[GRASS-user] vectors: how to find the gravitation centre of point data ?

["Peter Löwe"]

-------- Original-Nachricht --------
> Datum: Fri, 05 Dec 2008 13:40:08 +0100
> Von: Moritz Lennert <mlennert at club.worldonline.be>
> An: "Peter Löwe" <peter.loewe at gmx.de>
> CC: grass-user at lists.osgeo.org
> Betreff: Re: [GRASS-user] vectors: how to find the gravitation centre of point data ?

> On 05/12/08 12:27, Peter Löwe wrote:
> > [...]
> >>> is there an easy way to derive the centre of a cloud of points ?
> >>>
> >>> I am aware that the set of vector points ("cloud of points") could be
> >>> used to stake out a polygon/boundary and use v.centroid etc etc to
> >>> derive in turn its' centroid, but I am hoping for an easier solution
> >>> .. ?
> >>>
> >>> This issue stems from working with v.to.db: currently it is not
> >>> possible to upload xy coordinates for "multiploygons" into a
> >>> database, i.e: Within a vector layer, there might be several
> >>> boundaries which all share the same category value. If the
> >>> "gravitational centre/ super-centroid" for these boundaries could be
> >>> (conveniently) calculated, the v.to.db issue could be taken care of.
> >>
> >> Why not use v.dissolve on these polygons and then get the centroids of 
> >> the result ?
> > 
> > This option fails if the polygons don't have common borders. An example
> for this would be multiple polygons describing spatial exclaves of a
> territory/county/soil type, etc. 
> > 
> >> Another (very wild) guess: what about the mean of the coordinates of
> the 
> >> individual polygons' centroids ?
> > 
> > That would be great if it could be automated.
> 
> Using a "real" DB-backend, it should be as easy as (assuming that when 
> you say boundaries, you mean areas, each of which has a centroid):
> 
> - v.to.db type=centroid option=coor
> - SELECT cat, avg(X), avg(Y) from TABLE group by cat
> 

Very good point.
If this is combined with a previous step to create individual CATs for the exclave areas while preserving the original CAT in another column, we can successfully apply v.to.db even while several areas share the same ID (having the centroids xy-coordinates for all exclave-area) _plus_ having the coordinates for the super-centroid of all exclaves.

Northern winter might be the perfect time to write GRASS scripts for this kind of stuff..

Thanks,
Peter

> 
> If you have boundaries with categories, but no centroids, I imagine that 
> v.centroids + v.distance with upload=cat should do the trick to transfer 
> boundary cats to their centroids.
> 
> Moritz

-- 
Dr. Peter Löwe
<peter.loewe at gmx.de>





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