[GRASSLIST:5240] Local density analysis script for GRASS 5.7

[Michael Barton]

Here is something I've wanted to do for awhile.

This script calculates Ian Johnson's Local Density Analysis for vector
points. It is used in archaeology to examine the degree of clustering at
varying neighborhood radii of sites or artifacts or the degree of clustering
of one artifact class in the vicinity of another artifact class. It is a
nice routine, but rarely available.

I've been futzing with this for awhile in GRASS and finally worked out how
to do it. The problem was determining the number of points within a given
radius of a point. This should not be difficult in a GIS like GRASS, but it
is surprisingly tricky. R.neighbors doesn't have a count option and hacking
it by giving all relevant cells a cat of one and doing a sum doesn't give a
correct answer. Buffering also doesn't give the same answer as summing the
count of point around each point as the latter can count some points twice
(correctly), whereas buffering only will count them once. V.neighbors
produces an output that is NOT a count of points around each point but
something quite different. Anyway, I was able to use v.distance with the -a
and -p flags to get this. This method may be useful for other kinds of
similar clustering or neighborhood analyses.

For simply looking at the degree of clustering in a points file, use the
same file for the base and analysis files. For looking at the degree of
clustering for points of type 2 in the neighborhood of points of type 1, use
type 1 for the base file and type 2 for the analysis file.

You can select minimum and maximum neighborhood radii, along with a value to
increment the neighborhood radius from minimum to maximum.

The main output is in a text file, to which you can append a series of
analyses. For a single analysis, you can also have a line graph of the
results displayed. 

I've used this kind LDA with an algorithm described by Ian Johnson for
MapInfo and in Keith Kintigh's archaeological analysis program. The results
look correct here. But I'd appreciate it if anyone cares to test it and let
me know what they think.

I'll post this on my website in the script exchange area.

Cheers and Happy New Year
Michael Barton
____________________
C. Michael Barton, Professor of Anthropology
School of Human Evolution and Social Change
PO Box 872402
Arizona State University
Tempe, AZ  85287-2402
USA

Phone: 480-965-6262
Fax: 480-965-7671
www: <www.public.asu.edu/~cmbarton>

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