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: [EMAIL PROTECTED]
> To: [EMAIL PROTECTED]
> CC: [EMAIL PROTECTED]; r-sig-geo@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: [EMAIL PROTECTED]
> > > To: [EMAIL PROTECTED]
> > > CC: [EMAIL PROTECTED]; r-sig-geo@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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