[R-sig-Geo] merge sliver polygons

[Roger Bivand]

On Wed, 13 Aug 2008, Murray Richardson wrote:

> Hello again r.sig.geo list,
>
> Thanks Roger, for help on my previous question regarding iterating through a 
> shapefile.
>
> I'm sure once I receive my copy of  "Applied Spatial Data Analysis with R" I 
> will find answers to simple questions like this on my own, but in the 
> meantime....
>
> Is it possible to merge sliver polygons that fall below a certain threshold 
> area with adjacent neighbours (e.g. perhaps using  unionSpatialPolygons but 
> without aggregating any  polygons?).  If a sliver shares edges with more than 
> one polygon, it doesn't really matter which one it merges with, but if I had 
> to choose a rule I would have it merge with the largest one.

Not such a simple question ...

Both the Polygon and Polygons objects in the SpatialPolygons object have 
"area" slots, with different roles. The Polygon objects have a correct 
naive area in the geometry of the coordinates taken as planar. The 
Polygons objects use the "gross" area of Polygon objects belonging to 
them, but "only" to provide the plot order (plot from largest to smallest 
to avoid over-painting).

If you "trust" the area slot of the Polygons objects (beware of hole 
Polygon objects), you can first find your candidate slivers by retrieving 
the areas by:

Polygons_areas <- sapply(slot(SPobj, "polygons"),
   function(x) slot(x, "area"))

and set a cutoff. Then use poly2nb(SPobj, queen=FALSE) in spdep to find 
the neighbours (rook criterion). Next use the output object to identify 
the largest neighbours of the sliver candidates, and build a "new 
Polygons" ID vector. Finally, use unionSpatialPolygons(). I'm assuming you 
wouldn't have asked if there was useful data in the slivers!

Hope this helps,

Roger

>
> Thanks in advance,
>
> Murray Richardson
>
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>

-- 
Roger Bivand
Economic Geography Section, Department of Economics, Norwegian School of
Economics and Business Administration, Helleveien 30, N-5045 Bergen,
Norway. voice: +47 55 95 93 55; fax +47 55 95 95 43
e-mail: Roger.Bivand at nhh.no