problem calculating the proportion of overlap between two geographic rasters

Hello list,

Im trying to calculate the porportion of overlap among two rasters. I think
i got it done, but when I apply the script on identical rasters, the
proportion obtained is not 1!
Someone could please explain me what I could be missing here?

Here comes a reproducible example:

library(raster)

#generate 2 identical  raster layers b and d that take values 0 and 1.

wgs84 <- "+proj=longlat +datum=WGS84 +ellps=WGS84 +towgs84=0,0,0"
d <- raster()
extent(d) <- extent(119.4993, 141.0993, -36.65831, -29.85831)
res(d) <- c(0.08, 0.08)
projection(d) <- CRS(wgs84)
values(d) <- sample(c(0, 1), ncell(d), replace=TRUE)
b=d

# calculate each layers' area
a=raster::area(d)
s=cellStats(a, 'sum')

#sum both layers, so as to have value 2 were both layers present value 1
soma=sum(b,d)
overl=sum(values(area(soma))[which(values(soma)==2)])# sum cell sizes of
cells with value 2 within soma

overl/s# i think this should give 1 but it does not.

# visualizing
library(maps)
map(regions = "australia")
image(d, add = TRUE, col = "blue")
image(soma, add = TRUE, col = "green")


Thanks in advance!
Agus
--
Agust?n Camacho Guerrero.
Doutor em Zoologia.
Laborat?rio de Herpetologia, Departamento de Zoologia, Instituto de
Bioci?ncias, USP.
Rua do Mat?o, trav. 14, n? 321, Cidade Universit?ria,
S?o Paulo - SP, CEP: 05508-090, Brasil.

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