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

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, but popular) belief that you should use a planar coordinate reference system. Where applicable, the transformation to lon/lat is done automatically, if transform=TRUE.

Note that it is important that polygon geometries are valid. If they are not valid, the computed area may be wrong. You can check for validity with is.valid and fix some problems with makeValid

Usage

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

# S4 method for class '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")
#>   layer      area
#> 1     1 273986501

# all cells 
expanse(rast(r), unit="km")
#>   layer      area
#> 1     1 510065622

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

expanse(m, unit="km")
#>   layer      area
#> 1     1 498751903
expanse(m, unit="km", transform=FALSE)
#>   layer       area
#> 1     1 1241591858

m2 <- c(m, m)
values(m2) <- cbind(c(1,2,NA,NA), c(11:14))
#> Warning: [setValues] values were recycled
expanse(m2, unit="km", byValue=TRUE, wide=TRUE)
#>   layer        1        2       11       12       13       14
#> 1     1 10182145 10182145        0        0        0        0
#> 3     2        0        0 10182145 10182145 10182145 10182145


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)
#>   layer  value       area
#> 1     1 forest   50778375
#> 2     1  water  715780779
#> 3     1  urban 1118216460
#> 4     1  crops  617965652
#> 5     1  grass   60868836

g <- rasterize(v, r, "NAME_1")
expanse(r, byValue=TRUE, zones=g, wide=TRUE)
#>   layer         zone   forest     water     urban     crops    grass
#> 1     1     Diekirch  1110530 146734320 332045014 584531527 58106847
#> 2     1 Grevenmacher 46327739 247619382 219364571   1670053        0
#> 5     1   Luxembourg        0 315854211 555664543  25106417        0


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

a <- expanse(v)
a
#>  [1] 312283206 218674025 259454806  76200409 263174257 188282143 128991500
#>  [8] 210354494 185630770 251322021 237113004 233329960
sum(a)
#> [1] 2564810595