Apply a function to layers of a SpatRaster ("overlay").
The number of arguments in function fun
must match the number of layers in the SpatRaster (or the number of sub-datasets in the SpatDataSet). For example, if you want to multiply two layers, you could use this function : fun=function(x,y){return(x*y)}
percentage: fun=function(x,y){return(100 * x / y)}
. If you combine three layers you could use fun=function(x,y,z){return((x + y) * z)}
Before you use the function, test it to make sure that it works for vectors (not only for single numbers). That is, it must return the same number of elements as its input vectors, or multiples of that. Make sure it also returns the same number of values when some or all input values are NA
. Also, the function must return a vector or matrix, not a data.frame
.
Use app
for summarize functions such as sum
, that take any number of arguments; and tapp
to so for groups of layers.
# S4 method for SpatRaster
lapp(x, fun, ..., usenames=FALSE, filename="", overwrite=FALSE, wopt=list())# S4 method for SpatDataSet
lapp(x, fun, ..., recycle=FALSE, filename="", overwrite=FALSE, wopt=list())
SpatRaster or SpatDataSet
function to be applied
additional arguments to be passed to fun
logical. Use the layer names to match the function arguments? If FALSE
matching is by position
logical. Recycle layers to match the subdataset with the largest number of layers
character. Output filename. Optional
logical. If TRUE
, filename
is overwritten
list. Options for writing files as in writeRaster
SpatRaster
# NOT RUN {
s <- rast(system.file("ex/logo.tif", package="terra"))
ss <- s[[2:1]]
fvi <- function(x, y){ (x - y ) / (x + y) }
x <- lapp(ss, fun=fvi )
# which is the same as supplying the layers to "fun"
# in some cases this will be much faster
y <- fvi(s[[2]], s[[1]])
f2 <- function(x, y, z){ (z - y + 1) / (x + y + 1) }
p1 <- lapp(s, fun=f2 )
p2 <- lapp(s[[1:2]], f2, z=200)
# the usenames argument
fvi2 <- function(red, green){ (red - green ) / (red + green) }
names(s)
x1 <- lapp(s[[1:2]], fvi2, usenames=TRUE)
x2 <- lapp(s[[2:1]], fvi2, usenames=TRUE)
# x1 and x2 are the same, despite the change in the order of the layers
# x4 is also the same, but x3 is not
x3 <- lapp(s[[2:1]], fvi2, usenames=FALSE)
x4 <- lapp(s, fvi2, usenames=TRUE)
# while this would give an error because
# there are too many layers in s
# x5 <- lapp(s, fvi2, usenames=FALSE)
pairs(c(x1, x2, x3, x4))
## SpatDataSet
x <- sds(s, s[[1]]+50)
lapp(x, function(x, y) x/y, recycle=TRUE)
# }
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