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## odd griddata error message

 From: Søren Hauberg Subject: odd griddata error message Date: Tue, 13 Jun 2006 23:03:56 +0200

```Hi,
I'm using griddata from octave-forge in octave 2.1.72 to interpolate
unevenly spaced data. However, I get the following long error message
from QHull. I don't know what this message means, so if anybody has some
insight I would be very interested...

Søren

qhull precision error: initial simplex is not convex. Distance=2.1e-17

While executing:  | qhull d Qbb T0
Options selected for Qhull 2003.1 2003/12/30:
delaunay  Qbbound-last  _pre-merge  _zero-centrum  Pgood  _max-width
1.6e+02
Error-roundoff 2.2e-13  _one-merge 1.6e-12  _near-inside 7.8e-12
Visible-distance 4.4e-13  U-coplanar-distance 4.4e-13  Width-outside
8.9e-13
_wide-facet 2.7e-12

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 points:
- p588 (v3): 1.2e+02 1.6e+02 1.6e+02
- p469 (v2):     1 1.6e+02 1e+02
- p119 (v1): 1.2e+02     1    57
- p0 (v0):     1     1 5.6e-19

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 2.2e-13.  The center point, facets and distances
to the center point are as follows:

center point     60.5     80.5     79.5

facet p469 p119 p0 distance= -4.4e-15
facet p588 p119 p0 distance= -4.4e-15
facet p588 p469 p0 distance= -7.7e-15
facet p588 p469 p119 distance= -7.7e-15

These points either have a maximum or minimum x-coordinate, or
they maximize the determinant for k coordinates.  Trial points
are first selected from points that maximize a coordinate.

The min and max coordinates for each dimension are:
0:         1       120  difference=  119
1:         1       160  difference=  159
2:  5.565e-19       159  difference=  159

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 2.2e-13.
- trace execution with 'T3' to see the determinant for each point.

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
error: invalid column index = 2
error: evaluating argument list element number 1
error: evaluating assignment expression near line 75, column 7
error: evaluating if command near line 55, column 3
error: called from `griddata' in file
`/usr/share/octave/site/m/octave-forge/geometry/griddata.m'
error: evaluating assignment expression near line 101, column 15

```