-----BEGIN PGP SIGNED MESSAGE----- Hash: SHA1 did you read the documentation of argument fit.method in function gstat::fit.variogram?
On 09/04/2013 02:00 AM, Moshood Agba Bakare wrote: > Dear All, I tried to compare the result obtained by fitting the > spherical variogram model using fit.variogram and nls functions. > The large difference in the results is a great concern for me > knowing that the two functions use Weighted Least Squares (WLS) > approach for estimating parameters. The steps taken are as > follows: > > ## Fit empirical semivariogram using gstat empvar<-variogram(yield > ~ 1,canmod.sp,cutoff= 400,width = 25, Cressie=TRUE) > > # Fitting Spherical variogram model to sample variogram > > sph.var<- vgm(psill =130, model = "Sph", range = 65, nugget = 180) > sph.mod<-fit.variogram(empvar, model = sph.var) print(sph.mod) > > The result obtain from fitting the spherical model to sample > variogram is sph.mod model psill range 1 Nug > 230.917736411 0.0000000000 2 Sph 108.323055319 87.6889385431 > > The non-linear least squares (NLS) approach use by default s > Gauss-newton algorithm in an iterative search process. I used the > initial values obtained from the empirical variogram above > (psill=130, range=65, and nugget=180) as starting values for the > iterative procedure. > > ## Define Spherical Variogram functions for NLS > > sph.vgram <- function(dist, range, psill, nugget){ dist <- > dist/range nugget + psill*ifelse(dist < 1, (1.5 * dist - 0.5 * > dist^3),1) } > > ## Fitting spherical with NLS > > fit.var <- nls(gamma~sph.vgram(dist,range,psill,nugget),data = > empvar, start=list(psill=130, range=65, nugget=180),trace=T) > > The result obtained from the nls fitting is > > Nonlinear regression model model: gamma ~ sph.vgram(dist, range, > psill, nugget) data: empvar psill range nugget > 90.7071423 342.1025007 278.9542178 residual sum-of-squares: > 1140.67875 > > Number of iterations to convergence: 18 Achieved convergence > tolerance: 9.79837651e-06 > > Could anyone explain why large difference in the two result? Is the > R script for fitting the NLS right? I am worried for having such > disparity. Thanks while looking forward to reading your suggestion, > comments and advice. Moshood. > > [[alternative HTML version deleted]] > > _______________________________________________ R-sig-Geo mailing > list R-sig-Geo@r-project.org > https://stat.ethz.ch/mailman/listinfo/r-sig-geo > - -- Edzer Pebesma Institute for Geoinformatics (ifgi), University of Münster Heisenbergstraße 2, 48149 Münster, Germany. Phone: +49 251 83 33081, Fax: +49 251 8339763 http://ifgi.uni-muenster.de -----BEGIN PGP SIGNATURE----- Version: GnuPG v1.4.11 (GNU/Linux) Comment: Using GnuPG with Thunderbird - http://www.enigmail.net/ iQEcBAEBAgAGBQJSJw4gAAoJEM1OCHCtOnfxjeMH/0CmRhzIYBhS44vTS4617G7G h8E/owvIS9wWMBEDe85qRhnnDSHOu8NRpcsO0QFNIYjzGtzf6Crporf9RAvAbdkO 0m+3ZDQgqIOmwmDp+svQ41f9HvxDcszJWv/ijuSjIrFG5cRG6PzgYKwamwTw5miR iVTUV2Syhxzpb72Or3TuadcBLF2JFEMMt/r3CMGZ2CszdIKSddMZDPf5ki0co3V3 nyR0pYNfq4dJdKzvTH8u0R9C6pCnTlYRoUFHfss3SSQfgbVEwZinVt2nBoUE4m41 ZSrYWLFkUVBOEHHbfGWb+WOc9Moz1ApCygmX5GYnPR9HZuPKHHcSgU2ErLRD30w= =Bd64 -----END PGP SIGNATURE----- _______________________________________________ R-sig-Geo mailing list R-sig-Geo@r-project.org https://stat.ethz.ch/mailman/listinfo/r-sig-geo