[R-sig-Geo] variogram question

[Paul Hiemstra]

...in addition, any feedback on the package would be more than welcome!

Paul

Edzer Pebesma schreef:
> In general: no, in special cases: yes.
>
> Fitting variograms involves non-linear regression for most models 
> (Sph, Exp, Gau, ...) for the range parameter, so you need starting 
> values. Given the initial range, linear regression is sufficient to 
> find the nugget/sill component(s), as they are linear. In principle, 
> gstat could be made simpler in that respect, I'd say.
>
> For an initial range, you could use some heuristics (20% of the 
> longest distance in your data?), but it is often not so hard to think 
> of cases where this would fail.
>
> Another issue is automatic values for the width and cutoff.
>
> You could have a look at package automap, by Paul Hiemstra, which 
> tries to do some of these heuristics--good or bad, who will tell.
> -- 
> Edzer
>
> Wesley Roberts wrote:
>> Dear r-sig-geo users,
>>
>> I am currently analyzing some Lidar data we have collected over our 
>> study area. I am interested in identifying the range of the 
>> semi-variogram as this value will determine the width of 
>> pseudo-flight lines I intend to use to sample the lidar data. Our 
>> point density is upwards of 5 points per square meter captured over 
>> even-aged managed Eucalyptus plantations with an espacement of 2 
>> meters between trees and 3 meters between rows.
>> I have imported an x,y,z data set containing canopy height and 
>> coordinates and successfully run the experimental variogram using the 
>> "variogram" module in gstat.
>> cpy.pts2 <- variogram(dbl_5 ~ 1, cutoff=50, width=2, D)
>>
>> I have also managed to fit several models using the
>> cpy.pts2.fit <- fit.variogram(cpy.pts2, model = vgm(2, "Sph", 4, 5))
>>
>> command as suggested by the gstat manual. I would like to fit the 
>> various models "Sph, Exp..." etc without having to specify the nugget 
>> psill and range. Essentially I would like an objective method to 
>> measure and record these values as I will be running several hundred 
>> variograms. Is it possible to perform this type of analysis using gstat?
>>
>> Many thanks for all your help and suggestions
>> Wesley
>>
>> Wesley Roberts MSc.
>> Researcher
>> Earth Observation (Ecosystems)
>> Natural Resources and the Environment
>> CSIR
>> Tel: +27 (21) 888-2490
>> Fax: +27 (21) 888-2490
>>
>> "To know the road ahead, ask those coming back."
>> - Chinese proverb
>>
>>   
>


-- 
Drs. Paul Hiemstra
Department of Physical Geography
Faculty of Geosciences
University of Utrecht
Heidelberglaan 2
P.O. Box 80.115
3508 TC Utrecht
Phone:     +31302535773
Fax:    +31302531145
http://intamap.geo.uu.nl/~paul