It is difficult for us to know exactly what happens as you are not
providing a reproducible example. But my guess would be that this is
related to your use of a Gaussian variogram without nugget for the first
part of the variogram. This can cause rather large weights (positive and
negative) if you have observations in close vicinity of each other,
leading to predictions far away from your range of observations. See if
a small nugget effect (~0.01 or less) can solve the problem, or if you
can change to a different variogram model.
Cheers,
Jon
On 12/4/2014 8:19 AM, Bingwei Tian wrote:
Dear list,
I am doing a 3D estimation of logtransfered subsurface temperature (with a
strong vertical trend) with a Nested 3D variogram, but the results show very
high value over than origin data.
This is not normal and absolutely wrong if I back transfer logged data.
I attached the
<http://r-sig-geo.2731867.n2.nabble.com/file/n7587514/vgm.png> and anyone
who knows what is the reason for the very high value differ from origin
data?
Or what kind of processing I should to do for the data back transfer? Thanks
in advance for any help.
Data:
summary((spdf$logt))
Min. 1st Qu. Median Mean 3rd Qu.
0.741937344729 3.487375077900 3.908014984030 3.952886346280 4.452019006490
Max.
5.733988316710
Model: Nested 3D varigram
uk.eye1 <- vgm(psill = 0.155, model = "Gau", range=700, nugget=0)
uk.eye <- vgm(psill = 0.125, model = "Sph", range=35000, nugget=0,
add.to=uk.eye1)
model psill range
1 Nug 0.000 0
2 Gau 0.155 700
3 Nug 0.000 0
4 Sph 0.125 35000
UK:
logt.uk <- krige(log(t)~z, spdf, grid, model = uk.eye, nmax = 20)
Result:
summary((logt.uk$var1.pred))
Min. 1st Qu. Median Mean 3rd Qu.
-1.66562650678 3.30346488250 3.76836777085 3.81376070431 4.24457939254
Max.
15.05945622140
-----
Bingwei
Ph.D. Student
Kyoto University
C-1-2-225, Katsura Campus, Kyoto University,
Nishikyo-ku, 〒615-8530, Kyoto, Japan
--
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--
Jon Olav Skøien
Joint Research Centre - European Commission
Institute for Environment and Sustainability (IES)
Climate Risk Management Unit
Via Fermi 2749, TP 100-01, I-21027 Ispra (VA), ITALY
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and do not necessarily represent official views of the European Commission.
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