Cool U.S. Government Map Site

I'm a bit of a map geek. I'm not expert, but I love the things. Always have. In college I even took a class in cartography. Then I came across this:

http://nationalatlas.gov

The United States government has an agency called the "U.S. Geologic Survey" (USGS) and their mission is to compile and collate information on ... well, nearly everything. What we see here is the confluence of several technologies:

• Computer database
• Computer-generated imagery
• Broad-based internet access

They have maps on all manner of things -- geology, precipitation, aquifers, West Nile Virus incidents, presidental elections, etc., etc. And they have a way to "layer" different data types on top one another.

I printed off the precipitation map for the State of Arizona. There are some truly dry regions of the state -- less than 5" rain a year -- but Tucson is not that dry. In fact, the shade of light-brown / green around the Tucson area is not that much dryer than some areas of my old state of Michigan. It would be interesting to "layer" the precipitation map with the ground-water supply map. A third layer of population would show, I'm sure, a strong correlation.