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## 255 results from messages in new_ai_geostats

• ### [ai-geostats] Re: about cross-validation

...variogram model. You can start with my 1986 paper, "The art of cross validation", which can be downloaded at http://uk.geocities.com/drisobelclark/resume (follow publications link) and/or the 1979 paper "Does geostatistics work?", Isobel "M. Nur Heriawan" wrote: Dear everyone...
Isobel Clark Apr 27, 2006
• ### [ai-geostats] Re: Puzzling question

Hi. At last one I can answer (i.e. an easy one): If the phenomenon is non-stationary then the semi-variogram is NOT: (1) 2\gamma(x,x')=Var[Y(x)-Y(x')]^2 but (2) 2\gamma(x,x')=Var[(Y(x)-m(x))-(Y(x')-m(x'))]^2 The form (1) is a simplification of the full definition (2) when m(x)=m(x'). This is why, if you do have a non-stationary mean, you get a parabola added to the expected shape...
Isobel Clark Apr 19, 2006
• ### [ai-geostats] Re: Regional estimation - block kriging or conditional simulation?

Tom > Would it be wise to state that if you only want the mean and variance = > use block kriging, if you want a pdf = use conditional simulation? Oh, yes, please do. There are two ways to apply "discretisation". One is to estimate each of the fine grid of points and store the weights for each sample. You can then aggregate (average) the estimates and compute the estimation...
Isobel Clark Mar 15, 2006
• ### [ai-geostats] RE: kriging without a nugget

Hello all The real issue here is not what your philosophy is but what your software does with the semi-variogram model at zero distance. There are (to my knowledge) two possibilities in current software packages: (a) force the model to go through zero at zero distance, that is gamma(0)=0 (b) allow the model to hit the vertical axis, that is gamma(0)=nugget effect Option (a) makes...
Isobel Clark Mar 7, 2006
• ### [ai-geostats] Software for Automatic Semivariogram Estimation

Thanks Behrang, I see you are using Cressie weights. Isobel http://uk.geocities.com/drisobelclark Behrang Kushavand wrote: Dear Prof. Clark Here is the paper: http://www.ansinet.org/fulltext/jas/jas581405-1407.pdf Formula (4...
Isobel Clark Mar 2, 2006
• ### [ai-geostats] Re: Software for Automatic Semivariogram Estimation

Hi All It is difficult to have an automatic best fit semi-variogram until you define what you mean by "best fit". Noel Cressie's goodness of fit statistic goes a long way towards the ideal, but is very insensitive to changes in nugget effect and pretty insensitive to fairly large changes in the ranges. Optimal Cressie fits aren't always optimal visually, either. None of the...
Isobel Clark Mar 1, 2006
• ### [ai-geostats] Re: Software for Automatic Semivariogram Estimation

Behrang What weighting do you use in the weighted least squares? Isobel http://www.kriging.com Behrang Kushavand wrote: hi, I have a software for Variogram AUTO Modeling (winvam) that works with gslib (GAMV.exe). First you must calculate experimental variogram (omni or directional) with gamv.exe and then by using winvam, you can fit the best model by least square and weights least...
Isobel Clark Mar 1, 2006
• ### [ai-geostats] Re: Software for Automatic Semivariogram Estimation

Ed I use the Cressie statistic to four significant figures as a guide in the interactive fitting, but generally end up using a visual judgement. It tracks as you drag the model around, so you can watch it change. I think the 'real' visual objective function is probably the perpendicular (to tangent) distance to the model line, which is effectively the combination of both gamma and...
Isobel Clark Mar 1, 2006
• ### Re: [ai-geostats] spherical model

...using something like Cressie's goodness of fit test (analagous to a sort of chi-squared statistic). Isobel http://uk.geocities.com/drisobelclark "M. Nur Heriawan" wrote: Dear list, I have small query. Why almost all kind of spatial data set (ore grade data, sea surface temperature...
Isobel Clark Feb 24, 2006
• ### [ai-geostats] Re: Some simple questions

Jan, you sent this to me personally not to the list - although you may have posted it earlier to the list and I din't see it. You lost the right to my response when you turned down my invitation 12 years ago. Your relentless attempts to denigrate a subject simply because you do not understand it does not improve the situation. I repeat my invitation. Come down to Reno, sit in my...
Isobel Clark Feb 24, 2006