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I have a stack of monthly temperature raster data and would like to calculate the global spatial mean per area using the terra package in R to obtain a monthly time series.

I tried terra::app and terra::mean but they both give me a raster with a temporal average per gridcell, not the spatial average. terra::extract might be another solution but I can't seem to get it to work, since I am interested in the global mean.

This feels like a very basic thing to do so apologies if it is obvious. Any tips appreciated.

Thanks

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You are after the function terra::global. Make sure to use na.rm=TRUE else wise, if there are NA' present the result will be NA. If the SpatRaster object is multi-layer, results will be returned for each layer.

library(terra)
r <- rast(ncols=100, nrows=100)
  r[] <- runif(ncell(r))
    r[c(50,100,1000)] <- NA

global(r, "mean")
global(r, "mean", na.rm=TRUE)

If you need to return the specific value to use as a scalar a given band can be indexed. Otherwise the returned object is a data.frame and a function that is expecting a numeric value will fail.

class(global(r, "mean", na.rm=TRUE))
global(r, "mean", na.rm=TRUE)[,1] 
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  • Worked like a charm, thank you! I was also wondering if there is an elegant way to store the time object with it? I can of course add it to the data frame using terra::time but it feels a bit hacky to me.
    – LuleSa
    Oct 4, 2023 at 22:33

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