raster (version 1.0.0-1)

polygonValues: Extract values from a RasterLayer by polygons

Description

This method returns the values of the cells of a Raster object that is covered by a polygon.

Usage

polygonValues(p, r, ...)

Arguments

p

SpatialPolygons* object

r

Raster object

...

additional arguments. See details

Value

• a list as long as the number of polygons in p

Details

There are two related additional arguments: weights. If TRUE, the function returns, for each polygon, a matrix with the cell values and the fraction of each cell coverd (the weight that can be used for averaging; see examples). This option can be useful if the polygons are small relative to the cells size of the RasterLayer cellnumbers. If cellnumbers=TRUE and weights=TRUE, cellnumbers will also be returned.

See Also

cellValues, xyValues, getValues, SpatialPolygons

Examples

r <- raster(nrow=18, ncol=9)
r[] <- 1:ncell(r)

cds1 <- rbind(c(-180,-20), c(-160,55), c(-60, 0), c(-160,-60), c(-180,-20))
cds2 <- rbind(c(-80,0), c(-20,60), c(0,5), c(-60,-45), c(-80,0))
cds3 <- rbind(c(80,0), c(100,60), c(120,0), c(120,-55), c(80,0))
hole <- rbind(c(-150,-20), c(-100,-10), c(-110,20), c(-150,-20))
polys <- SpatialPolygons(list(Polygons(list(Polygon(cds1)), 1), Polygons(list(Polygon(cds2)), 2), Polygons(list(Polygon(hole)), 3), Polygons(list(Polygon(cds3)), 4) ))
polys@polygons[[3]]@Polygons[[1]]@hole <- TRUE

#plot(r)
#plot(polys, add=TRUE)
v <- polygonValues(polys, r)
v
# mean for each polygon
unlist(lapply(v, function(x) if (!is.null(x)) mean(x, na.rm=TRUE) else NA ))

v <- polygonValues(polys, r, weights=TRUE)
v
# weighted mean for each polygon
unlist(lapply(v, function(x) if (!is.null(x)) {sum(apply(x, 1, prod)) / sum(x[,2])} else NA  ))