The following methods are used to measure the average precipitation over an area: 1. Arithmetic Mean Method 2. Thiessen polygon method 3. Isohyetal method 4. Inverse distance weighting. Precipitation. Mean precipitation over an area. 1. Arithmetic Mean Method. Simplest method for determining areal average where, P. i : rainfall at the i. th ADVERTISEMENTS: Read this article to learn about the following three methods of calculating average depth of precipitation upon the area of the basin, i.e., (1) Arithmetic Mean, (2) Theissen Polygon Method, and (3) Iso-Hyetal Method. 1. Arithmetic Mean: When the area of the basin is less than 500 km2 this method implies summing up of [] Table 2.1a: Calculation of areal average by the Thiesen Polygon method. Note. The Thiessen method attempts to allow for non-uniform distribution of gauges by providing a weighting factor for each gauge. The stations are plotted on a map and connecting lines form polygons around each station. The magnitude and extent of the resultant rainfall areas of coverage are then considered versus the area in question in order to estimate the areal precipitation value. 3) Thiessen Polygon - This is another graphical technique which calculates station weights based on the relative areas of each measurement station in the Thiessen polygon Thiessen polygons (otherwise known as Voronoi polygons or Voronoi diagrams), are an essential method for the analysis of proximity and neighborhood. Thiessen polygons (cf. figure below on the right side) are used to allocate space to the nearest point feature. One is the Thiessen polygon method, a graphical technique named for Alfred H. Thiessen, the American meteorologist (1872-1956) who developed it. Thiessen polygons are used to calculate areas in relationship to specifically placed rain gauges and thereby compute the average amount of precipitation that fell in a specific basin during a storm closest to two centroids; Thiessen edges may be delimited by two