raster (version 2.3-40)

extract: Extract values from Raster objects

Description

Extract values from a Raster* object at the locations of other spatial data. You can use coordinates (points), lines, polygons or an Extent (rectangle) object. You can also use cell numbers to extract values. If y represents points, extract returns the values of a Raster* object for the cells in which a set of points fall. If y represents lines, the extract method returns the values of the cells of a Raster* object that are touched by a line. If y represents polygons, the extract method returns the values of the cells of a Raster* object that are covered by a polygon. A cell is covered if its center is inside the polygon (but see the weights option for considering partly covered cells; and argument small for getting values for small polygons anyway). It is also possible to extract values for point locations from SpatialPolygons.

Usage

## S3 method for class 'Raster,matrix':
extract(x, y, method='simple', buffer=NULL, small=FALSE, cellnumbers=FALSE, 
   fun=NULL, na.rm=TRUE, layer, nl, df=FALSE, factors=FALSE, ...)

## S3 method for class 'Raster,SpatialLines':
extract(x, y, fun=NULL, na.rm=FALSE, cellnumbers=FALSE, df=FALSE, layer,
   nl, factors=FALSE, along=FALSE, sp=FALSE, ...)

## S3 method for class 'Raster,SpatialPolygons':
extract(x, y, fun=NULL, na.rm=FALSE, weights=FALSE, 
   normalizeWeights=TRUE, cellnumbers=FALSE, small=TRUE, df=FALSE, layer, nl, 
   factors=FALSE, sp=FALSE, ...)


## S3 method for class 'SpatialPolygons,SpatialPoints':
extract(x, y, ...)

Arguments

x

Raster* object

y

points represented by a two-column matrix or data.frame, or SpatialPoints*; SpatialPolygons*; Spat

method

character. 'simple' or 'bilinear'. If 'simple' values for the cell a point falls in are returned. If 'bilinear' the returned values are interpolated from the values of the four nearest raster cells.

buffer

numeric. The radius of a buffer around each point from which to extract cell values. If the distance between the sampling point and the center of a cell is less than or equal to the buffer, the cell is included. The buffer can be specified as a single val

small

logical. If TRUE and y represents points and a buffer argument is used, the function always return a number, also when the buffer does not include the center of a single cell. The value of the cell in which the point

fun

function to summarize the values (e.g. mean). The function should take a single numeric vector as argument and return a single value (e.g. mean, min or max), and accept a na.rm argument. Thus, standard R functions not including a

na.rm

logical. Only useful when an argument fun is supplied. If na.rm=TRUE (the default value), NA values are removed before fun is applied. This argument may be ignored if the function used has a ... argument and ignores

cellnumbers

logical. If cellnumbers=TRUE, cell-numbers will also be returned (if no fun argument is supplied, and when extracting values with points, if buffer is NULL)

df

logical. If df=TRUE, results will be returned as a data.frame. The first column is a sequential ID, the other column(s) are the extracted values

weights

logical. If TRUE, the function returns, for each polygon, a matrix with the cell values and the approximate fraction of each cell that is covered by the polygon(rounded to 1/100); unless normalizeWeights=TRUE, in which case they

normalizeWeights

logical. If TRUE, weights are normalized such that they add up to one for each polygon

factors

logical. If TRUE, factor values are returned, else their integer representation is returned

layer

integer. First layer for which you want values (if x is a multilayer object)

nl

integer. Number of layers for which you want values (if x is a multilayer object)

along

boolean. Should returned values be ordered to go along the lines?

sp

boolean. Should the extracted values be added to the data.frame of the Spatial* object y? This only applies if y is a Spatial* object and, for SpatialLines and SpatialPolygons, if fun is not NULL. In this case the re

...

additional arguments (none implemented)

Value

• A vector for RasterLayer objects, and a matrix for RasterStack or RasterBrick objects. A list (or a data.frame if df=TRUE) if y is a SpatialPolygons* or SpatialLines* object or if a buffer argument is used (but not a fun argument). If sp=TRUE and y is a Spatial* object and fun is not NULL a Spatial* object is returned. The order of the returned values corresponds to the order of object y. If df=TRUE, this is also indicated in the first variable ('ID').

See Also

getValues, getValuesFocal

Examples

r <- raster(ncol=36, nrow=18)
r[] <- 1:ncell(r)

###############################
# extract values by cell number
###############################
extract(r, c(1:2, 10, 100))
s <- stack(r, sqrt(r), r/r)
extract(s, c(1, 10, 100), layer=2, n=2)

###############################
# extract values with points
###############################
xy <- cbind(-50, seq(-80, 80, by=20))
extract(r, xy)

sp <- SpatialPoints(xy)
extract(r, sp, method='bilinear')

# examples with a buffer
extract(r, xy[1:3,], buffer=1000000)
extract(r, xy[1:3,], buffer=1000000, fun=mean)

## illustrating the varying size of a buffer (expressed in meters) 
## on a longitude/latitude raster
 z <- extract(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) <- "+proj=utm +zone=17" 
 xy[,1] <- 50
 z <- extract(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)

###############################
# extract values with lines
###############################

cds1 <- rbind(c(-50,0), c(0,60), c(40,5), c(15,-45), c(-10,-25))
cds2 <- rbind(c(80,20), c(140,60), c(160,0), c(140,-55))
lines <- spLines(cds1, cds2)

extract(r, lines)

###############################
# extract values with polygons
###############################
cds1 <- rbind(c(-180,-20), c(-160,5), c(-60, 0), c(-160,-60), c(-180,-20))
cds2 <- rbind(c(80,0), c(100,60), c(120,0), c(120,-55), c(80,0))
polys <- spPolygons(cds1, cds2)

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

# v <- extract(r, polys, cellnumbers=TRUE)

# weighted mean
# v <- extract(r, polys, weights=TRUE, fun=mean)
# equivalent to:
# v <- extract(r, polys, weights=TRUE)
# sapply(v, function(x) if (!is.null(x)) {sum(apply(x, 1, prod)) / sum(x[,2])} else NA)


###############################
# extract values with an extent
###############################
e <- extent(150,170,-60,-40)
extract(r, e)
#plot(r)
#plot(e, add=T)