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How to Calculate Rainfall With the Thiessen Polygon Method

Thiessen polygons, also known as Voronoi diagrams, are a method used to approximate the relative significance of data from points scattered arbitrarily across an area. They are very useful for calculating rainfall in a given area because in most cases rainfall is reported from weather stations representing unevenly distributed points in the area for which average rainfall is to be calculated. Construct Thiessen polygons by drawing intersecting perpendicular bisector lines between each pair of points.

Things You'll Need

Weather station map
10-by-10 grid paper
Pencil
Ruler
Paper

Instructions

- 1
  Copy the positions of the weather stations onto the grid paper, along with the edges of the area whose rainfall you are calculating.
- 2
  Draw dotted lines connecting each point to the points around it. Measure and mark the midpoints of the dotted lines.
- 3
  Draw the perpendicular bisectors of each of the dotted lines as solid lines. The points where the solid lines meet are the corners of the Thiessen polygons.
- 4
  Draw a chart with five columns. In the first column write the weather station names or numbers. In the second, write the rainfall reported at each station. In the third, write the number of squares covered by the polygon surrounding that station. In the fourth, write the percentage of the total area the value in the third column represents. For the fifth, find the precipitation from each station weighted by area by multiplying the values in the second and fourth columns.
- 5
  Total the values in the fifth column to find an approximation of the weighted rainfall over the area the polygons cover.

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