[R-sig-Geo] Coercing multiband readGDAL objects to a matrix/vector...

[Michael Sumner]

Can you provide more detail as to what you mean?

If you want more than one band in the matrix you'll need to pre-allocate 
the result and use sub-indexing into that matrix (2D).

In one step you could access the bands from the Spatial*DataFrame 
directly as columns/vectors, but you'd have to handle the orientation 
specially.

Perhaps you actually want the 3 bands as a 3D array?

Here's an example of that.

library(sp)
data(meuse.grid)
m = SpatialPixelsDataFrame(points = meuse.grid[c("x", "y")], data = 
meuse.grid)

dimXY <- getGridTopology(m)@cells.dim  ## the X/Y dimensions of the result

## the 3D result
result <- array(NA, c(dimXY, 3))

## slot each band matrix into the 3D array
for (i in 1:3) {
    result[,,i] <- as.image.SpatialGridDataFrame(m[i])$z
}
## check the result

image(result[,,1])
image(result[,,2])
image(result[,,3])

Regards, Mike.

Jonathan Greenberg wrote:
> Michael:
>
>    Thank you for your prompt reply -- if I wanted to extract multiple 
> bands in a single call, how would I do that?  I suppose I could do it 
> with a for-next loop, but it seems there is probably some simple way 
> of doing this:
>
> as.matrix(x[1:3]) (doesn't work, but the idea is I want bands 1-3 in 
> this matrix).
>
> --j
>
>
>
>
> Michael Sumner wrote:
>> Hello,
>> You can use either as.imageSpatialGridDataFrame(x[1]) or 
>> as.matrix(x[1]), where "x" is a SpatialGridDataFrame or 
>> SpatialPixelsDataFrame.
>>
>> The "1" index assumes you want the first band, replace as appropriate.
>> The first gives you a list with x, y, z components in the convention
>> expected by image(), so
>> as.imageSpatialGridDataFrame(x[1])$z is the matrix.
>> The second returns the matrix directly but in a different orientation.
>> HTH
>>
>> Regards, Mike.
>>
>> ==============Original message text===============
>> On Fri, 23 Jan 2009 8:59:02 +1100 Jonathan Greenberg wrote:
>>
>> I was hoping I could get some info on how to coerce the type of 
>> object readGDAL produces, assuming its a multiband object, into an R 
>> matrix (I just want the raster values in the matrix)?  Thanks!
>>
>> --j
>>
>>   
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