1. A tangible computer readable medium comprising machine readable instructions which, when executed, cause a machine to at least: divide an image of a geographic area of interest into a plurality of geographical zones, the geographical zones being representative of different geographical areas having approximately equal physical areas; measure a geographical feature represented in the image for corresponding ones of the plurality of geographical zones; store descriptions for the geographical zones in a computer memory; store values representative of the geographical feature of the geographical zones; sort the geographical zones into bins based on a comparison of the values to ranges of values, the ranges of values corresponding to the bins; and determine a number of zones to be sampled in one of the bins based on the range of values corresponding to the one of the bins.