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

[Edzer Pebesma]

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> 
>
> --------------------------------------------
>
>
>
>
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>
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>   

-- 
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/