need to calculate autocorrelation length of a matrix

[chaski]

Working with a n x n matrix of non-negative real values, I'm trying to
calculate a 2d analog of 'autocorrelation length' -- that is, I want to find
the minimum euclidean (or similar) distance at which, on average, two
entries in the matrix (separated by that critical distance) will show a
mutual correlation of less than 1/e (or 1/2, or some other such 'natural'
threshold).  

Analogous measures are used as metrics of landscape 'ruggedness' or
'roughness', I think, but I've looked in vain for an established method to
calculate them. Does anyone know of a script that could (perhaps with
tweaking at my novice level) be used to do so?

Thanks.
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