On Tue, 7 Sep 2010, christiaan pauw wrote:
Hallo Everybody
I have a SpatialPolygonsDataFrame where residential stands in a town
are given as polygons. The streets are "represented" by the empty
space between the polygons. I want to dissolve the stand boundaries
within a street block to get just the outer extent of a street block.
My aim is this to move from a map of individual houses to a map of
street blocks.
The criterion for merging is that they touch an neighbouring polygon
(queen). Merging should continue until there are no polygons left
touch each other i.e until you only have the street blocks separated
by the empty space where the streets are. I have tried to use
unionSpatialPolygons but have not found a way in which is can define
which polygons to merge since I do not know how to define the "IDs"
argument correctly. I had a feeling that I could specify which
polygons to merge with poly2nb but poly2nb does not work recursively -
i.e. the same polygon can be found in more than one entry on the nb
list.
I was also looking for a data set to build a self-contained example
with (like a map of the world where one can dissolve the country
boundaries and end up only with continents) but could not find one
Can anyone offer me some advice
Please look at ?n.comp.nb in spdep. This finds the number of separate
graph components in an nb object, returns the count, and a vector saying
which observation belongs to which disjoint connected subgraph. If there
are no contiguities across the streets, you should see each subgraph
matching your street blocks. Try using the subgraph vector to colour a map
of the residential polygons, or nodes in a plot of the nb object (see the
example for n.comp.nb. From there on unionSpatialPolygons() if n.comp.nb()
makes the classification you need.
Hope this helps,
Roger
Regards
Christiaan
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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.biv...@nhh.no
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