On 06/12/2010 09:11 AM, v v wrote: > Hi, Paul, > > Thank you for the reply. > > I have attempted to write some code to test out what you've suggested but ran > into some errors. > > library(fields) > k <- Krig(ozone$x, ozone$y, theta=20) > > # define objective function > kfunc <- function(para,k) { > xcoord=para[1] > ycoord=para[2] > > minimum <- NULL > The command above is not needed. > testpoint <- c(xcoord, ycoord) > minimum <- predict(k, testpoint) > btw, this predict command fails. I have no experience with Fields and no time right now to find out why. > return(minimum) > } > > initialpar <- c(max(ozone$x), max(ozone$y)) > initialpar > > # optimization > best <- optim(initialpar, kfunc, NULL, method = "BFGS", hessian=TRUE) > The costfunction for optim needs k as an input, which you didn't pass on. This passes on k:
best <- optim(initialpar, kfunc, NULL, k = k, method = "BFGS", hessian=TRUE) But as I mentioned above, the predict for Fields still does not work. cheers, Paul > > # Error in predict(k, testpoint) : argument "k" is missing, with no default > > > regards, > Jeff > > ---------------------------------------- > >> Date: Fri, 11 Jun 2010 15:04:32 +0200 >> From: p.hiems...@geo.uu.nl >> To: vivac...@hotmail.sg >> CC: r-sig-geo@stat.math.ethz.ch >> Subject: Re: [R-sig-Geo] How to locate local minima points on a Krige-fitted >> surface? >> >> Dear Jeff, >> >> I think you cannot escape discretizing your kriging surface as there is >> no mathematical expression that captures the kriging surface. To use >> optim you could make an objective function that was two inputs, x and y >> location. The functions calls the fields package to estimate the kriging >> prediction at this point and returns it. This ofcourse assumes that the >> variogram model is known. You have to play with the settings of optim to >> get it working and prevent a local minimum. You could take a look at >> SANN (simulated anealing) which is part of optim. >> >> cheers, >> Paul >> >> > _________________________________________________________________ > Hotmail: Powerful Free email with security by Microsoft. > https://signup.live.com/signup.aspx?id=60969 > -- Drs. Paul Hiemstra Department of Physical Geography Faculty of Geosciences University of Utrecht Heidelberglaan 2 P.O. Box 80.115 3508 TC Utrecht Phone: +3130 274 3113 Mon-Tue Phone: +3130 253 5773 Wed-Fri http://intamap.geo.uu.nl/~paul http://nl.linkedin.com/pub/paul-hiemstra/20/30b/770 _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo