Message-ID: <alpine.LRH.2.00.0910232125090.31851@reclus.nhh.no>
Date: 2009-10-23T19:28:52Z
From: Roger Bivand
Subject: basic polyfon shapefile subsetting
In-Reply-To: <4AE20167.3010109@u.washington.edu>
On Fri, 23 Oct 2009, Chris Fowler wrote:
> I swore I would figure this out without going to the list, but I just don't
> understand the data structure well enough to make this work. Easy pickings
> for many of you on this list.
>
> I have an ArcGIS shapefile "base.shp" that has 23034 polygons (U.S. census
> tracts). I want to end up with a subset of this file that has no 'islands'
> and a neighbor list to accompany it. What am I doing wrong?
>
> baseShp<-readShapePoly("Export_Output.shp") #read in a shapefile
> Q1B<-poly2nb(baseShp, row.names = baseShp$FIPS) #1st order Queen
> summary(Q1B) #731 with 0 neighbors we need to remove these
> x<-card(Q1B) #create a vector with the number of neighbors
> baseShp$numNeighbors<-x #assign that vector as an additional data column
> length(baseShp$numNeighbors) #check to make sure all 23034 have value
> summary(baseShp$numNeighbors) #...and that those values are what I
> #expect
> submap <- baseShp[baseShp$attr.data$numNeighbors >=1] #pull out those
> # with at least 1 neighbor
Use
baseShp[baseShp$numNeighbors >=1, ]
There are two mistakes, one the missing comma, the other the use of a
non-existing column, which will result in baseShp[NULL >=1] being
evaluated.
Roger
> plot(submap) #something plots....
> length(submap$numNeighbors) #0, no data transferred
> subm<-as.data.frame(submap) # convert to data frame for easier looking
> str(subm) #we still have 23034 observations and now we have no data
>
>
> So, why doesn't my subseting code bring data with it and why doesn't it
> actually do the subsetting?
>
> Thanks in advance,
>
> Chris
>
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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