[R-sig-Geo] variogram question

[Edzer Pebesma]

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
>
>   

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