Patrick,
Natgrid does not have a feature to do what you want. The only
parameter that controls extrapolation is "ext" which controls
whether extrapolation is allowed outside the convex hull (such
extrapolation is allowed in the default environment). As you
probably know you are on shaky ground when you extrapolate.
Estimating how a function will behave when the only knowledge
is in one direction works well only when the function is simple
and consistent.
The algorithm that Natgrid uses to extrapolate is as follows.
It determines three points in the function domain space such that
the triangle that connects these three points contains all points
in the domain of the input function (these are called pseudo-points);
it then calculates a plane that is a least-squares fit to the input
data values; it then assigns function values for the pseudo-points
such that these values lie on the least-squares fitted plane; it
then considers the pseudo-points and function values as if they were
in the original dataset. In effect extrapolated values asymptote to
the least-squares fitted plane.
Fred Clare
---------------------------------------------
Fred Clare fred AT unknown 303-497-1284
Visualization & Enabling Technologies Section
NCAR Scientific Computing Division
---------------------------------------------
> Is it possible to use the extrapolation mode while telling
> it (the natural neighbor interpolation code) that for all extrapolated point
> which are beyond a certain range (say 50 meters) of the convex hul (the
> polygon created by the data I'm using as input for the natural neighbor
> operation), no extrapolation should be done, the default "no data" value
> should be used for those points?
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