[R-sig-Geo] Constructing Polygons using bboxes from SpatialPolygonsDataFrame

[Roger Bivand]

On Mon, 8 Aug 2011, Cam Plouffe wrote:

> Hi everyone,
>
> I'm trying to make a function that constructs rectangular polygons based on
> the bounding boxes of other polygons from a SpatialPolygonsDataFrame.  I am
> having a number of errors right now, which I am unsure how to fix.  My code
> is basically as follows:
>
> temp = bbox(spdf_u[1,])
> temp = matrix(temp, ncol = 2)
>
> mat = matrix(nrow = 5, ncol = 2)
> mat[1,1] = temp[1,1]
> mat[1,2] = temp[2,1]
> mat[2,1] = temp[1,1]
> mat[2,2] = temp[2,2]
> mat[3,1] = temp[1,2]
> mat[3,2] = temp[2,2]
> mat[4,1] = temp[1,2]
> mat[4,2] = temp[2,1]
> mat[5,1] = temp[1,1]
> mat[5,2] = temp[2,1]
> rm(temp)
>
> temppoly = Polygon(mat)
> polylist = list(temppoly)
> temppolys = Polygons(polylist)
>
> I'm first encountering an error after I have constructed the Polygon object
> "temppoly".  It seems to have no problems at first, but if I try to plot
> "temppoly", I get the following error:
>
> Error in as.double(y) :
>  cannot coerce type 'S4' to vector of type 'double'

There is not plot method for "Polygon" objects.

>
> I'm also encountering an error when attempting to construct the Polygons
> object "temppolys":
>
> Error in Polygons(polylist) : Single ID required
>

See ?Polygons, and ID is required.

You can re-use things which are already there, and benefit from the *apply 
vectorisation way of doing things (which is worth learning, as it makes 
it easier to do things repeatedly):

library(sp)
library(maptools)
xx <- readShapeSpatial(system.file("shapes/sids.shp",
   package="maptools")[1], IDvar="FIPSNO",
   proj4string=CRS("+proj=longlat +ellps=clrk66"))
# giving some data to play with
bb0 <- lapply(slot(xx, "polygons"), bbox)
# make a list of the bounding boxes of the Polygons objects in the 
# "polygons" slot of xx
bb0[[1]] # show first component
library(splancs)
bb1 <- lapply(bb0, bboxx)
# turn these into matrices using a helper function in splancs
bb1[[1]] # show first component
bb2 <- lapply(bb1, function(x) rbind(x, x[1,]))
# close the matrix rings by appending the first coordinate
bb2[[1]] # show first component
rn <- row.names(xx)
# get the IDs
bb3 <- vector(mode="list", length=length(bb2))
# make somewhere to keep the output
for (i in seq(along=bb3)) bb3[[i]] <- Polygons(list(Polygon(bb2[[i]])),
   ID=rn[i])
# loop over the closed matrix rings, adding the IDs
bb3[[1]] # show first component
bbs <- SpatialPolygons(bb3, proj4string=CRS(proj4string(xx)))
plot(bbs)
# make a "SpatialPolygons" object

You were almost there, thanks for a clear explanation.

Hope this helps,

Roger

> I'm very new to R, so these could be very simply problems, but I'm unable to
> find any reference material about these issues.  If anyone could offer me
> some insight as to why I'm having these errors, and perhaps how to get
> around them, it would be greatly appreciated.  If more code is needed, let
> me know and I'll provide it.
>
> Thanks a lot,
> Cam
>
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-- 
Roger Bivand
Department of Economics, NHH Norwegian School of Economics,
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