[R-sig-Geo] why are idw and gstat different? [SEC=UNCLASSIFIED]

[Jin.Li at ga.gov.au]

Edzer,

Thanks for identifying the problem. The problem showed how big the impact of
the choice of nmax value on the estimation is.

In both krige and gstat, the default value for nmax is Inf. I am doing a
simulation experiment now to assess the performance of several spatial
interpolators using a few hundreds datasets. It is easy to do it by using the
default value. But I am wondering what the best guess for nmax is?

I assumed that the default value Inf for nmax would take the maximum value of
the number of samples available. After a few trials, I found my assumption
was wrong as evidenced below. Obviously, it took a value between 20 and 30. 
> system.time(x <- krige(log(zinc)~1, meuse, meuse.grid, model = m))
[using ordinary kriging]
   user  system elapsed 
   0.32    0.00    0.31 
> system.time(x <- krige(log(zinc)~1, meuse, meuse.grid, model = m, nmax=20))
[using ordinary kriging]
   user  system elapsed 
   0.25    0.02    0.27 
> system.time(x <- krige(log(zinc)~1, meuse, meuse.grid, model = m, nmax=30))
[using ordinary kriging]
   user  system elapsed 
   0.41    0.02    0.43
> system.time(x <- krige(log(zinc)~1, meuse, meuse.grid, model = m, nmax=50))
[using ordinary kriging]
   user  system elapsed 
   0.89    0.00    0.89 

Please clarify this. 
Thanks,
Jin 

-----Original Message-----
From: Edzer Pebesma [mailto:edzer.pebesma at uni-muenster.de] 
Sent: Tuesday, 8 July 2008 5:35
To: Li Jin
Cc: r-sig-geo at stat.math.ethz.ch
Subject: Re: [R-sig-Geo] why are idw and gstat different? [SEC=UNCLASSIFIED]

Jin, you specified nmax = 7 in the second call, but not in the first. 
Comparing idp values is easiest when other things remain equal.
--
Edzer

Jin.Li at ga.gov.au wrote:
> Dear there,
>
> I tried to compare idw and gstat in library(gstat). I specified idp as 0.5
in
> both functions, I expected identical results, but what I got are different
as
> shown for the first five samples.  Please help. Thanks.
>
>  
>
>   
>> data(meuse)
>>     
>
>   
>> coordinates(meuse) = ~x+y
>>     
>
>   
>> data(meuse.grid)
>>     
>
>   
>> gridded(meuse.grid) = ~x+y
>>     
>
>   
>> x <- idw(zinc~1, meuse, meuse.grid, idp=0.5)
>>     
>
> [inverse distance weighted interpolation]
>
>   
>> as.data.frame(x)[1:5,]
>>     
>
>   var1.pred var1.var      x      y
>
> 1  482.8753       NA 181180 333740
>
> 2  487.0065       NA 181140 333700
>
> 3  483.9747       NA 181180 333700
>
> 4  481.2115       NA 181220 333700
>
> 5  494.8720       NA 181100 333660
>
>  
>
>   
>> data(meuse)
>>     
>
>   
>> data(meuse.grid)
>>     
>
>   
>> meuse.gstat <- gstat(id = "zinc", formula = zinc ~ 1, locations = ~ x + y,
>>     
>
> +         data = meuse, nmax = 7, set = list(idp = .5))
>
>   
>> meuse.gstat
>>     
>
> data:
>
> zinc : formula = zinc`~`1 ; data dim = 155 x 12 nmax = 7
>
> set idp = 0.5; 
>
> ~x + y
>
>   
>> z <- predict(meuse.gstat, meuse.grid)
>>     
>
> [inverse distance weighted interpolation]
>
>   
>> z[1:5,]
>>     
>
>        x      y zinc.pred zinc.var
>
> 1 181180 333740  626.3628       NA
>
> 2 181140 333700  645.9319       NA
>
> 3 181180 333700  629.7041       NA
>
> 4 181220 333700  615.1368       NA
>
> 5 181100 333660  682.3401       NA
>
>  
>
> Cheers,
>
>  
>
> Jin
>
> --------------------------------------------
>
> Jin Li, PhD
>
> Spatial Modeller/
>
> Computational Statistician
>
> Marine & Coastal Environment
>
> Geoscience Australia
>
>
>
> Ph: 61 (02) 6249 9899
>
> Fax: 61 (02) 6249 9956
>
> email: jin.li at ga.gov.au <mailto:jin.li at ga.gov.au> 
>
> --------------------------------------------
>
>
>
>
> 	[[alternative HTML version deleted]]
>
> _______________________________________________
> R-sig-Geo mailing list
> R-sig-Geo at stat.math.ethz.ch
> https://stat.ethz.ch/mailman/listinfo/r-sig-geo
>   

-- 
Edzer Pebesma
Institute for Geoinformatics (IfGI), University of Münster, Weseler
Straße 253, 48151 Münster. Phone: +49 251 8333081 Fax: +49 251 8339763
email: edzer.pebesma at uni-muenster.de http://ifgi.uni-muenster.de/