map algebra with ascii grid
As Roger mentioned, we did try to make the Spatial*DataFrame objects behave like a data.frame but indeed not with respect to applying direct arithmetric on them. I do agree with Tim that if a is a single-band SpatialGridDataFrame, then a = a + 1 is more straightforward than a$band1 = a$band1 + 1 although the latter does better reveal what's going on inside; the full story is of course that a at data$band1 = a at data$band1 + 1 which make things pretty unreadable. In addition, if b is a single-band SpatialGridDataFrame only a = a + b could allow for checking that a and b are identical spatial grids, in contrast to a$band1 = a$band1 + b$band1 So, yes, I like it, and it will break where a or b contain mixed double and factor variables, as it should. The full map algebra, as I have been taught, includes besides arithmetical and logical operation the full set of useful functions (for both floats and factors) that take neighbouring cells into account, where neighbourhood can be global, a regular window or defined by another map (e.g. the land use category we're in). For an example of a list of functions, see for instance the work of my Utrecht colleagues: http://pcraster.geog.uu.nl/documentation/pcrman/r2850.htm where e.g. distance calculation is called "spread" with a constant friction map. Now that would be nice to have in R. -- Edzer
Tim Keitt wrote:
Any algebraic operation supported by R can easily be applied to a GDAL image using rgdal -- size is virtually unlimited if you pull in data as raster lines or tiles. The approach is to create an output image and then use getRasterData and putRasterData to read chunks of data, operate on them, and then write back to disk. I can work up an example if anyone is interested. THK On 7/16/07, Denham Robert <Robert.Denham at nrw.qld.gov.au> wrote:
Philip,
this sounds like an interesting approach. Do you do anything in
particular to handle larger datasets? How large are your images?
Robert Denham
Environmental Statistician
Remote Sensing Centre
Telephone 07 3896 9899
www.nrw.qld.gov.au
Department of Natural Resources & Water
QScape Building, 80 Meiers Road, Indooroopilly Qld 4068
----------------------------------------------------------------------
Message: 1
Date: Sun, 15 Jul 2007 07:40:12 -0600
From: Philip Riggs <priggs at cnr.colostate.edu>
Subject: [R-sig-Geo] map algebra with ascii grid
To: r-sig-geo at stat.math.ethz.ch
Message-ID: <3513F1E2-D6E1-4B2B-89E8-D0B00C2804B7 at cnr.colostate.edu>
Content-Type: text/plain; charset=US-ASCII; delsp=yes; format=flowed
Dear R Users,
could you please advise me on doing map algebra with spatial grids?
It's the first time I am using spatial objects in R. I have imported
an ascii grid file and wanted to round the values in it or sum with a
value, but it gives an error message:
I use Python with Numpy/GDAL python bindings all the time to do spatial
algebra and manipulate grids.
It has all the math functions, and in the case of categorical data you
can reassign values. Then I
use rpy to do any statistical analyses in R. You could also do your
manipulation in Python, then
save the grid in an appropriate format for import into R to do the
statistical analyses.
Philip
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