I read that stuff - still does not get to the heart of overlaying data into one map. The earth is round, maps are flat. Not all data layers use the same map projection. Any photographic data has lens distortion. Also all digital data has its smallest unit - not all digital data units are equal size. And then there are those pesky shadows.

Layering all that data actually creates phantom data. There are lots of people working on lots of methods to make the process more accurate. Google's "human" input with an army of people with I-phones or whatever, improves accuracy where there are hundreds of people. I am mighty glad there are not hundreds of people in each square mile in the wilderness! The accuracy of google earth in the wilderness vs cities is vastly less. I have spent many hours looking at google earth maps in area where I have traveled and although pretty good, it still does not equal a geo-referenced stereo pair of low altitude air photos. We used those eons ago even in the 1970's to make geologic maps. BUT, the air photo was just the base map - we walked every inch of every geologic unit boundary and plotted on the map. That is "ground truth". Seldom can I find on google earth, the off-trail route that I took. A 10-foot cliff may stop you cold, but not show up on google earth.

A few good things though. With the date of the data, I found out that a stream I was hoping to walk down was bone dry on the October 2012 photo! Not that there are not small pockets of water, but it gave me a "heads up".