Gradient calculation with ESRI grid (.asc) format

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You could try the focalFilter function in the raster package:
install.packages("raster", repos="http://R-Forge.R-project.org")

with this argument for the Sobel filter:
filter=matrix(c(1,2,1,0,0,0,-1,-2,-1) / 4, nrow=3)

(For using raster this way you should upgrade to R 2.9 first, if you
have not done so yet; otherwise you will get an old version; this is
an unpleasant feature of R-forge)

Robert
I'm looking to calculate gradients from oceanographic variables (ie. sea
surface temperature, chlorophyll, etc.) for which I have downloaded in ESRI
grid format (.asc). ?From some research on gradient analysis, I have found a
few people who have used a Sobel filter to estimate gradients in images (ie.
presumably .tif or similar files). ?However, I have tons of data in ESRI
grid format, and all of it is unprojected (ie. lat/lon coordinates). ?I'd
appreciate suggestions on this if anyone has ideas. ?One of the primary
issues to overcome is that the units of my grids are different (ie.
x=longitude, y=latitude, z=temperature) and my grid files span the entire
south Pacific Ocean.

Thanks,

Tim

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