xyValues: Extract values at xy coordinates

Description

These methods return the values of a Raster* object for the cells in which a set of points fall. Values can be extracted through bilinear interpolation of values of the four cells nearest to each point. It is also possible to supply a buffer to select values for cells within a certain distance around each point (see Details).

Usage

xyValues(object, xy, ...)

Arguments

object

Raster* object

Coordinates. A n*2 matrix or dataframe with the first column having the x values, and the second column has the y values. It can also be a SpatialPoints* object, or a vector of length

...

Additional arguments. See Details

Value

A vector (object is a RasterLayer) or a matrix (object is a RasterStack or RasterBrick); Or a list if a buffer is supplied without a function.

Details

These are the additional arguments that can be suplied to this function: 1) method. If method='simple' (the default), values for the cell a point falls in are returned. The alternative is method='bilinear', in which case the returned values are interpolated from the values of the four nearest raster cells. 2) buffer. The radius of a buffer around each point for which to extract cell values. It can be a single value, or a vector of the length of the number of points. If the data are not projected (latitude/longitude), the unit should be meters. Otherwise it should be in map-units (typically also meters). 3) fun. Only useful when non-zero buffers are used. The values of all the cells are summarized with the function. The function should take a single numeric vector as argument and return a single values (e.g. mean, min or max). 4) na.rm. Only useful when fun is supplied. If na.rm=TRUE (the default value), NA values are removed before fun is applied.

Examples

Run this code

r <- raster()
r[] <- 1:ncell(r)
xy <- cbind(-50, seq(-80, 80, by=20))
xyValues(r, xy)
xyValues(r, xy, method='bilinear')

# examples with a buffer
xyValues(r, xy[1:3,], buffer=100000)
xyValues(r, xy[1:3,], buffer=100000, fun=mean)

## illustrating the varying size of a buffer (expressed in meters) on a longitude/latitude raster
 z <- xyValues(r, xy, buffer=1000000)
 s <- raster(r)
 for (i in 1:length(z)) { s[z[[i]]] <- i }
## compare with raster that is not longitude/latitude
 projection(r) <- NA 
 xy[,1] <- 50
 z <- xyValues(r, xy, buffer=8)
 for (i in 1:length(z)) { s[z[[i]]] <- i }
 plot(s)
# library(maptools)
# data(wrld_simpl)
# plot(wrld_simpl, add=TRUE)

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