[R-sig-Geo] merge sliver polygons

[Roger Bivand]

On Fri, 5 Sep 2008, Murray Richardson wrote:

> Hi Roger
>
> I'm just getting around to trying this out. 
> I must say, I'm not clear on how to work with the neighbour list object to 
> accomplish this.  Specifically, how would I identify the largest neighbours 
> of the sliver candidates?
>
> Furthermore, do I first subset the polygons according to the area threshold, 
> and then construct the neighbour list for only these candidate slivers?  I'm 
> not clear on how to identify neighbours for a subset of polygons.

My idea (IIRC!) was to look at a distribution of the areas of the Polygon 
objects in the dataset. A good deal will depend on whether the slivers are 
represented as separate Polygons objects (note the _s_), or whether they 
are Polygon objects belonging to Polygons objects. So nimbleness will be 
needed!

library(maptools)
xx <- readShapeSpatial(system.file("shapes/sids.shp", package="maptools")[1],
       IDvar="FIPSNO", proj4string=CRS("+proj=longlat +ellps=clrk66"))
pls <- slot(xx, "polygons")
AREAS <- sapply(pls, function(x) sapply(slot(x, "Polygons"), function(y)
       slot(y, "area")))
summary(sapply(AREAS, length))
summary(unlist(AREAS))

is not a very good example, because the "areas" are planar in non-planar 
coordinates, but I hope you see the point. If you have a clear break in 
the distribution, you can find out which they are, and where they are. If 
all the Polygons objects (the ones in the "polygons" slot of your 
SpatialPolygons object, pls) are singletons, things get much easier, 
because the merging will be between Polygons objects, provided directly in 
maptools.

This should be a start - once you have singleton Polygons objects with 
areas as attributes, we can continue.

Hope this helps,

Roger

>
> Sorry to bother you with this again.  It will be very useful if it works as I 
> would also like to use it for region merging based on other attributes.
>
> Still waiting on that book...!

PS. Edzer, Virgilio and I did see the only copy in Europe in Dortmund at 
useR! three weeks ago. It turns out that it was printed in the US, and is 
at or awaited at the publisher's warehouse for dispatch. Things seem to 
take time in the "real" world!

>
> Murray
>
>
>
> Roger Bivand wrote:
>>  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
>> > 
>>> _______________________________________________
>> >  R-sig-Geo mailing list
>> >  R-sig-Geo at stat.math.ethz.ch
>> >  https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>> > 
>> > 
>> 
>

-- 
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