Math Forum / Math Software / MATLAB / July 2008

Hi all,

If you can help with the following that would be very useful;

I have several interpolated surfaces of the same region, however they
are on slightly different grids (as the loess interpolation uses the
min and max of the data set) and so the X,Y positions vary. I want to
compare the change in the surfaces i.e. the Z values, however i
obviously need corresponding X,Y positions for each surface.

To do this is have simply done the following;

[Xg,Yg]= meshgrid(XX , YY);                      %Defined a grid based
on pre-set co-ords
Zg = griddata (Xi,Yi,Zi,Xg,Yg);                     %Extract the data
from the loess interp surface (Xi,Yi,Zi)

So my aim is for all the surfaces to have the same X,Y positions and
extract the Z values for these points. This works sometimes, however i
also get the error at the base of this message, although there does
not seem to be an obvious reason as to why it works with some but not
others.

If anyone has an suggestions as to how i can do this in a better way
it would be very helpful.

Thanks

Ponch

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??? qhull precision error: initial facet 2 is coplanar with the
interior point

ERRONEOUS FACET:

While executing: | qhull d Qt Qbb Qc

Options selected for Qhull 2002.1 2002/8/20:
 delaunay Qtriangulate Qbbound-last Qcoplanar-keep _pre-merge
 _zero-centrum Pgood Qinterior-keep _max-width 3e+002
 Error-roundoff 6e-013 _one-merge 4.2e-012 Visible-distance 1.2e-012
 U-coplanar-distance 1.2e-012 Width-outside 2.4e-012 _wide-facet
7.2e-012

The input to qhull appears to be less than 3 dimensional, or a
computation has overflowed.

Qhull could not construct a clearly convex simplex from input points.

The center point is coplanar with a facet, or a vertex is coplanar
with a neighboring facet. The maximum round off error for
computing distances is 6e-013.

If the input should be full dimensional, you have several options that
may determine an initial simplex:
 - use 'QJ' to joggle the input and make it full dimensional
 - use 'QbB' to scale the points to the unit cube
 - use 'QR0' to randomly rotate the input for different maximum
points
 - use 'Qs' to search all points for the initial simplex
 - use 'En' to specify a maximum roundoff error less than 6e-013.

If the input is lower dimensional:
 - use 'QJ' to joggle the input and make it full dimensional
 - use 'Qbk:0Bk:0' to delete coordinate k from the input. You should
   pick the coordinate with the least range. The hull will have the
   correct topology.
 - determine the flat containing the points, rotate the points
   into a coordinate plane, and delete the other coordinates.
 - add one or more points to make the input full dimensional.

This is a Delaunay triangulation and the input is co-circular or co-
spherical:
 - use 'Qz' to add a point "at infinity" (i.e., above the paraboloid)
 - or use 'QJ' to joggle the input and avoid co-circular data
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