fitting a surface to widely spaced data

[Simon Cox]

Though idw _can_ be used for interpolation, it is really best used for
_smoothing_.

Use s.surf.tps for interpolation - it uses splines.

If you have any known discontinuities (ruptures?) you will have to deal
with them carefully to avoid the interpolated surface "ringing" near to the
tear.  You may have to use another way - typically by using masks.  Also,
check the mail archives for a LOT of chat about this.  Also, see my paper
at
http://www.dem.csiro.au/simon/snowys/12agc.cox.html
for an example covering some of this.

Simon Cox

At 10:56 PM 25/5/95 -0700, Eric Small wrote:
>Hi.
>
>I am trying to fit a surface to widely spaced data, that is concentrated in
>particular areas.  In other words, i have a ton of points in a cluster,
>but lots of space between clusters.  In general, the distance between
>each data cluster is on the order of several to 10km, while the
>grid spaceing is 50-100m.
>
>When i use the r.surf.idw (or 2) the results are rather ugly.
>In addition to being unattractive, the fitted surfaces does not make
>much sense either -- it has scarps and holes.
>
>Does anyone know a way to fit surfaces not using inverse distance weighting?
>How about fitting a 3rd or 4th order polynomial to the data?
>I know this can be done with other GIS software, but I am working with
>GRASS and have not found a way.
>
>Any help would be great!
>
>Eric Small

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