tapp: Apply a function to subsets of layers of a SpatRaster

In terra: Spatial Data Analysis

Apply a function to subsets of layers of a SpatRaster

Description

Apply a function to subsets of layers of a SpatRaster (similar to tapply and aggregate). The layers are combined based on the index.

The number of layers in the output SpatRaster equals the number of unique values in index times the number of values that the supplied function returns for a single vector of numbers.

For example, if you have a SpatRaster with 6 layers, you can use index=c(1,1,1,2,2,2) and fun=sum. This will return a SpatRaster with two layers. The first layer is the sum of the first three layers in the input SpatRaster, and the second layer is the sum of the last three layers in the input SpatRaster. Indices are recycled such that index=c(1,2) would also return a SpatRaster with two layers (one based on the odd layers (1,3,5), the other based on the even layers (2,4,6)).

The index can also be one of the following values to group by time period (if x has the appropriate time values): "years", "months", "yearmonths", "week" (the ISO 8601 week number, see Details), "yearweeks", "days", "doy" (day of the year), "7days" (seven-day periods starting at Jan 1 of each year), "10days", or "15days". It can also be a function that makes groups from time values.

See app or Summary-methods if you want to use a more efficient function that returns multiple layers based on all layers in the SpatRaster.

Usage

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

Arguments

x	SpatRaster
index	factor or numeric (integer). Vector of length nlyr(x) (shorter vectors are recycled) grouping the input layers. It can also be one of the following values: "years", "months", "yearmonths", "days", "week" (ISO 8601 week number), or "doy" (day of the year)
fun	function to be applied. The following functions have been re-implemented in C++ for speed: "sum", "mean", "median", "modal", "which", "which.min", "which.max", "min", "max", "prod", "any", "all", "sd", "std", "first". To use the base-R function for say, "min", you could use something like fun = \(i) min(i)
...	additional arguments passed to fun
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. Ignored for functions that are implemented by terra in C++ (see under fun)
filename	character. Output filename
overwrite	logical. If TRUE, filename is overwritten
wopt	list with named options for writing files as in writeRaster

Details

"week" follows the ISO 8601 definition. Weeks start on Monday. If the week containing 1 January has four or more days in the new year, then it is considered week "01". Otherwise, it is the last week of the previous year (week "52" or "53", and the next week is week 1.

Value

SpatRaster

See Also

app, Summary-methods

Examples

r <- rast(ncols=10, nrows=10)
values(r) <- 1:ncell(r)
s <- c(r, r, r, r, r, r)
s <- s * 1:6
b1 <- tapp(s, index=c(1,1,1,2,2,2), fun=sum)
b1
b2 <- tapp(s, c(1,2,3,1,2,3), fun=sum)
b2

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