Remark 2

Optimum sampling grid. The minor axis of the ellipse (Figure 4.5) is the direction in which the phenomenon varies fastest, and the major axis the direction in which variability is slowest. Thus, the optimum sampling grid for this phenomenon is a rectangular one, aligned in the directions of these two main axes ($Oy_1$ and $Oy_2$) with sides in the ratio $/lambda$, the smallest side corresponding to the direction $Oy_2$ of fastest variability.