- Hi Malin,

In theory and under stationary of order 2,

indicator semivariogram values should not exceed

0.25 which is the maximum variance that you could obtain

for indicator variables, for a proportion of 50%.

I am wondering whether you have used the option

"standardize sill" in gamv, which could explain

these values larger than 1.

Pierre

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On Tue, 16 Jan 2001, Malin Fahller wrote:

> Hi everybody,

>

> I feel confused and really need some help to straighten things out.

>

> I am currently working with geostatistical methods for marine geological mapping. My sourcedata consists of sediment samples, interpreted seismic profiles, bathymetric data and sonardata. In my first, and easiest, case I only use the sedimentsamples to get a rough picture of the sedimentary boundaries.

>

> So this question concerns the use of one single source of data, sedimentary samples. The source data is an ascii_file with three columns: (x_location, y_location, category (i.e soiltype)).

>

> What I have done so far is that for each category (i.e soiltype), I have done an indicator transform (for example: value 1 = sand, value 0 = not sand). Then I have used GSLIB, gamv, to make an omnidirectional semivariogram for my indicator transformed data. This has worked really nicely but I get semivariogram values that is larger than 1 and this troubles me. How can I possibly get semivariogram values that is outside the range 0 - 1 when I have indicator transformed data? Is there an error in the program or have I misunderstood the mathematics of semivariograms? Can someone PLEASE help me out.

>

> I reccon this is a basic question, but I just cant proceed if I dont get this straightened out.

>

> Many thanks in advance

> Malin

>

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* Support to the list is provided at http://www.ai-geostats.org - Dear Malin:

The semivariogram of an indicator variable is always less than 1.

Proof:

The experimental semivariogram is (1/2) of the mean of the squared

differences Z(xi+h)-Z(xi) but the values of Z are 0 or 1, then the

semivariogram is (1/2) of the mean of the absolutes values of

Z(xi+h)-Z(xi). Now you use the triangular inequality and you get that

the semivariogram is less than 1.

Regards

Marco Alfaro

Malin Fahller wrote:

> Hi everybody, I feel confused and really need some help to straighten

[Non-text portions of this message have been removed]

> things out. I am currently working with geostatistical methods for

> marine geological mapping. My sourcedata consists of sediment samples,

> interpreted seismic profiles, bathymetric data and sonardata. In my

> first, and easiest, case I only use the sedimentsamples to get a rough

> picture of the sedimentary boundaries. So this question concerns the

> use of one single source of data, sedimentary samples. The source data

> is an ascii_file with three columns: (x_location, y_location, category

> (i.e soiltype)). What I have done so far is that for each category

> (i.e soiltype), I have done an indicator transform (for example:

> value 1 = sand, value 0 = not sand). Then I have used GSLIB, gamv, to

> make an omnidirectional semivariogram for my indicator transformed

> data. This has worked really nicely but I get semivariogram values

> that is larger than 1 and this troubles me. How can I possibly get

> semivariogram values that is outside the range 0 - 1 when I have

> indicator transformed data? Is there an error in the program or have I

> misunderstood the mathematics of semivariograms? Can someone PLEASE

> help me out. I reccon this is a basic question, but I just cant

> proceed if I dont get this straightened out. Many thanks in

> advanceMalin