lapp: Apply a function to layers of a SpatRaster, or sub-datasets...

In terra: Spatial Data Analysis

Apply a function to layers of a SpatRaster, or sub-datasets of a SpatRasterDataset

Description

Apply a function to a SpatRaster, using layers as arguments.

The number of arguments in function fun must match the number of layers in the SpatRaster (or the number of sub-datasets in the SpatRasterDataset). 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 is vectorized. That is, it should work for vectors longer than one, not only for single numbers. Or if the input SpatRaster(s) have multiple layers, it should work for a matrix (multiple cells) of input data (or matrices in the case of a SpatRasterDataSet). The function must return the same number of elements as its input vectors, or multiples of that. Also make sure that the function is NA-proof: it should returns the same number of values when some or all input values are NA. And the function must return a vector or a matrix, not a data.frame. To test it, run it with do.call(fun, data) (see examples).

Use app for summarize functions such as sum, that take any number of arguments; and tapp to do so for groups of layers.

Usage

## S4 method for signature 'SpatRaster'
lapp(x, fun, ..., usenames=FALSE, cores=1, filename="", overwrite=FALSE, wopt=list())

## S4 method for signature 'SpatRasterDataset'
lapp(x, fun, ..., usenames=FALSE, recycle=FALSE, 
    filename="", overwrite=FALSE, wopt=list())

Arguments

x	SpatRaster or SpatRasterDataset
fun	a function that takes a vector and can be applied to each cell of x
...	additional arguments to be passed to fun
usenames	logical. Use the layer names (or dataset names if x is a SpatRasterDataset) to match the function arguments? If FALSE, argument matching is by position
cores	positive integer. If cores > 1, a 'parallel' package cluster with that many cores is created and used. You can also supply a cluster object
recycle	logical. Recycle layers to match the subdataset with the largest number of layers
filename	character. Output filename
overwrite	logical. If TRUE, filename is overwritten
wopt	list with named options for writing files as in writeRaster

Value

SpatRaster

Note

Use sapp or lapply to apply a function that takes a SpatRaster as argument to each layer of a SpatRaster (that is rarely necessary).

See Also

app, tapp, math

Examples

s <- rast(system.file("ex/logo.tif", package="terra")) + 1  
ss <- s[[2:1]]

fvi <- function(x, y){ (x - y ) / (x + y) } 
# test the function
data <- list(c(1:5,NA), 6:1)
do.call(fvi, data)

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 fail because 
# there are too many layers in s
# x5 <- lapp(s, fvi2, usenames=FALSE)

pairs(c(x1, x2, x3, x4))

## SpatRasterDataset
x <- sds(s, s[[1]]+50)
fun <- function(x, y) { x/y }

# test "fun"
data <- list(matrix(1:9, ncol=3), matrix(9:1, ncol=3))
do.call(fun, data)

lapp(x, fun, recycle=TRUE)

# the same, more concisely
z <- s / (s[[1]]+50)

terra documentation built on Oct. 14, 2024, 5:07 p.m.