Anisotropic variogram models are similar to those for isotropic variograms but includes directional information in the range parameter. GS+ calculates geometric anisotropy, in which the range changes with direction but the sill remains constant. Consequently the anisotropic model includes a directional component only for the range term below.
Four types of anisotropic variogram models are provided in GS+: Linear, Spherical, Exponential, and Gaussian. Each model can be described using the following terms:
• Nugget Variance or Co – the y-intercept of the model; this value is the same for all directions.
• Sill or Co+C – the model asymptote; this value is the same for all directions.
• Range or A – the separation distance over which spatial dependence is apparent for the direction examined. It is the sum of:
• A1 – the range parameter for the major axis of variation f, and
• A2 – the range parameter for the minor axis (f + p/2);
adjusted for the angle between pairs q as noted in the formulas for individual models.
GS+ calculates default values for each parameter of the four models. You may change any of these four model parameters from the Anisotropic Variogram Model dialog window.