Thank you very much Sebastiano and Piere Goovaerts for your suggestions.
First of all I did not see any special trend. As Piere Goovaerts said I took
log of data and semivariogram of logarithm still show a moderate spatial
correlation however the nugget effect is higher. I have a sparse sampling of
data values with areas of high values located mostly in the north and south of
the area. I tried to divide the area to three more homogenous sub-areas. But
the semivariograms for these sub-areas show less spatial correlation than for
whole area.
What can I do now? By the way I already removed a few very suspicious values
from the data sat.
Should I stick with ordinary kriging only?
Regards
Suzaneh
Pierre Goovaerts <[EMAIL PROTECTED]> wrote: Hi Suzanne,
I am surprised that you don't obtain a well-structured indicator
variogram for the median threshold at least. This might indicate that
the structure you see in the variogram of raw data is caused by
a cluster of extreme values. These data are distinguished only
for extreme quantile thresholds, which should explain why you
don't see any correlation for middle thresholds.
I would suspect that taking the log of the data would also reduce
the structure you see on your variogram.
Hope it helps,
Pierre
Pierre Goovaerts
Chief Scientist at BioMedware Inc.
Courtesy Associate Professor, University of Florida
President of PGeostat LLC
Office address:
516 North State Street
Ann Arbor, MI 48104
Voice: (734) 913-1098 (ext. 8)
Fax: (734) 913-2201
http://home.comcast.net/~goovaerts/
________________________________
From: [EMAIL PROTECTED] on behalf of Suzanne
Sent: Mon 10/15/2007 4:03 AM
To: [email protected]
Subject: AI-GEOSTATS: Smoothness of indicator kriging over ordinary kriging
Dear list
I have a data set of highly positively skewed.
I tried to use indicator kriging to improve the estimation accuracy over OK.
But I found out some difficulties:
1- The omnidirectional semivariogram show a strong to moderate spatial
correlation whereas indicator semivariograms except for 0.1 and 0.8 quantiles
do not show any spatial correlation.
2- I tried to use some quantiles, which their indicator kriging show a weak
spatial correlation. I run IK with 5 possible cutoffs. The estimation accuracy
goes a little bit higher however the map produced using IK is much smoother
than OK.
I do not know why this happen? And what should I do now?
I really need help. Please let me know your opinion about that.
Best regards
Suzaneh
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