stAni: A spatio-temporal anisotropy; the number of space units
equivalent to one time unit.
and it is only used in spatio-temporal variograms that have a metric to them. By this it is meant that you can compute a distance between two points
For the "separable" variograms, the semi-variance is a product of a spatial and a temporal component:
Y(S, S') = s((x,y),(x',y')).t(t,t'). There's no real concept of a single-measure "distance" between
S' - suppose they are 6km apart and two days apart. Are they closer or further apart than two points 4km apart and three days apart? That often doesn't make sense, and a separable space-time variogram avoids this question by assuming the variance depends independently on the spatial distance and the temporal distance.
The "metric" variograms are different. The
stAni parameter says how many space units are equivalent to a time unit - and in this way define a speed.
So a point at
(0,0,0) is five spatial units from a point at
(3,4,0), and if
stAni is 1, it is at the same distance from a point
(0,0,5) - ie in the same location but but five days later. In a metric variogram those pairs of points will have the same variance.
If you want to explore space-time anisotropy in data then you should probably plot a space-time variogram and see what that tells you.