Raster algebra

Many generic functions that allow for simple and elegant raster algebra have been implemented for SpatRaster objects, including the normal algebraic operators such as +, -, *, /, logical operators such as >, >=, <, ==, !} and functions such as abs, round, ceiling, floor, trunc, sqrt, log, log10, exp, cos, sin, max, min, range, prod, sum, any, all. In these functions you can mix terra objects with numbers, as long as the first argument is a terra object.

library(terra)
# create an empty SpatRaster
r <- rast(ncol=10, nrow=10)
# assign values to cells
values(r) <- 1:ncell(r)
s <- r + 10
s <- sqrt(s)
s <- s * r + 5
values(r) <- runif(ncell(r))
r <- round(r)
r <- r == 1

You can also use replacement functions (not yet supported)

s[r] <- -0.5
s[!r] <- 5
s[s == 5] <- 15

If you use multiple SpatRaster objects (in functions where this is relevant, such as range), these must have the same resolution and origin. The origin of a SpatRaster object is the point closest to (0, 0) that you could get if you moved from a corners of a SpatRaster object towards that point in steps of the x and ``y resolution. Normally these objects would also have the same extent, but if they do not, the returned object covers the spatial intersection of the objects used.

When you use multiple multi-layer objects with different numbers or layers, the ‘shorter’ objects are ‘recycled’. For example, if you multiply a 4-layer object (a1, a2, a3, a4) with a 2-layer object (b1, b2), the result is a four-layer object (a1b1, a2b2, a3b1, a3b2).

r <- rast(ncol=5, nrow=5)
values(r) <- 1
s <- c(r, r+1)
q <- c(r, r+2, r+4, r+6)
x <- r + s + q
x
## class       : SpatRaster
## dimensions  : 5, 5, 4  (nrow, ncol, nlyr)
## resolution  : 72, 36  (x, y)
## extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
## coord. ref. : lon/lat WGS 84
## source(s)   : memory
## names       : lyr1, lyr2, lyr3, lyr4
## min values  :    3,    6,    7,   10
## max values  :    3,    6,    7,   10

Summary functions (min, max, mean, prod, sum, Median, cv, range, any, all) always return a SpatRaster object. Perhaps this is not obvious when using functions like min, sum or mean.

a <- mean(r, s, 10)
b <- sum(r, s)
st <- c(r, s, a, b)
sst <- sum(st)
sst
## class       : SpatRaster
## dimensions  : 5, 5, 1  (nrow, ncol, nlyr)
## resolution  : 72, 36  (x, y)
## extent      : -180, 180, -90, 90  (xmin, xmax, ymin, ymax)
## coord. ref. : lon/lat WGS 84
## source(s)   : memory
## name        :      sum
## min value   : 17.33333
## max value   : 17.33333

Use global if instead of a SpatRaster you want a single number summarizing the cell values of each layer.

global(st, 'sum')
##              sum
## lyr.1    25.0000
## lyr.1.1  25.0000
## lyr.1.2  50.0000
## lyr1    100.0000
## lyr2    108.3333
## lyr1.1   50.0000
## lyr2.1   75.0000
global(sst, 'sum')
##          sum
## sum 433.3333