[R-sig-Geo] Technique required: create a new shapefile from polygon vertices list

[Roger Bivand]

Sorry, one line missing marked ###:

On Tue, 21 Jun 2005, Roger Bivand wrote:

> On Mon, 20 Jun 2005, Rick Reeves wrote:
> 
> > 
> > 
> > 
> > I have a collection of polygons, expressed as ordered lists 
> > or line segments defined as x/y coordinate pairs. In the line
> > set for each polygon, the last coordinate of the last line
> > connects to the first coordinate of the first line. 
> > 
> > I can plot these using the graphics package method 'polygon'. 
> > 
> > Now, I would like to export them as a shapefile. Seemed like
> > this would be simple. So far, I have examined the maptools,
> > shapefiles,sp, geoR packages and found no way to write a 
> > polygon shapefile that does not require the input data to 
> > have been in a shapefile (e.g., maptools.write.polylistShape).
> 
> Assuming you would like each list element to be a separate shape:
> 
> > library(sp)
> > pl <- list(s1=cbind(c(2,4,4,1,2), c(2,3,5,4,2)), s2=cbind(c(5,4,2,5), 
> +   c(2,3,2,2)), s3=cbind(c(4,4,5,10,4), c(5,3,2,5,5)))
> > str(pl)
> List of 3
>  $ s1: num [1:5, 1:2] 2 4 4 1 2 2 3 5 4 2
>  $ s2: num [1:4, 1:2] 5 4 2 5 2 3 2 2
>  $ s3: num [1:5, 1:2] 4 4 5 10 4 5 3 2 5 5
> > plot(c(0,11), c(1,6), asp=1, type="n")
> > for (i in pl) polygon(i)
> > names(pl)
> [1] "s1" "s2" "s3"
> 
> replicates your list (but here I've named the polygons in the list with 
> unique names, you can use names(pl) <- c(<unique names>) if need be.
> 

###
Srsl <- vector(mode="list", length=length(pl))
###

> > for (i in 1:length(pl)) Srsl[[i]] <- Srings(list(Sring(pl[[i]])), 
> +   names(pl)[i])
> > shps <- SpatialRings(Srsl, pO=1:length(pl))
> > plot(shps)
> 
> That gives us a SpatialRings object, the plot order pO= argument is
> required, but will not be needed later for this case. Now for the 
> attribute data - it is required, because shapefiles have to have a DBF 
> file:
> 
> > df <- data.frame(x1=rbinom(3,6,0.1), x2=sample(letters,3), x3=runif(3), 
> +    row.names=getSRSringsIDSlots(shps))
> > df
>    x1 x2         x3
> s1  1  d 0.78274192
> s2  0  k 0.07043669
> s3  1  o 0.09974697
> > SRDF <- SpatialRingsDataFrame(shps, df)
> 
> Now that we have a SpatialRingsDataFrame object, we can write it as a 
> shapefile (in fact using the facilities in the maptools package inside a 
> wrapper in the spmaptools package in the repository at sourceforge):
> 
> # rSpatial <- "http://r-spatial.sourceforge.net/R"
> # install.packages("spmaptools", repos=rSpatial)
> > library(spmaptools)
> > writeSRDFShape(SRDF, "mySRDF")
> > getinfo.shape("mySRDF")
> Shapefile Type: Polygon   # of Shapes: 3 
> 
> The sp package mechanisms also let you subset your SRDF, use the polygons 
> from the maps package, and use ArcInfo e00 files too if you feel like it. 
> You can also use gpclib through the spgpc wrapper to dissolve internal 
> borders when aggregating polygons if you need to. More ideas will be 
> welcome too ... (we still don't have a way to write a *.prj file from the 
> PROJ.4-format coordinate reference system information that we can embed in 
> the sp objects, that does need doing).
> 
> Hope this helps
> 
> Roger 
> 
> > 
> > Someone suggests using read.table in geoR. But how do you 
> > specify that you want to create polygons? 
> > 
> > Can anyone suggest a method using R methods or otherwise (e.g. gdal)
> > for generating a polygon shapefile as I describe? Even
> > the Arc GIS documentation is sketchy on thiis topic. 
> > 
> > Thanks, Rick Reeves
> > 
> > 
> > Scientific Programmer / Quantitative Analyst
> > National Center for Ecological Analysis and Synthesis
> > University of California, Santa Barbara
> > 805 892 2534
> > reeves at nceas.ucsb.edu
> > 
> > _______________________________________________
> > 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