[R-sig-Geo] Convert SpatialGridDataFrame to polygons...?

[Roger Bivand]

On Thu, 15 Jul 2010, Crowe, Andrew wrote:

> Gavin
>
> Gavin
>
> You can convert the raster grid to a polygon grid by coercing the 
> SpatialGridDataFrame to a SpatialPixelsDataFrame then from that to a 
> SpatialPolygonsDataFrame.
>
> You should then be able to recode the polygon data for each particular 
> depth class and union them using unionSpatialPolygons.  This then needs 
> joining with a data frame created from the depth classes to create a 
> SpatialPolygonsDataFrame for output to a shapefile using writePolyShape

In addition, you can massage the output of contourLines() to make a less 
jagged version:

library(maptools)
data(volcano)
x2 <- ContourLines2SLDF(contourLines(volcano))
# volcano is a regular "image" style object, SpatialGridDataFrame
# objects can be coerced with as.image.SpatialGridDataFrame()
# we don't (yet) have contourLines() for sp objects
x3 <- x2[(as.character(x2$level) > "140" & as.character(x2$level) <
  "170"),]
cx3 <- coordinates(x3)
str(cx3)
crds <- rbind(cx3[[1]][[1]], c(NA, NA), cx3[[2]][[1]], c(NA, NA),
   cx3[[2]][[2]], c(NA, NA), cx3[[1]][[2]])
# this could be automated in a loop
colnames(crds) <- c("x", "y")
pols <- map2SpatialPolygons(as.data.frame(crds), IDs=rep("1", 4))
# number of IDs like input polygons
pls <- slot(pols, "polygons")
gpclibPermit()
pls1 <- lapply(pls, checkPolygonsHoles)
slot(pols, "polygons") <- pls1
plot(pols, col="grey", pbg="yellow")

Both the use of checkPolygonsHoles() and unionSpatialPolygons require the 
use of gpclib with its bad licence. We're at mid-term with the GSoC rgeos 
project now, which provides similar facilities.

Using the contour lines may give a visually more pleasing, and maybe more 
helpful rendering than the blocky polygonised raster cells (which anyway 
will have been interpolated).

Please let us know how you get on,

Roger


>
> Hope that helps
>
> Andrew
>
> Dr Andrew Crowe
>
> Lancaster Environment Centre
> Lancaster University
> Lancaster    LA1 4YQ
> UK
>
> Tel: +44 (0)1524 595879
>
> ________________________________
>
> From: r-sig-geo-bounces at stat.math.ethz.ch on behalf of Gavin Simpson
> Sent: Thu 15/07/2010 12:28 PM
> To: r-sig-geo at stat.math.ethz.ch
> Subject: [R-sig-Geo] Convert SpatialGridDataFrame to polygons...?
>
>
>
> Dear list,
>
> I have been given a set of ASCII grids which I read into R using
> readAsciiGrid(). The grids represent an interpolated water depth data
> set for a set of lakes; each grid represents a separate lake. For some
> reason I have been asked to convert this gridded data into a set of
> polygons. For example; given the gridded data, I want to generate a
> polygon that covers the area of the lake (from the grid) that is 1-2m
> deep say. I would then need to write out this polygon as an ESRI shape
> file.
>
> Are there tools in R to do this sort of conversion?
>
> It is a bit more complex that the simple example, as lakes can have
> multiple basins (areas of deeper water). Say a lake has two deep areas
> so a cross section through the lakes might look like a W, for such a
> lake I would require an object that contained two polygons representing
> the area of the lake between 4 and 5 meters say in *each* of the basins.
> These would then be read out into a shape file. My GIS colleague
> indicates that ArcGIS/ArcMAP has a function to polygonise a raster grid
> in such a way. Is there something similar in the R spatial software
> stack?
>
> An alternative approach would be to contour (at appropriate depth
> intervals) the interpolated grid and convert the set of contours into
> polygons for individual lakes. Is there a function to do this?
>
> As you can see, this is very much a look-see at the moment. We are
> evaluating whether it would be quicker to work out how to do this in R
> as opposed to doing it by hand in ArcGIS.
>
> Any suggestions would be most gratefully received indeed!
>
> All the best,
>
> Gavin
> --
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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