[R-sig-Geo] variogram using gstat

[Edzer Pebesma]

I don't know of any general guidelines here. I would settle with a 
reasonable number of point pairs (preferably over 20) per lag.
--
Edzer

PUJAN RAJ REGMI wrote:
> Dear sir,
> Thank you very much for the help. I would like to ask another question 
> regarding the "width" selection. For omni directional which 
> width should I choose in case when data set are in grid pattern?(in my 
> case 12m in each x and y dir. i.e. 16.97m in diagonal) I saw 
> selectiong different width (12 or 16.97m) resulted the plot varied 
> slightly which can be even vivid when i got the parameters for the 
> sill and range of the model semivariograms of such plots. I suppose 
> the 12 m would be the good choce as the curve will be more contninous 
> but in the same time ecah "dist" will have less no. of pairs but 
> tradeoff between no. pairs and  no."dist" should be done?? While for 
> directional semivariogram like in E-W and N-S is that necessary I 
> should choose 12m and in 45 and 135 16.97m?
> Thanking you
> Pujan
>  
>
>
> ------------------------------------------------------------------------
> > Date: Thu, 8 May 2008 08:10:48 +0200
> > From: edzer.pebesma at uni-muenster.de
> > To: regmi_pujan at hotmail.com
> > CC: thierry.onkelinx at inbo.be; r-sig-geo at stat.math.ethz.ch
> > Subject: Re: [R-sig-Geo] variogram using gstat
> >
> > From the gslib book, I understand that there is a lag, and a lag
> > tolerance, and a cutoff. lag 0 is 0, the lag 1 is <0,tol], lag 2
> > <lag2-tol,lag2+tol>, lag3 <lag3-tol,lag3+tol>, etc. This allows for
> > overlap in the lags, if tolerance is larger than half the difference
> > between lags.
> >
> > gstat uses regular lag (the first one is not shorter than the latter):
> > lag1 = [0,width>
> > lag2 = [width,2*width>
> > lag3 = [2*width,3*width>
> > up to the cutoff. So, there's no possibility of overlapping lags, and a
> > different organization of them.
> >
> > tol_hor is the horizontal direction tolarance, and has nothing to do
> > with width and cutoff.
> >
> > note that gstat does not have a bandwidth concept, so for comparison
> > purposes you should set this to a very large value.
> > --
> > Edzer
> >
> > PUJAN RAJ REGMI wrote:
> > >
> > > Dear sir,
> > > I spent lot of time to figured it out what causing the difference in
> > > result of R/gstat and gslib but didn't get any logical 
> explanation. In
> > > my opinion the source of difference is the cutoff and width.
> > > I would be thankful if you give me the in depth explanation of these
> > > two parameters used in gstat or you can suggest me the suitable value
> > > i should adopt for my case. In my data set the diagonal is 2771.89 so
> > > 1/3 of it is 923.96 i believe this is the default cutoff. but what
> > > about width, i don't get the logic of this function? where as in 
> gslib
> > > we have to define the limit to what level pairs will be compared and
> > > the distance between the pairs and its tolerance. for my case in case
> > > of directional semivariogram the distance between is 12m and 
> tolerance
> > > 6m and the extent is 923.96m that is i have to define numbers of lag
> > > as 923.96/12 ~77 nos. i didn't see the tolerance for lag in gstat, i
> > > think this has been done with build in script which incorporates with
> > > tol.hor command?? One more querry; is it correct to use same cutoff
> > > and width for omnidirectional and directional semivariogram
> > > (especially in 45 and 135 directions)?? as the lag in 0 and 90
> > > directions is 12 where as in 45 and 135 is 16.97m.
> > > Though my question seems very basic, hope you find sometime to 
> answer them
> > > Thanking you
> > > Pujan
> > >
> > > 
> ------------------------------------------------------------------------
> > > > Date: Wed, 7 May 2008 18:27:44 +0200
> > > > From: edzer.pebesma at uni-muenster.de
> > > > To: regmi_pujan at hotmail.com
> > > > CC: thierry.onkelinx at inbo.be; r-sig-geo at stat.math.ethz.ch
> > > > Subject: Re: [R-sig-Geo] variogram using gstat
> > > >
> > > > PUJAN RAJ REGMI wrote:
> > > > >
> > > > > Dear sir,
> > > > > Ya, Now I am comparing the results from gslib and R/gsat. The
> > > shape of
> > > > > variograms exactly overlaps but the position of points varies, I
> > > don't
> > > > > know why? or it is normal! I first checked the result of
> > > > > omnidirectional variogram but still to have checked for 
> directional
> > > > > cases. I hope the plot will overlap exactly on these cases too!!
> > > > > Thanking you
> > > > > Pujan
> > > > >
> > > > Does gslib also compute average distances over the point pairs, 
> or does
> > > > it give the middle point of the distance bin?
> > > > --
> > > > Edzer
> > >
> > > 
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