about ST kriging

Dear all,

I am working about space-time kriging. Before I asked again about ST kriging
but now the problem is 
different and I could not overcome it!

During kriging operation I got this error:

pred = krigeST(prec~1, w, STS(grid,time,index), v)

Error in chol.default(A$Sm, LINPACK = TRUE) : 
  non-positive definite matrix in 'chol'

I explained the data set below:

# prec is annual precipitation measured from 257 meteorological stations

# w is STSDF object

summary(w)

Object of class STSDF
[[Spatial:]]
Object of class SpatialPoints
Coordinates:
         min     max
Xnew  140556 1730472
Ynew 4309064 4971309
Is projected: NA 
proj4string : [NA]
Number of points: 257
[[Temporal:]]
     Index                      object at time  
 Min.   :1970-01-01 12:00:00   Min.   : 1.0  
 1st Qu.:1979-07-03 00:00:00   1st Qu.:10.5  
 Median :1989-01-01 12:00:00   Median :20.0  
 Mean   :1989-01-01 02:46:09   Mean   :20.0  
 3rd Qu.:1998-07-03 00:00:00   3rd Qu.:29.5  
 Max.   :2008-01-01 12:00:00   Max.   :39.0  
[[Data attributes:]]
   Min. 1st Qu.  Median    Mean 3rd Qu.    Max. 
  114.5   413.7   533.6   633.0   735.6  3332.0 

# I used grid to obtain predictions over it. It has 25 km spatial resolution

# time is temporal entity of study. It contains 39 annual observation.

# index is required for STSDF, and I used this code to create index.

index <- cbind(as.integer(as.factor(data$Station)),

as.integer(as.factor(data$Year)))

# v is space-time variogram

How can I solve the problem?
Thank you in advance!


Pinar Aslantas Bostan
Research Assistant
Department of Geodetic and
Geographic Information Technologies (GGIT) Middle East Technical University
06531 Ankara/TURKEY
aslantas at metu.edu.tr