R: Examples for 'raster::extract'

### ** Examples

r <- raster(ncol=36, nrow=18, vals=1:(18*36))

###############################
# extract values by cell number
###############################
extract(r, c(1:2, 10, 100))

[1]   1   2  10 100

s <- stack(r, sqrt(r), r/r)
extract(s, c(1, 10, 100), layer=2, n=2)

       layer.2 layer.3
[1,]  1.000000       1
[2,]  3.162278       1
[3,] 10.000000       1

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

[1] 626 554 482 410 338 266 194 122  50

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

[1] 607.5 535.5 463.5 391.5 319.5 247.5 175.5 103.5  31.5

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

[[1]]
 [1] 586 587 588 589 590 591 592 593 620 621 622 623 624 625 626 627 628 629 630
[20] 631

[[2]]
[1] 517 518 552 553 554 555

[[3]]
[1] 445 446 481 482

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

[1] 611.1 541.5 463.5

## 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
 crs(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
###############################
r <- raster(ncol=36, nrow=18, vals=1:(18*36))
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)

[[1]]
 [1] 126 127 161 162 163 164 196 197 200 201 231 232 237 266 267 273 274 302 310
[20] 311 338 346 381 382 414 417 450 451 452 453 487 488

[[2]]
 [1] 139 140 141 174 175 177 208 209 210 213 243 244 249 250 279 286 322 358 359
[20] 394 429 430 465 501 537

###############################
# 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)

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

[1] 387.8158 329.3913

# 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)

[1] 502 503 538 539

#plot(r)
#plot(e, add=T)

Examples for 'raster::extract'

Extract values from Raster objects