The United States Geological Survey National Geospatial Program “provides leadership for USGS geospatial coordination, production and service activities.” The National Map is one of several initiatives administered by the program . The National Map puts orthoimagery (aerial photographs), elevation, geographic names, hydrography, boundaries, transportation, structures, and land cover at the fingertips of users through its web viewer. The National Map is transitioning to a new method for delivering its products, making it ever more useful for professionals, teachers, students, and the general public. The new National Map Beta Viewer and Download platform recently made its debut. In addition to better product access, the new viewer lets you preview and download all National Map data and new US TOPO maps from one site. It will be interoperable with map viewers such as Google Maps, Bing!Maps, Google Earth using WMS, KML or ArcGIS. The viewer provides GIS tools to identify features, perform measurements and reverse geocode. For more information about features Introduction Letter Beta Information Sheet User Help/Support (QuickStart, FAQs, Contact Us). Useful tools like area measurement is provided free of charge, unlike the free version of Google Earth. The video below illustrates some of these features.
I encourage you to check out the new National Map beta viewer.