[R-sig-Geo] Clipping a large image without reading in the entire image

[Andrew Niccolai]

Greetings Spatial R users/developers,

I was curious as to whether or not it would be possible to describe a
bounding box for a large image so that what is read into R would be just the
contents of the image inside the bounding box.  Conceptually, it would be
akin to "clipping" the image but without the image read into the program.
My issue is that I have a very large 3-band orthophoto of several hundreds
of hectares of forest (roughly 8MB image).  I am running an analysis of
LIDAR data that requires the user to select a smaller area of a highly
degraded lidar image.  I would like the bounding box of this LIDAR area to
be used to "clip" a georegistered orthophoto that complements the LIDAR
data.  However, the complete ortho image seems to crash my WindowsXP, R
v2.4.1 setup. When I take the bounding box coordinates into ERDAS Imagine
and subset the orthophoto the following code works fine to bring it into R:

## Read in Tif file
setwd(anypathway)
tif.image <- "Pre_Subset_Image.tif"
LIDAR.tif <- GDAL.open(tif.image)
dim(LIDAR.tif)
image(LIDAR.tif[,,1])

## This converts the rgdal class to an SpatialGridDataFrame class
LIDAR.tif.1 <- LIDAR.tif[,]
bbox(LIDAR.tif.1)
image(LIDAR.tif.1,main="")
mtext("Orthophoto of Preloaded Fusion Example", side=3, line = 3, font=3,
cex=1.25)


However, this would be impractical with the code that I am building to jump
out of R and subset images in Imagine (plus very expensive for the end
user).  Therefore, does anyone have any solutions for reading in an image
that would work in theory like the following:

setwd(anypathway)
tif.image <- "VERY_LARGE_IMAGE.tif"
Bounded.tif <- GDAL.open(tif.image, bbox=c(UpL.X, LwR.X, UpL.Y, LwR.Y))

By the way, I am not set on GDAL.open as the solution but it seems to work
well reading in Tifs & Jpegs.

Again, thanks in advance


Andrew Niccolai
Doctoral Candidate
Yale School of Forestry