rasterizing digitized contour maps

[Greg Koerper]

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 at heart.cor.epa.gov
ManTech Environmental Technology, Inc.  UUCP:     hplabs!hp-pcd!orstcs!koerper
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