extract: Extract values from a SpatRaster
In terra: Spatial Data Analysis

extract

R Documentation

Extract values from a SpatRaster

Description

Extract values from a SpatRaster for a set of locations. The locations can be a SpatVector (points, lines, polygons), a matrix with (x, y) or (longitude, latitude – in that order!) coordinates, or a vector with cell numbers.

When argument y is a SpatVector the first column has the ID (record number) of the SpatVector used (unless you set ID=FALSE).

Alternatively, you can use zonal after using rasterize with a SpatVector (this may be more efficient in some cases).

Usage

## S4 method for signature 'SpatRaster,SpatVector'
extract(x, y, fun=NULL, method="simple", cells=FALSE, xy=FALSE,
    ID=TRUE, weights=FALSE, exact=FALSE, touches=is.lines(y),
	layer=NULL, bind=FALSE, raw=FALSE, ...)

## S4 method for signature 'SpatRaster,SpatExtent'
extract(x, y, cells=FALSE, xy=FALSE)

## S4 method for signature 'SpatRaster,matrix'
extract(x, y, cells=FALSE, method="simple")

## S4 method for signature 'SpatRaster,numeric'
extract(x, y, xy=FALSE)

## S4 method for signature 'SpatVector,SpatVector'
extract(x, y)

Arguments

`x`	SpatRaster or SpatVector of polygons
`y`	SpatVector (points, lines, or polygons). Alternatively, for points, a 2-column matrix or data.frame (x, y) or (lon, lat). Or a vector with cell numbers
`fun`	function to summarize the data by geometry. If `weights=TRUE` or `exact=TRUE` only `mean`, `sum`, `min` and `max` are accepted).
`method`	character. method for extracting values with points ("simple" or "bilinear"). With "simple" values for the cell a point falls in are returned. With "bilinear" the returned values are interpolated from the values of the four nearest raster cells
`cells`	logical. If `TRUE` the cell numbers are also returned, unless `fun` is not `NULL`. Also see `cells`
`xy`	logical. If `TRUE` the coordinates of the cells are also returned, unless `fun` is not `NULL`. Also see `xyFromCell`
`ID`	logical. Should an ID column be added? If so, the first column returned has the IDs (record numbers) of input SpatVector `y`
`weights`	logical. If `TRUE` and `y` has polygons, the approximate fraction of each cell that is covered is returned as well, for example to compute a weighted mean
`exact`	logical. If `TRUE` and `y` has polygons, the exact fraction of each cell that is covered is returned as well, for example to compute a weighted mean
`touches`	logical. If `TRUE`, values for all cells touched by lines or polygons are extracted, not just those on the line render path, or whose center point is within the polygon. Not relevant for points; and always considered `TRUE` when `weights=TRUE` or `exact=TRUE`
`layer`	character or numeric to select the layer to extract from for each geometry. If `layer` is a character it can be a name in `y` or a vector of layer names. If it is numeric, it must be integer values between `1` and `nlyr(x)`
`bind`	logical. If `TRUE`, a SpatVector is returned consisting of the input SpatVector `y` and the `cbind`-ed extracted values
`raw`	logical. If `TRUE`, a matrix is returned with the "raw" numeric cell values. If `FALSE`, a data.frame is returned and the cell values are transformed to factor, logical, or integer values, where appropriate
`...`	additional arguments to `fun` if `y` is a SpatVector. For example `na.rm=TRUE`. Or arguments passed to the `SpatRaster,SpatVector` method if `y` is a matrix (such as the `method` and `cells` arguments)

Value

data.frame, matrix or SpatVector

Examples

r <- rast(ncols=5, nrows=5, xmin=0, xmax=5, ymin=0, ymax=5)
values(r) <- 1:25
xy <- rbind(c(0.5,0.5), c(2.5,2.5))
p <- vect(xy, crs="+proj=longlat +datum=WGS84")

extract(r, xy)
extract(r, p)

r[1,]
r[5]
r[,5]

r[c(0:2, 99:101)]

f <- system.file("ex/meuse.tif", package="terra")
r <- rast(f)

xy <- cbind(179000, 330000)
xy <- rbind(xy-100, xy, xy+1000)
extract(r, xy)

p <- vect(xy)
g <- geom(p)
g

extract(r, p)

x <- r + 10
extract(x, p)

i <- cellFromXY(r, xy)
x[i]
r[i]

y <- c(x,x*2,x*3)
y[i]

## extract with a polygon
f <- system.file("ex/lux.shp", package="terra")
v <- vect(f)
v <- v[1:2,]
z <- rast(v, resolution=.1, names="test")
values(z) <- 1:ncell(z)

rf <- system.file("ex/elev.tif", package="terra")
x <- rast(rf)
extract(x, v, mean, na.rm=TRUE)

e <- extract(z, v)
e
tapply(e[,2], e[,1], mean, na.rm=TRUE)

x <- c(z, z*2, z/3)
names(x) <- letters[1:3]

e <- extract(x, v)
de <- data.frame(e)
aggregate(de[,2:4], de[,1,drop=FALSE], mean)

terra documentation built on Jan. 17, 2023, 5:18 p.m.