[R-sig-Geo] Extracting informations from overlay

[Edzer Pebesma]

On 12/06/2011 12:06 PM, sandra werb wrote:
> Dear all,
> 
> I am working with a Lidar dataset, which contains thousands of
> undistributed points. What I want to do is to count the number of points
> within a raster cell and extract this information. Not just as single
> numbers, but together with the ID or the coordinates of the raster. I came
> to this so far:
> 
> - Create an empty raster, all cells are 0
> 
> empty.r <- raster(nrows=733, ncols=732, xmn=-158, xmx=-121, ymn=-28.3,
> ymx=8.35, crs=NA)
> empty.r[] <- 0
> 
> - read in the dataset of points containing only xy information and define
> these as spatialpoints
> 
> r1_points <- read.table("09_16_B.txt", sep="\t", dec=".", header=F)
> p <- r1_points[,-3]
>  p<-SpatialPoints(data.frame(p))
> 
> - create an overlay of the raster and the spatial points and use function
> table to
> 
> o <- overlay(as(empty.r, "SpatialGrid"),p)
> tab <- table(o)

untried, from ?over:

g = as(empty.r, "SpatialGrid")
o = sapply(over(as(g, "SpatialPolygons"), p, returnList = TRUE), length)
gdf = SpatialGridDataFrame(g, data.frame(o = o))
spplot(gdf)

> 
> tab gives me the numbers of points within one cell, but I don't know how to
> get this information together with the ID or XY coordinates as one table or
> else. Can anyone help?
> 
> Thank you very much in advance!
> 
> Leni
> 
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-- 
Edzer Pebesma
Institute for Geoinformatics (ifgi), University of Münster
Weseler Straße 253, 48151 Münster, Germany. Phone: +49 251
8333081, Fax: +49 251 8339763  http://ifgi.uni-muenster.de
http://www.52north.org/geostatistics      e.pebesma at wwu.de