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I'm trying to combine bioclim raster layers to create the following layer.

(Q) = AnnPrec/[(Tmax+Tmin)*(Tmax-Tmin)]*1000

For that, I import a bioclim stack:

 envirc = stack("C:/Users/Agus/Dropbox/wiens SL paper/clime.grd")

you can get the stack here: https://www.dropbox.com/sh/n8qfu9cmuowondp/AADcFHTnCqhhoYA43UeckVS-a?dl=0

separate specific layers

 AnnPrec=subset( envirc,"bio12") 
 Tmax=subset( envirc,"bio5") 
 Tmin=subset( envirc,"bio6")

I could use overlay function , but this seems to work

 den=((Tmax+Tmin)*(Tmax-Tmin))*1000

but now, this operation leads me to a constant value, which is incorrect, it # should be a range of values 

 Q=(AnnPrec/den)

using overlay function gives me the same value

 Q=overlay(AnnPrec,Tmax,Tmin, fun=function(x,y,z){return(x/((y+z)*(y-z))*1000)})

What I'm doing wrong?

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  • 1
    Any chance of an example that doesn't require a 1GB download? Or at least summary on your layers so we can be sure your layers aren't constant values. I just tried on three random number rasters and I get a raster with some varying random numbers in. So my money is on a problem related to your data...
    – Spacedman
    Jan 29, 2016 at 18:20
  • You state that ` this operation leads me to a constant value` but that is not correct. Do show(Q) and you will see the range of values. Because of the odd range of values (mostly very close to zero, but with some outliers, the default legend is not helpful. But see plot(log(Q)) or plot(Q, zlim=c(-0.0001, 0.0001)) (and fdetsch's answer)
    – Robert Hijmans
    Jan 31, 2016 at 19:57

1 Answer 1

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Hm, I don't really see the problem. overlay works just fine on my machine. Here's the code.

# (Q)=AnnPrec/[(Tmax+Tmin)*(Tmax-Tmin)]*1000
Q <- overlay(AnnPrec, Tmax, Tmin, fun = function(x, y, z) {
  x / ((y + z) * (y - z)) * 1000
}, filename = "q", overwrite = TRUE)

And here's the resulting image including the range of values.

range(Q[], na.rm = TRUE)
[1] -11629.63  12039.74

spplot(Q, at = seq(-500, 500, 10), 
       colorkey = list(space = "top", width = .6), 
       scales = list(draw = TRUE))

spplot

If that doesn't work, maybe you can provide the actual contents of AnnPrec, Tmax and Tmin as suggested by @Spacedman.

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