More useful are data from a recording raingage which gives a record of accumulation as a function of time. Hence, intensities can be determined from recording gages. Areal rainfall is obtained from point data by an arithmetic or area-weighted average of the rainfall amounts. The two accepted methods for applying this formula are the Thiessen polygon method and the isohyetal method. Each uses a different technique to delineate the sub-areas for which individual precipitation measurements are presumed to be applicable.
Rainfall amounts from a set of gages are plotted on a map of the region.
Obviously, this works best when there are numerous raingages. This technique has the advantage of being quick to apply for multiple storms because it uses fixed sub-areas. It is based on the hypothesis that, for every point in the area, the best estimate of rainfall is the measurement physically closest to that point. This concept is implemented by drawing perpendicular bisectors to straight lines connecting each two raingages. This yields, when the watershed boundary is included, a set of closed areas known as Thiessen polygons.