|Click to enlarge (This is not the wall size map, see below for link)|
Ground conductivity plays a major part in the effectiveness of your Part 15 AM broadcast range (or any other type of AM broadcast). Knowing the ground conductivity of your particular area is a useful indicator to determine what level of signal performance to expect.
Via the FCC you can download a high resolution ground conductivity map broke up in 48 parts to optimize for printing on multiple 8 1/2 inch by 14 inch papers and assembled to the full size 43 x 69 inches map. Free download available at: http://www.fcc.gov/encyclopedia/m3-map-effective-ground-conductivity-united-states-wall-sized-map-am-broadcast-stations
Here's an example of one part of the map: