[R-sig-Geo] Specify merging mode

[Etienne Bellemare Racine]

Thanks 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)

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
>>
>>        [[alternative HTML version deleted]]
>>
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>>