Wrong values computed by terra::median if SpatRaster has more than 3 layers

[MatteaE]

Hi all

Trying to combine multiple layers of a SpatRaster with the median() function, the resulting grid has some wrong (seemingly semi-random?) values, as soon as the original SpatRaster has more than 3 layers.

Minimal example: terra_median_bug.zip

Result on my system:

R 4.1.2, terra 1.6.47 on Ubuntu 22.04.01.

[rhijmans]

Thank you for reporting this. This is a bug in the CRAN version but it works correctly in the development version of "terra", With that version I get:

library(terra)
#terra 1.5.48
lf <- list.files(pattern = "\\.tif$")
r <- rast(lf)

Three ways to compute the median

# built in
x <- median(r)
y <- app(r, median)
# "manual"
z <- app(r, \(i) median(i))

Compare

s <- c(x,y,z)
diff(s)
#class       : SpatRaster 
#dimensions  : 13, 13, 2  (nrow, ncol, nlyr)
#resolution  : 80, 80  (x, y)
#extent      : 717180, 718220, 4392180, 4393220  (xmin, xmax, ymin, ymax)
#coord. ref. : WGS 84 / UTM zone 42N (EPSG:32642) 
#source(s)   : memory
#names       : median, median 
#min values  :      0,      0 
#max values  :      0,      0 

Cell 6 had the largest difference, but it is good now.

r[6]
#  North/South North/South North/South North/South North/South North/South
#1   -5.855352    -4.82694    2.915545    3.260163   -109.9052  -0.2166536
#  North/South North/South North/South North/South
#1     4.42027   -3.505593    1.284746   -1.696935
s[6]
#      median     median     median
#1 -0.9567944 -0.9567944 -0.9567944