[R-sig-Geo] Specify merging mode

[Etienne Bellemare Racine]

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
>>>
>>>        [[alternative HTML version deleted]]
>>>
>>> _______________________________________________
>>> R-sig-Geo mailing list
>>> R-sig-Geo at stat.math.ethz.ch
>>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>>>
>>>