Dear geoUseRs, New in the world of variograms, I ask for some advices !
My general question is : Is there a methodology to find the best distance to be used as a limit for the calculation of a semi-variogram ? Here is an example that is problematic for me : I have a 300 km by 300 km grid regularly spaced points containing water current speed. I first tested for spatial trend making a linear model between current speed value and coordinates. As there were a high effect, I removed this trend making my variogram on the residuals of the model. I calculated the semi-variogram on my variable from 0 to 300 km. The variance first increase, attain a local maximum then decrease, and re-increase acquiring a wave form, never really converging to a sill value ... 1) do you have ever met such a pattern ? And if yes what was the reason of it ? 2) I agree that value of points located at 300 km from each other are certainly not linked but how to know if there is no sill in the semi-variogram ? Do I have to consider the distance where the first decrease occurred in the semi-variance plot as the limit of the range ? 3) With that kind of shape what kind of variogram model would you recommend to fit on it ? _______________________________________________ R-sig-Geo mailing list R-sig-Geo@stat.math.ethz.ch https://stat.ethz.ch/mailman/listinfo/r-sig-geo
