expanse {terra}

Get the expanse (area) of individual polygons or for all (summed) raster cells

Description

Compute the area covered by polygons or for all raster cells that are not NA.

This method computes areas for longitude/latitude rasters, as the size of the cells is constant in degrees, but not in square meters. But it can also be important if the coordinate reference system is planar, but not equal-area.

For vector data, the best way to compute area is to use the longitude/latitude CRS. This is contrary to (erroneous) popular belief that suggest that you should use a planar coordinate reference system. This is done automatically, if transform=TRUE.

Usage

## S4 method for signature 'SpatRaster'
expanse(x, unit="m", transform=TRUE, byValue=FALSE,
			zones=NULL, wide=FALSE, usenames=FALSE)

## S4 method for signature 'SpatVector'
expanse(x, unit="m", transform=TRUE)

Arguments

x

SpatRaster or SpatVector

unit

character. Output unit of area. One of "m", "km", or "ha"

transform

logical. If TRUE, planar CRS are transformed to lon/lat for accuracy

byValue

logical. If TRUE, the area for each unique cell value is returned

zones

NULL or SpatRaster with the same geometry identifying zones in x

wide

logical. Should the results be in "wide" rather than "long" format?

usenames

logical. If TRUE layers are identified by their names instead of their numbers

Value

SpatRaster: data.frame with at least two columns ("layer" and "area") and possibly also "value" (if byValue is TRUE), and "zone" (if zones is TRUE). If x has no values, the total area of all cells is returned. Otherwise, the area of all cells that are not NA is returned.

SpatVector: numeric (one value for each (multi-) polygon geometry.

See Also

cellSize for a the size of individual cells of a raster, that can be summed with global or with zonal to get the area for different zones.

Examples

### SpatRaster 
r <- rast(nrows=18, ncols=36)
v <- 1:ncell(r)
v[200:400] <- NA
values(r) <- v

# summed area in km2
expanse(r, unit="km")

# all cells 
expanse(rast(r), unit="km")

r <- rast(ncols=90, nrows=45, ymin=-80, ymax=80)
m <- project(r, "+proj=merc")

expanse(m, unit="km")
expanse(m, unit="km", transform=FALSE)

m2 <- c(m, m)
values(m2) <- cbind(c(1,2,NA,NA), c(11:14))
expanse(m2, unit="km", byValue=TRUE, wide=TRUE)


v <- vect(system.file("ex/lux.shp", package="terra"))
r <- rast(system.file("ex/elev.tif", package="terra"))
r <- round((r-50)/100)
levels(r) <- data.frame(id=1:5, name=c("forest", "water", "urban", "crops", "grass"))
expanse(r, byValue=TRUE)

g <- rasterize(v, r, "NAME_1")
expanse(r, byValue=TRUE, zones=g, wide=TRUE)


### SpatVector
v <- vect(system.file("ex/lux.shp", package="terra"))

a <- expanse(v)
a
sum(a)