Variography

Spatial analysis or variography is carried out in order to understand how sample values relate to each other in space, which can then help to infer possible similarities between known samples and points that have not been sampled. A variogram is calculated by plotting the average variability for all sample pairs at a certain distance apart against that separation distance.

Variograms can be calculated either purely by separation distance or by separation distance and direction. If direction is not taken into account and all sample pairs at a given separation distance are used in the calculation of the average variability, it is referred to as an isotropic or omni-directional variogram. If direction and distance are used to select sample pairs, then it is referred to as a directional variogram.

When calculating variograms, the separation distance is termed the lag or h (for example, 10 m). When calculating directional variograms, “h” refers to the distance and direction vector (for example 10 m north–south). The gamma symbol (ɣ) is the standard symbol for variability in a variogram. On the variogram we plot ɣ(h) being the average variability (or variogram value) of all sample pairs separated by vector h. The average variability is calculated for a series of lags and plotted against lag distance to create a variogram plot. The separation distance at which the sill is reached is called the range or range of continuity and indicates the distance at which there is no longer correlation between the samples.

To get started with variography, see Generating Variograms.