[R-sig-Geo] modifying shpfiles in maptools

[Roger Bivand]

On Thu, 24 May 2007, Markus Loecher wrote:

> Hello,
> I am trying to manipulate a shpfile (actually subsetting it) but am 
> not getting anywhere. I would like to plot the census blocks of San 
> Francisco county (ID = 075) only but the shpfile I have is for all of CA.
> So I am executing the following lines of code, where shpfile.CA is 
> the big one for CA:
> 
> shpfile.CA <- read.shape("bg06_d00.shp", dbf.data=TRUE, verbose=TRUE);
> #keep only those entries for SF county:
> shpfile.CA$att.data[,"COUNTY"] <- as.character(shpfile.CA$att.data[,"COUNTY"]);
> i.keep <- shpfile.CA$att.data[,"COUNTY"] == "075";
> shpfile$att.data <- shpfile.CA$att.data[i.keep,];
> shpfile$Shapes <- shpfile.CA$Shapes[i.keep];
> 
> To me this seems like a correct subsetting of a list, but when I try 
> to plot the new, smaller shpfile, I get the error message:
> 	Error in 1:attr(theMap$Shapes, "nshps") : NA/NaN argument
> What foolish mistake am I committing ?
> 
> Also, it seems that plot.Map is deprecated. I wished I had an example 
> of how to use plot.Spatial instead, with a shpfile...

Yes, the Map object structure is too close to the input shapefile to be 
easy to manipulate. Try rather:

shpfile.CA <- readShapePoly("bg06_d00.shp")
shpfile.CA$COUNTY <- as.character(shpfile.CA$COUNTY)
shpfile.SF <- shpfile.CA[shpfile.CA$COUNTY == "075",]

should work if shpfile.CA$COUNTY is correct - I'd do a 
table(shpfile.CA$COUNTY) first to check that the values came out right.

The examples are in maptools (and rgdal) for reading and writing, and in 
sp for manipulating and plotting - for easy plots see spplot().

Roger

> 
> Thanks!
> 
> markus
> 
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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