Specify merging mode

A correction to my previous example, a line was missing :

Thanks (again) Robert,

From what you showed, I tried to wrap the operations in a generic function
for my own purpose, but I don't know why, I can only apply some functions
e.g. a mean function is ok, but not a max function. Could you point what's
wrong ? What's the better way to apply a function to a raster list ? My
problem is that I have a variable number of rasters to merge.

r <- raster()
r1 <- crop(r, extent(-10, 10, -10, 10))
r2 <- crop(r, extent(0, 20, 0, 20))
r3 <- crop(r, extent(10, 30, 10, 30))

r1[] <- 1:ncell(r1)
r2[] <- 1:ncell(r2)
r3[] <- 1:ncell(r3)
rasters <- list(r1, r2, r3)

fmerge <- function(rasters, fun, ...){
?ex <- raster(unionExtent(rasters))
?res(ex) <- res(rasters[[1]])
?ex[] <- NA
? ?for( i in 1:length(rasters) )
? ? ?rasters[[i]] <- merge(rasters[[i]], ex)
? ?do.call( fun, c(rasters, ...) )
}

rfm <- fmerge(rasters, mean, na.rm=T) ?#ok
rfx <- fmerge(rasters, max, na.rm=T) ? #wrong

Etienne

Robert J. Hijmans wrote :

Dear Etienne,

This is not in raster (as a single function) but it is a good idea
that I'll look at (and merge needs to be improved for speed also). A
function that would blend overlapping layers based on distance to
their edges would also be useful.

Here are two ways in which you can accomplish what you asked for:

r <- raster()
r1 <- crop(r, extent(-10, 10, -10, 10))
r2 <- crop(r, extent(0, 20, 0, 20))

r1[] <- 1:ncell(r1)
r2[] <- 1:ncell(r2)

# the trick, first merge with empty RasterLayers
r1 <- merge(r1, raster(r2))
r2 <- merge(raster(r1), r2)

# Now combine with a function
rm = mean(r1, r2, na.rm=TRUE)
plot(rm)


# alternative solution:

r <- raster()
r1 <- crop(r, extent(-10, 10, -10, 10))
r2 <- crop(r, extent(0, 20, 0, 20))
r1[] <- 1:ncell(r1)
r2[] <- 1:ncell(r2)

x = (r1 + r2) / 2 ? # this returns the spatial intersection or r1 & r2
rm = merge(x, r1, r2)
plot(rma)

Robert


On Mon, Mar 29, 2010 at 1:39 PM, Etienne Bellemare Racine
<etiennebr at gmail.com> wrote:

Hi list,

In the raster package, is it possible to specify which way I want to
blend the two rasters. e.g. :

r1 <- raster(xmn = -10, ymn = -10, xmx = 10, ymx = 10)
r2 <- raster(xmn = 0, ymn = 0, xmx = 20, ymx = 20)

r1[] <- 1:ncell(r1)
r2[] <- 1:ncell(r2)

rm <- merge(r1, r2)
plot(rm)

You get the first raster on top (r1), but I would like to merge them
with a function.

I would like to use something like e.g. fun = mean or max. If it's not
in the raster package, is there any package that allow to merge images
and specify the way it merge.

Thanks,
Etienne

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