[R-sig-Geo] Problem with cell numbers when using a raster extent object

[Ned Horning]

Robert,

Here are examples of the two versions.

### This one does not select points within the extent ###

library(raster)
crs_predImage <- CRS("+proj=utm +zone=33 +datum=WGS84 +units=m +no_defs 
+ellps=WGS84 +towgs84=0,0,0")
predImage <- raster(nrows=7561, ncols=8521, xmn=240585, xmx=496215, 
ymn=5297985, ymx=5524815)
projection(predImage) <- crs_predImage
predImage[] <- 1

commonExt <- extent(366760.6 , 425233.5, 5396052, 5450158)

sampleCells <- sampleRandom(predImage, size=20, sp=TRUE, extent=commonExt)

plot(predImage)
plot(commonExt, add=T)
plot(sampleCells, add=T)


### This one does work ###

library(raster)
crs_predImage <- CRS("+proj=utm +zone=33 +datum=WGS84 +units=m +no_defs 
+ellps=WGS84 +towgs84=0,0,0")
predImage <- raster(nrows=7561, ncols=8521, xmn=240585, xmx=496215, 
ymn=5297985, ymx=5524815)
projection(predImage) <- crs_predImage
predImage[] <- 1

commonExt <- extent(366760.6 , 425233.5, 5396052, 5450158)

sampleCells <- sampleRandom(crop(predImage,commonExt), size=20, sp=TRUE, 
ext=commonExt, na.rm=TRUE)

plot(predImage)
plot(commonExt, add=T)
plot(sampleCells, add=T)


On 11/25/2013 01:04 PM, Robert J. Hijmans wrote:
> Ned,
>
> Hard to see what does not work here. Perhaps it is just the [i] in
> xyFromCell(predImage, sampleCells[i])?
> Can you please provide a self-contained example, perhaps based on the below?
>
> library(raster)
> r <- raster(system.file("external/rlogo.grd", package="raster"))
> e <- extent(r) + 100
> s <- sampleRandom(r, size=100, cells=TRUE, extent=e)[,1]
> xy <- xyFromCell(r, s)
>
> plot(e)
> plot(r, add=T, legend=F)
> points(xy)
>
>
>
> On Mon, Nov 25, 2013 at 9:42 AM, Ned Horning <horning at amnh.org> wrote:
>> Hi -
>>
>> I have a script that calculates a common extent between two images than then
>> uses the resulting extent to restrict the random selection of cells from a
>> raster using the “sampleRandom” function. I use the following function:
>>
>> sampleCells <- sampleRandom(predImage, size=numSamps, cells=TRUE,
>> extent=commonExt)[,1]
>>
>> Next I use "xyFromCell(predImage, sampleCells[i])" to calculate the
>> coordinates of the center of a cell and all of the cell centers fall outside
>> of the extent that I am using. Using "sp=TRUE" I get a similar result - all
>> of the points are outside of the extent:
>>
>> sampleCells <- sampleRandom(predImage, size=numSamps, sp=TRUE,
>> ext=commonExt)
>>
>> To get around this problem I tried to physically clip the image before
>> running “sampleRandom” and the results are similar. The following line also
>> produces cells that are outside of the extent “commonExt” when I use
>> “xyFromCell”:
>>
>> sampleCells <- sampleRandom(crop(predImage,commonExt), size=numSamps,
>> ext=commonExt, na.rm=TRUE)
>>
>> However, if I run the following line using"clip" and "sp=TRUE" the sample
>> points fall within the extent:
>>
>> sampleCells <- sampleRandom(crop(predImage,commonExt), size=numSamps,
>> sp=TRUE, ext=commonExt, na.rm=TRUE)
>>
>> So, there is a work-around but I'd prefer not to have the overhead of
>> clipping the image.
>>
>> This seems somewhat similar to a problem I reported a few months ago and was
>> fixed. Can anyone replicate this problem? Am I doing something wrong?
>>
>> I am using R version: 3.0.2 and Raster package version: 2.1-49 on Ubuntu
>> 12.04 64 bit
>>
>> Ned
>>
>> _______________________________________________
>> R-sig-Geo mailing list
>> R-sig-Geo at r-project.org
>> https://stat.ethz.ch/mailman/listinfo/r-sig-geo