Semi-variogram for point-based data in two dimensions
variog2 yvar coord1 coord2 [if exp] [in range] , width(#) [ generate(newvar) lags(#) list graph_options ]
Description
variog2 calculates, graphs, and optionally lists the first lags values of the semi-variogram for a series of observations in two spatial dimensions. That is, we have measurements of a response yvar (rainfall, ore grade, soil depth) at points whose horizontal map coordinates are coord1 and coord2. The response must be named first; the order of the two coordinate variables is immaterial.
The semi-variogram is a plot of the semi-variance
1 2 -------- SUM (y - y ) = gamma(h) 2 #pairs i, j in i j same band against the lag or band number h = 1, ... , lags. In words, it shows half the mean difference squared for pairs of points in various distance bands. Given width w, successive bands are for distances up to w, between w and 2w, between 2w and 3w, and so forth. Note that the units of the semi-variogram are the units of the response variable, squared.
Options
width() specifies the width of successive distance bands in the units linked to the coordinate variables. It is a required option.
generate(newvar) saves the semi-variances in newvar.
lags() specifies the number of lags. If not specified, the first int(_N/2) semi-variances are graphed: that is, the number of lags is about half the number of observations.
list lists the semi-variances and the number of pairs of measurements on which they are based. This may help in identifying parts of the variogram based on rather few pairs of data.
graph_options refers to any of the options of line.
Examples
. variog2 height easting northing, width(10) . variog2 height easting northing, w(10) list . variog2 height easting northing, w(10) recast(scatter)
Author
Nicholas J. Cox, University of Durham, U.K. n.j.cox@durham.ac.uk
Also see
On-line: help for variog