[R-sig-Geo] summing rasters with a condition given by other rasters

[Ben Tupper]

Hi,

On May 10, 2015, at 7:37 AM, Martin Brandt <martin.brandt at mailbox.org> wrote:

> Hi Ben,
> 
> many thanks for the detailed answer. The code works fine, but when I use
> rasters for eos and sos instead of numbers, i get an error:
> 
> 
> 
> b <- brick(system.file("external/rlogo.grd", package="raster"))
> 
> b <- addLayer(b,b,b)
> b
> 
> sos <- raster(system.file("external/rlogo.grd", package="raster"))
> eos <- raster(system.file("external/rlogo.grd", package="raster"))
> 
> 
> sum_segment <- function(x, from = sos, to = eos, ...) {
>  sum(x[from:to],...)
> }
> 
> s <- calc(b, sum_segment)
> 
> Error in .calcTest(x[1:5], fun, na.rm, forcefun, forceapply) : 
>  cannot use this function
> 
> am I doing something wrong?

Yes, as tempting as it is to do otherwise you really do have to use numeric position indices.  Inside the function sum_segment the value of x is simple a numeric vector. Within the scope of the function the context of the pixels embedded in the raster object is temporarily "lost" - they are just a vector of numbers.  

To work with the calc() function you must first compute the position indices for sos and eos using which().

sos <- which(names(b) == "blue.1")  # you would substitue the name of your layer for blue.1
eos <- which(names(b) == "green.3") # and again for green.3

Cheers,
Ben


> 
> 
> 
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Ben Tupper
Bigelow Laboratory for Ocean Sciences
60 Bigelow Drive, P.O. Box 380
East Boothbay, Maine 04544
http://www.bigelow.org