The Anisotropic Semivariance Surface or Variogram Map provides a visual picture of semivariance in every compass direction. This allows one to more easily find the appropriate principal axis for defining the anisotropic variogram model. A transect in any single direction (e.g. 330 degrees north) is equivalent to the variogram in that direction: the surface (z-axis) is semivariance; the x and y axes are separation distances in E-W and N-S directions, respectively. The center of the map corresponds to the origin of the variogram g(h) = 0 for every direction. See Isaaks and Srivastava (1989; page 150) and Goovaerts (1997; page 98). The map can also be projected as a 3d surface via the Edit Graph command.
A map will appear incomplete (checkered cells, as below) when there are not at least 3 pairs of points to provide an average semivariance for a particular lag class and direction. Cells with fewer than 3 pairs will appear to be missing. Cell size is dependent on the lag class distance interval.
Change the graph projection to 3d from 2d or to 2d from 3d.
When the check box is marked, a transect on the map surface illustrates the azimuth angle. The angle is specified in the text box (e.g. 330º in the window above), and also displayed graphically in the area under the azimuth angle. The average semivariance for the transect is displayed as A:nnn beneath the Set command. Note that there is no difference between an angle and that angle plus 180º.
Make the azimuth angle displayed in this window the Principal Anisotropic Axis in the Semivariance Analysis window. Pressing Set will reset the Principal Anisotropic Axis (for consistency, the angle in the Semivariance Analysis window will display a value between 0º and 180º, so that a value >180º will appear as that value less 180º (e.g. 225º will be reset to 90º in the Semivariance Analysis window).
Set will force a Rebuild of the anisotropic variograms (and models, if selected) in the Autocorrelation Window. The Principal Anisotropic Axis (the Major Axis of the anisotropic model) should be the direction of major spatial continuity (or major axis of the anisotropic variogram model), corresponding to the direction with the longest range.
The average semivariance for the azimuth transect is displayed as A:nnn beneath the Set command, where nnn is the average semivariance. A should be smaller in the major direction (lower average semivariance) and largest in the minor (90º-offset) direction.
• Off – returns the mouse to normal operation.
• Rotate – turns the cursor into a rotator cuff when the left mouse button is pushed, allowing the image to be rotated as desired. This option is available only for 3d map projections.
• Move – allows the graph to be moved within the window by clicking the left mouse button and dragging the cursor.
• Scale – shrinks the graph image with the left mouse button.
• Zoom – allows you to zoom in on a particular graph area by using the left mouse button to define a rectangular zoom area. Within the zoomed area the location of the cursor is noted on the Mouse Location panel.
• Reset – resets the image to the default rotation angle and scale.
Provides information on the current location of the cursor when it is on the map surface. Units are map units as specified by the axes. Z is semivariance.
You may Print, Copy, Edit, Export, or List graph values for either graph using the menu commands of the main GS+ window, or via a right-click menu.