"“Once a year they pick cities like Denver or London and rescan them and they get it into their database – how often Google buys those images and updates its maps, is up to them.”
I'm surprised that Google is still buying DigitalGlobe imagery for the continental USA, ESPECIALLY for major metropolitan areas.
Most states have state-level orthoimagery collection programs, and as a result, there is high-quality aerial imagery significantly exceeding these satellites in quality over most of the USA, especially in metropolitan areas.
For example, New York State has 2 foot (24 inch) resolution across the entire state (only slightly worse than DigitalGlobe's best quality available), and over much of the state has 1 foot (12 inch) and even 0.5 foot (6 inch) resolution, the latter of which is better than what DG offers government customers. This data is under similar extremely permissive licensing to most other government GIS data such as TIGER. (Anyone can download NYGIS orthoimagery, and this same imagery is what Google uses for Maps/Earth for "satellite" which is really "aerial")
Pennsylvania has similar quality statewide imagery. Same for New Jersey (1 foot in the case of NJ).