📄 gslib help gam.htm
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<H2>GSLIB Help Page: GAM</H2></CENTER>
<DL>
<DT><IMG height=14 alt=o src="GSLIB Help GAM.files/ball.red.gif" width=14>
<STRONG>Description:</STRONG>
<UL>
<LI>Variogram calculation of gridded data </LI></UL>
<DT><IMG height=14 alt=o src="GSLIB Help GAM.files/ball.red.gif" width=14>
<STRONG>Parameters:</STRONG>
<UL>
<LI><B>datafl:</B> the input data in a simplified Geo-EAS formatted file.
The data are ordered rowwise (x cycles fastest, then y, then z).
<LI><B>nvar</B> and <B>ivar(1) ... ivar(nvar):</B> the number of variables
and their columns in the data file.
<LI><B>tmin</B> and <B>tmax:</B> all values, regardless of which variable,
strictly less than <B>tmin</B> and greater than or equal to <B>tmax</B> are
ignored.
<LI><B>outfl:</B> the output variograms are written to a single output file
named <B>outfl</B> The output file contains the variograms ordered by
direction and then variogram type specified in the parameter file (the
directions cycle fastest then the variogram number). For each variogram
there is a one-line description and then <B>nlag</B> lines each with the
following:
<UL>
<LI>lag number (increasing from 1 to <B>nlag</B>
<LI>average separation distance for the lag.
<LI>the {\em semivariogram</B> value (whatever type was specified).
<LI>number of pairs for the lag.
<LI>mean of the data contributing to the tail.
<LI>mean of the data contributing to the head.
<LI>the tail and head variances (for the correlogram). </LI></UL>The {\tt
vargplt</B> program documented in section VI.1.8 may be used to create
PostScript displays of multiple variograms.
<LI><B>igrid:</B> the grid or realization number. Recall that realizations
or grids are written one after another; therefore, if <B>igrid=2</B> the
input file must contain at least 2 x <B>nx</B> x <B>ny</B> x <B>nz</B>
values and the second set of <B>nx</B> x <B>ny</B> x <B>nz</B> values will
be taken as the second grid.
<LI><B>nx, xmn, xsiz:</B> definition of the grid system (<I>x</I> axis).
<LI><B>ny, ymn, ysiz:</B> definition of the grid system (<I>y</I> axis).
<LI><B>nz, zmn, zsiz:</B> definition of the grid system (<I>z</I> axis). One
or two dimensional data may be considered by setting the number of nodes in
some directions to 1. Often, <TT>gam</TT> is used to check the variogram
reproduction of realizations from a simulation program.
<LI><B>ndir</B> and <B>nlag:</B> the number of directions and lags to
consider. The same number of lags are considered for all directions and all
directions are considered for all of the <B>nvarg</B> variograms specified
below.
<LI><B>ixd, iyd</B> and <B>izd:</B> these three arrays specify the unit
offsets that define each of the <B>ndir</B> directions (see section III.2).
<LI><B>standardize:</B> if set to 1, the semivariogram values will be
divided by the variance
<LI><B>nvarg:</B> the number of variograms to compute.
<LI>The "variogram type" is specified by an <A
href="http://www.gslib.com/gslib_help/vtype.html">integer code</A>
<LI><B>ivtail, ivhead</B> and <B>ivtype:</B> for each of the <B>nvarg</B>
variograms one must specify which variables should be used for the tail and
head and which type of variogram is to be computed. For direct variograms
the <B>ivtail</B> array is identical to the <B>ivhead</B> array. Cross
variograms are computed by having the tail variable different from the head
variable, e.g., if <B>ivtail(i)</B> is set to 1, <B>ivhead(i)</B> is set to
2, and <B>ivtype(i)</B> is set to 2, then distance measure <B>i</B> will be
a cross semivariogram between variable 1 and variable 2. Note that
<B>ivtype(i)</B> should be set to something that makes sense (e.g., types
1,2, or 3); a cross relative variogram would be difficult to interpret.
Further, note that for the cross semivariogram (<B>ivtype</B>=2) the two
variables <B>ivtail</B> and <B>ivhead</B> are used at both the tail and head
locations. The <B>ivtype</B> variable corresponds to the integer code in the
list given in section III.1.
<LI><B>cut:</B> whenever the <B>ivtype</B> is set to 9 or 10, i.e., asking
for an indicator variogram, then a cutoff must be specified immediately
after the <B>ivtype</B> parameter on the same line in the input file. Note
that if an indicator variogram is being computed then the cutoff/category
applies to variable <B>ivtail(i)</B> in the input file (although the
<B>ivhead(i)</B> variable is not used it must be present in the file to
maintain consistency with the other variogram types). </LI></UL>
<DT><IMG height=14 alt=o src="GSLIB Help GAM.files/ball.red.gif" width=14>
<STRONG>Application notes:</STRONG>
<UL>
<LI>Regularly spaced data in 1-D can be handled by setting <I>ny, nz</I> to
one and <I>iyd, izd</I> to zero. </LI></UL></DT></DL><IMG height=8 alt=---
src="GSLIB Help GAM.files/line.blue.gif" width=652>
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