I want to create a maximum value composite raster by taking maximum values from each band of a raster stack. I am using the following code
library(terra)
library(RStoolbox)
rast1 <- rast(lsat)
#Create two raster stacks
rast2 <- rast(lsat)
random_nums <- runif(length(rast2), min = 0.3, max = 0.5) # a set of random numbers the size of the image
rast2[] <- rast2[] + random_nums
rast3 <- rast(lsat)
random_nums <- runif(length(rast3), min = 0.1, max = 0.4) # a set of random numbers the size of the image
rast3[] <- rast3[] - random_nums
#Create a stack of all the rasters
s <- c(rast1, rast2, rast3)
#Create a maximum value composite
b1 <- tapp(s, index=c(1,8,15,2,9,16,3,10,17,4,11,18,5,12,19,6,13,20,7,14,21),
fun=max)
b1
Now it is giving me 21 layers. But the output should have 7 bands with each band made of by taking the maximum of rast1, rast2 and rast3 i.e. the band1 of b1 should take the maximum of band1 of rast1, 2 and 3. Likewise for b2 to b7.
Example data
library(terra)
r1 <- rast(ncol=10, nrow=10, nlyr=7)
set.seed(1)
r1 <- init(r1, runif)
r2 <- init(r1, runif)
r3 <- init(r1, runif)
The most straightforward way to get the parallel maximum cell values (the maximum value for each cell and each layer, across SpatRasters):
m1 <- max(r1, r2, r3)
Or create a SpatRasterDataset and use app
d <- sds(r1, r2, r3)
m2 <- app(d, max)
I would not recommend it as it is unnecessarily complex, but you can also do that with tapp like this:
i <- rep(1:7, 3)
i
# [1] 1 2 3 4 5 6 7 1 2 3 4 5 6 7 1 2 3 4 5 6 7
s <- c(r1, r2, r3)
m3 <- tapp(s, i, fun=max)
m3
#class : SpatRaster
#dimensions : 10, 10, 7 (nrow, ncol, nlyr)
#resolution : 36, 18 (x, y)
#extent : -180, 180, -90, 90 (xmin, xmax, ymin, ymax)
#coord. ref. : lon/lat WGS 84
#source : memory
#names : X1, X2, X3, X4, X5, X6, ...
#min values : 1.808664e-01, 0.000000e+00, 2.803072e-309, -6.884876e-311, 0.000000e+00, 0.000000e+00, ...
#max values : 9.926841e-01, 3.131513e-294, 1.536768e+277, 3.202245e+307, 3.046950e+294, 1.047578e+296, ...
Note that the layers with the same index are grouped. So for what you want you should only have seven numbers, each occurring three times.