R help - May 2014 - Calculating the focal mean of a raster using an annulus

Hello esteemed R experts,

I am attempting the use the 'focal' function in the raster package to
calculate the mean of an annulus (as opposed to a focal mean of a circle or
square).
>From what I can tell, this requires me to generate a weights matrix and use
this matrix in the focal function.
The problem, however, is that there are edge effects because the weights to not
get readjusted along the boundary/edge of my raster. Therefore, the focal mean
of the annulus near the boundary of my rasters are lower than would be expected.

Code is below that repeats the problem. Please help if you can.

Thank you,
Sean Parks


##################################################################
##################################################################

library(raster)
the.raster <- raster(matrix(round(rep(seq(1,5, 0.0001), 250)), nrow=250,
ncol=250))

#############################################################
# Create annulus weights matrix
# There is actually an error in the weights matrix that I will figure out later
# Bonus points if you can identify and fix it
# This error does not affect the overall issue that I am attempting to address
# Please don't [publicly] make fun of me for this clunky code
#############################################################
w <- focalWeight(the.raster, d=0.025, type='circle')
w.rev <- w
for (i in 1:ncol(w)) {
               col <- w[,i]
               num.recs <- length(which(w != 0))

               first.rec <- match(1/num.recs, col)
               last.rec <- length(col) - (first.rec- 1)
               non.recs <- seq(1:length(col))
               non.recs <- non.recs[-c(first.rec, last.rec)]

               col[non.recs] <- 0
               w.rev[,i] <- col
}

count <- length(which(w.rev != 0))
w.rev[w.rev != 0] <- 1/count

###############################################################
# End of creating weights matrix
###############################################################

# Now make and view the annulus raster
# Note the edge effects in the top and bottom of the plot

annulus.raster <- round(focal(the.raster, w=w.rev, na.rm=T, pad=T))
plot(annulus.raster)

# END

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