Hi Julia, initial variogram values just need to be sufficiently reasonable from where to start fitting. I know about some known rule-of-thumb:
+ sill: the total variance of your data; + range: half the diagonal of your bbox; + nugget: the measurement error. You can also decide whether to keep some of these parameters fixed, instead of being shifted during model fitting: see arguments in ?fit.variogram! Make sure you have at least ~30 pairs of points per lag, and 10 lags minimum. You might also want to increase the resolution of the lags at short distances, check the `boundaries` arg of variogram()). Regarding anisotropy, you can read Notes and Examples in ?vgm! Piero On 25 September 2012 10:57, Julia Chacon Labella <juliacha...@gmail.com>wrote: > Please excuse these basic questions, but I am a bit lost in geostatistics. > > I am trying to make a kriging of the soil properties of a plot. I am > already using gstat with R and I don't have any problems with the scripts > and so on...but, I don't really know how to choose the nugget, psill and > range for the variogram model. Moreover, I am not sure about how to include > the anisotropy in the model, I have already include a linar model in some > cases, to remove any trend. > > I am following the steps recomended in the gstat help, but I am a bit > befuddled. > > Let me show you my scripts (the psill, range and nugget are choosen just to > make the test). > > coordinates(corg)= c("x", "y") > > v = variogram(SOC~1, corg) > > SSErr.a <- numeric(4) > names(SSErr.a) <- c("Exponencial","Esférico"," > Gausiano","Lineal") > v.fit = fit.variogram(v, vgm(psill=0.18, model="Exp", range=5, nugget=0.1)) > SSErr.a[1] = attr(v.fit, "SSErr") > v.fit = fit.variogram(v, vgm(psill=0.18, model="Sph", range=5, nugget=0.1)) > SSErr.a[2] = attr(v.fit, "SSErr") > v.fit = fit.variogram(v, vgm(psill=0.18, model="Gau", range=5, nugget=0.1)) > SSErr.a[3] = attr(v.fit, "SSErr") > v.fit = fit.variogram(v, vgm(psill=0.18, model="Lin", range=5, nugget=0.1)) > SSErr.a[4] = attr(v.fit, "SSErr") > SSErr.a > > v = variogram(SOC~x+y, corg) > > SSErr.b <- numeric(4) > names(SSErr.b) <- c("Exponencial","Esférico","Gausiano","Lineal") > v.fit = fit.variogram(v, vgm(psill=0.18, model="Exp", range=5, nugget=0.1)) > SSErr.b[1] = attr(v.fit, "SSErr") > v.fit = fit.variogram(v, vgm(psill=0.18, model="Sph", range=5, nugget=0.1)) > SSErr.b[2] = attr(v.fit, "SSErr") > v.fit = fit.variogram(v, vgm(psill=0.18, model="Gau", range=5, nugget=0.1)) > SSErr.b[3] = attr(v.fit, "SSErr") > v.fit = fit.variogram(v, vgm(psill=0.18, model="Lin", range=5, nugget=0.1)) > SSErr.b[4] = attr(v.fit, "SSErr") > SSErr.b > > Thanks a lot, > Cheers. > > -- > > Julia Chacon Labella [PhD candidate] > > Área de Biodiversidad y Conservación > > Escuela Superior de Ciencias Experimentales y Tecnología > > Universidad Rey Juan Carlos > > c/ Tulipán, s/n. > > E-28933 Móstoles (Madrid) > > España > > > -- > Julia > > [[alternative HTML version deleted]] > > > _______________________________________________ > R-sig-Geo mailing list > R-sig-Geo@r-project.org > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > > [[alternative HTML version deleted]]
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