[R-sig-Geo] What is the reason for Very high value by Universal Kriging based on Nested 3D varigram

[Jon Skoien]

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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> Sent from the R-sig-geo mailing list archive at Nabble.com.
>
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-- 
Jon Olav Skøien
Joint Research Centre - European Commission
Institute for Environment and Sustainability (IES)
Climate Risk Management Unit

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