffers around points with unique ID numbers. the idea was to
run (the predecessor to) v.rast.stats on each buffer to collect
some neighborhood context stats for each ID point, which could
be loaded into a table.  wherever the line of points was not
exactly straight the buffers overlapped a little and grass's
cleaning would treat it as a ven diagram and create a new
centroid in the overlapping sliver. no good for extracting the
buffer area where='NODE_ID = 1234'.
also you might consider a vector area map with multiple over-
lapping home ranges for different territorial animals. not 3d,
but still merging/flattening overlapping areas is not appropriate.

for things like neighboring land parcels, political lines, or
land/sea boundaries non-topological is a nightmare, but for
other tasks it can occasionally be very useful.


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