Allan,
Be aware of the distinction between r.surf.idw and r.surf.idw2. The latter
computes the distance to ALL non-zero cell values for EACH interpolated cell
value. r.surf.idw uses a matrix search algorithm which attempts to minimize
the number of distances calculated. The two utilities should give nearly
identical results (tie-breaking may vary), but when non-zero cells are
numerous, r.surf.idw will be faster, i.e. the cost of the matrix search will be
less than the cost of exhaustive distance comparisons.
Nevertheless, check r.surf.contour. Either r.surf.idw* function will produce a
discontinuous elevation gradient for the reasons you mention in question 1
unless the number of nearest neighbors included is set somewhat high.
Inverse distance-weighted interpolation is probably a poor choice for your
objective. It is better applied to quasi-random point data, such as
temperature measurements from meteorological stations.
greg
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Greg Koerper Internet: greg@heart.cor.epa.gov
ManTech Environmental Technology, Inc. UUCP: hplabs!hp-pcd!orstcs!koerper
200 SW 35th St., JSB
Corvallis, OR 97333 "The 90s will make the 60s look like the 50s.
(503) 754-4490 Just ask your kids."
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