[R-sig-Geo] How to calculate R2 coefficient of determination for variogram ?

[Sébastien Durand]

Hello, 

I found the following trend on the web, and I am facing the same problem.

I am trying to compute the r2 from my fit, but I end up with a negative value ... 

So I have two quick questions for you: 

1- What do you mean by  : "the same weighting scheme should have been used" and how would you code that?

2-I guess the code that was provide by Kili, should work for any variogram fit...

Thanks a lot for you time.

N.B.: I was unsure how to pursue this discussion on the web, so my apologies for contacting you directly!

Sébastien 


> It looks quite OK; I suspect however that instead of using
> mean(s.v$gamma) for SSTot, for this mean the same weighting scheme
> should have been used.
> --
> Edzer
> 
> On 09/02/2010 02:59 PM, 
> kili at grf.rs
>  wrote:
>>  Hi list,
>>  I'm not sure in my calculation of R2 coefficient of determination for
>>  variogram. Could someone check it, or give the better way.
>>  Here is an example:
>>  library(sp)
>>  library(gstat)
>>  data(meuse)
>>  coordinates(meuse) <- ~x+y
>>  s.v<- variogram(log1p(zinc)~sqrt(dist), meuse)
>>  vr.fit <- fit.variogram(variogram(log1p(zinc)~sqrt(dist), meuse), vgm(0,
>>  "Exp", 300, 1))
>>  ## SSErr is it summ of weighted squares of residuals ?
>>  (SSErr<-attr(vr.fit,"SSErr"))
>> 
>>  ## SStot total sum of weighted squares
>>  weig<-s.v$np/s.v$dist^2  #fit.method	 fitting method, used by gstat. The
>>  default method uses weights $N_h/h^2$ with $N_h$ the number of point pairs
>>  and $h$ the distance.
>>  (SStot<- sum(weig*(s.v$gamma-mean(s.v$gamma))^2)  )
>> 
>>  (R2<-1-SSErr/SStot    )    # R2 coefficient of determination
>>  Best regards,
>>  Kili
>> 
>