I am new to gstat. As a introduction to the software I have been trying to
reproduce some results that I have obtained using GSLIB. Unfortunately, I
have not been able to obtain the same results. One of the problems that I
have had is gstat produces negative (in some cases very large) estimates
for values that should all be positive (I am working with concentration
data). I know that in some cases, negative values can be generated by the
'screening effect' by which large sample data receive a negative weight.
However, this did not occur with GSLIB. I did suspect that the problem was
caused by my specification of the variogram model. However, the model I am
using is very simple. I have included the gstat command file and the GSLIB
parameter file below.
I have also tried the following with no appreciable difference in the results:
1) increased search distance to 400 feet
2) Using force option to ensure the minimum number of sample observations
(4) were included in the interpolation
3) Tried changing minimum number of points to 2
4) Changed gstat block discretization from default (Gauss quadrature) to 4
x 4 regular block discretization
Any ideas what I am doing wrong?
I would also like to hear from others that have compared results from the
two programs.
Thanks,
Bill
GSTAT Command File:
#
# Change of support: local ordinary block kriging on a mask
#
data(lead): 'Site4dat.eas', x=1, y=2, v=3,
min=4, max=8, radius=200;
variogram(lead): 32000000 Exp(260);
mask: 's4okpred.asc';
predictions(lead): 'proks4.asc';
variances(lead): 'prvaoks4.asc';
blocksize: dx=10, dy=10; # define block dimensions
GSLIB parameter file:
********************************************************
* File generated by WinGslib - modify at your own risk
* Version: 1.3.0
* Build code: 283d374/1/1_4.1_2222
* Date: Friday, February 2, 2001
* Time: 2:24:55 PM
* Author: THAYER
* Computer name:
* Comment:
********************************************************
Parameters for KB2D
**************************
START OF PARAMETERS:
C:\SITE4Weighted.DAT
2 3 4 - columns for X, Y, variable
-1E+21 1E+21 -trimming limits
1 -debug level (0-3)
C:\SITE4OK200.dbg
C:\SITE4GSLIB.out
280 -1200 10 -nx, xmin, xsize
120 -100 10 -ny, ymin, ysize
4 4 -x and y block discretization
4 8 -Min and max Data for kriging
200 -Maximum search radius
1 0 -0=SK, 1=OK, (mean if SK)
1 0 -nst, nugget effect
2 32000000 0 260 260 -it, cd, azm, a_max, a_min
