Hi,
I'm trying to calculate a sum of differences between two ascii grids.
I have the file names of the two grid files in a data.frame along with
other data. I'm then trying to add a new column to the data.frame with
the result of that calculation.
eg.
library(maptools)
difcount <- function(surfA, surfB) {
A <- readAsciiGrid(surfA, colname="a")
B <- readAsciiGrid(surfB, colname="b")
sum(A$a - B$b)
}
attach(mydataframe)
difcount(surfA[1],surfB[1]) #this test part works fine and gives me
the correct result
#But when I try to do a batch job by running the function against each
row of the table it fails
mydataframe["TotalDif"] <- difcount(surfA,surfB)
Error in file(fname, "r") : invalid 'description' argument
This error seems to be coming from the readAsciiGrid function, but its
being given the same entry so I don't understand why it fails. Any
ideas?
Thanks,
Colin
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