A Kansas farmer wants to determine the average rainfall on his sunflower crop. The sunflower field takes up an entire section (a square of 1 mile per side, or 640 acres). The farmer has rain gauges located at the four corners and in the center. The rainfall depths are NE = 3.2 in., NW = 3.6 in., SE = 3.8 in., SW = 4.0, and center = 3.8 in. Determine the average depth of rainfall using the Theissen polygon method.