Over at the Google Earth Library site, Matt has started a project to convert USGS topographic maps (1:250K, 1:100K and 1:24K scales) into Google Earth overlays.. These are “super” overlays, where views from higher elevations are at lower resolutions, while closer views load in high-resolution imagery; this speeds up display times significantly. Arizona and Nevada are available now, with Colorado and California coming soon.
The network link to access the overlays online is at the bottom of the web page; click on the “G-Earth” graphic. Once loaded in Google Earth, activate it, then expand it to select the state and map type you want, 1:24K maps for Arizona here: [...]
Read more: http://freegeographytools.com/2009/usgs-topographic-map-overlays-for-google-earth