[GRASS-user] More on buffering and overlays

Richard Chirgwin rchirgwin at ozemail.com.au
Thu Jun 26 20:21:58 EDT 2008

First, thanks to Mortiz for pointing out the buffering issue. By adding 
centroids to the buffered output, that now corresponds to the source set 
of points.

Overlaying is still causing some pain - I need to explain what I have to 
ask my question, so bear with me.

One input is a population map divided into adjacent areas. The second 
input is (mostly) non-overlapping polygons. The aim is to calculate the 
population which lies inside / outside these polygons (rather like John 
Field's query which was provided by Mortiz under the recent "strategy 
required" thread.

ainput = populations
binput = catchment

When I run v.overlay, I get a table output:
cat, a_cat, population, b_cat, catchment_name

However: there are areas within the catchment polygons for which b_cat 
and catchment_name are null. In this state, they can't be used to count 
populations, so I have to then run v.distance (dmax=0) to assign the 
correct b_cat and catchment_names to the nulls.

While it's merely inconvenent rather than being a crisis, I'd be 
interested to know whether the overlay problem is my own fault, or 
whether something is going wrong with v.overlay.


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