[GRASS-user] More on buffering and overlays

Moritz Lennert mlennert at club.worldonline.be
Fri Jun 27 11:53:13 EDT 2008

On 27/06/08 02:21, Richard Chirgwin wrote:
> 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. 

I'm not sure I understand: b_cat and catchment_name should only be null 
in areas where you have ainput, but no binput. In all places where you 
have binput, these two should be filled in, unless the original binput 
polygon has null values (i.e. does not have a centroid with a category).


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