It probably means that you have a spatial trend in your data. Remove any trend, then try again.
Dan Bebber Department of Plant Sciences University of Oxford South Parks Road Oxford OX1 3RB UK Tel. 01865 275000 > -----Original Message----- > From: Jose Luis Gomez Dans [mailto:[EMAIL PROTECTED] > Sent: 03 November 2004 16:19 > To: [EMAIL PROTECTED] > Subject: [ai-geostats] Understanding variograms > > > Hi all, > I'm new to the list, so bear with me :) > I'm trying to carry out some kriging of some (x,y,v) > data set on to a > regular grid. I have read some of the literature, and > I think that > this would be a good option (I have also examined the possibility of > interpolating using a TIN, but I feel a bit more > "safe" using a > statistical approach!). > > In order to do my kriging, I need to have a model > semi-variogram, and I > thought about using gstat for this (from the command > line, no R > interfaces, my files are just ASCII (x,y,z) triplets). > Given that my > data files are huge (of the order of millions of > points), I selected a > region of interest, and tried to produce a sample > semi-variogram, with > views to fitting a model. > > In gstat, I got something that resembles y=exp(x), > which looks > distinctly wrong. The semivariogram value is ever > increasing, and does > not reach a sill at all. This is strange, and rather > unexpected. I have > tried doing a log transform of the data, but the shape > of the curve is > mostly unchanged. > > I would be very grateful if anyone could shed any > light on what this > variogram means, how I could make progress modelling > it and so on. > > Many thanks! > Jos� > > -- > Jose L Gomez-Dans, Research Assistant > Bristol Glaciology Centre, Geographical Sciences/CPOM > University of Bristol, Bristol, UK > > > > > > > ___________________________________________________________ALL > -NEW Yahoo! Messenger - all new features - even more fun! http://uk.messenger.yahoo.com
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