[R-sig-Geo] Interpolcation option: IDW or OK?

[Yong Li]

Many thanks for the message, Edzer.

-----Original Message-----
From: Edzer Pebesma [mailto:edzer.pebesma at uni-muenster.de] 
Sent: Monday, 9 February 2009 7:08 PM
To: Yong Li
Cc: r-sig-geo at stat.math.ethz.ch
Subject: Re: FW: [R-sig-Geo] Interpolcation option: IDW or OK?

Yong Li wrote:
> Hi Edzer,
>
> I would say the spatial structure is regarded not significant when
c0/c0+c1 is very much greater than 75%. In my case I used even distance
intervals and calculated c0/c0+c1 for log(OLSENP) greater than 85%. I
knew this index sometimes is very fragile, very much depending on how we
fit the model.
>
> However when I zoomed in by using variable distance intervals
(boundaries=c(100,200,300,400,600,900,1000,1500,2000))and maxdist=2000
meters, I found a pretty good model-fitted experimental variogram. But
the local OK interpolation using such a fitted model did not make sense
when compared the predictions to the observations as in most areas
values of OLSENP were severely underestimated. You may have seen my code
with which I have tried the nested models, but unfortunately no luck
either. I maybe think the parameters for local ordinary kriging are not
optimized, but I have tried lots of sets of nmin, nmax and maxdist and
did see the hopeful end.
>
> The journal editor insists in OK being better than IDW. I need to
collect my evidence to defend my IDW choice. That is my intention raised
such a question in our forum here.
>   
I cannot find evidence in your data for such a claim; the cross 
validation statistics (rmse) seem to favour OK with your nested model.

YONGLI==================================================================
====
Could you have a look at the predictions generated by my nested model
and compare to the observations? The values were severely
underestimated. 
YONGLI==================================================================
====

In your first email, you stated the following:
>> Normally if we do not find a significant spatial structure for a soil
>> variable, we may choose IDW or other methods. 
What is the argumentation behind this? Who claimed this?

YONGLI==================================================================
====
Could you have a look at "Mueller et al. (2004) Map Quality for Ordinary
Kriging and Inverse Distance Weighted Interpolation. Soil Sci. Soc. Am.
J. 68:2042-2047."? 
YONGLI==================================================================
====

Regards,

Yong Li