griddata problem

[Jeremy Ardley]

I'm attempting to generate a data grid from a set of x,y,z  data vectors into a UTM grid defined by two coordinate matrices..

I use the command

agrid = griddata(plotx,ploty,plotc,moadXGUTM,moadYGUTM)

where plotx, ploty, and plotc are 671 point vectors of the spot data points and moadXGUTM and moadYGUTM are matrices of UTM coordinates.

I have checked there is no duplication in any of the data vectors using unique. I have previously used this technique with a different data set.

I have confirmed all the spot data points fall within the grid area.

The error message includes this diagnostic section

>>>>>>>>>>>>>>

While executing:  | qhull d Qt Qbb Qc
Options selected for Qhull 2003.1 2003/12/30:
  delaunay  Qtriangulate  Qbbound-last  Qcoplanar-keep  _pre-merge
  _zero-centrum  Pgood  Qinterior-keep  _max-width 1.2e+04
  Error-roundoff 3.5e-09  _one-merge 2.5e-08  Visible-distance 7e-09
  U-coplanar-distance 7e-09  Width-outside 1.4e-08  _wide-facet 4.2e-08
  _narrow-hull -2.2e-16

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:
- p610 (v3): 2.4e+05 3.7e+06     0
- p365 (v2): 2.5e+05 3.7e+06 1.2e+04
- p670 (v1): 2.5e+05 3.7e+06 1e+04
- p305 (v0): 2.4e+05 3.7e+06    15

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 3.5e-09.  The center point, facets and distances
to the center point are as follows:

center point 2.453e+05 3.709e+06     5560

facet p365 p670 p305 distance= -5.2e-10
facet p610 p670 p305 distance= -7.3e-10
facet p610 p365 p305 distance= -1e-09
facet p610 p365 p670 distance= -4e-10

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:  2.395e+05  2.514e+05  difference= 1.194e+04
  1:  3.706e+06  3.713e+06  difference= 7286
  2:         0  1.194e+04  difference= 1.194e+04

>>>>>>>>>>>>

Is there anything special about the particular set of points?
Is there any other test I can do on the data to find potential problems?

I've read some of the answers to similar questions but nothing that helps me.

The version of Octave is

octave.x86_64           6:3.4.3-1.el6   @epel

Running on a Centos 6.4 64 bit system.

Thanks in advance.

--
Jeremy Ardley