What are the different imagery layers in Virtual Earth?

From: Virtual Earth For Government: Top 10 Questions Asked About Virtual Earth (at Conferences): What are the different imagery layers in Virtual Earth? Okay, people don't really ask this outright but many do not understand the difference between satellite and aerial imagery and the different layers of Virtual Earth imagery so I thought I'd cover this here. Virtual Earth contains several types of imagery, beginning with low to medium resolution (1 km to 10 meter, depending on the zoom factor) satellite imagery providing a global "backdrop" for the digital globe. Microsoft then includes higher resolution (1 meter or better) Ikonos satellite imagery for areas of high interest that we haven't (or do not intend) to cover with aerial imagery (explained next). A big competitive advantage for Virtual Earth, is its aerial imagery: photography captured by aerial cameras mounted in small aircraft flying at low altitudes. For nationwide coverage, Virtual Earth features 30 cm to 1 meter orthorectified aerial photography. For denser, high interest urban areas, Microsoft provides 15 cm (or better) "orthos", increasingly provided by the leading digital aerial camera the UltraCam-X. (Compare this to the competition's "high resolution" satellite imagery of just .6 meter resolution.) The next layer is Virtual Earth's exclusive Bird's Eye imagery, which I have discussed before in this blog. This 15 cm resolution aerial photography is captured at 45 degrees and allows users to see the facades and entrances to structures from 4 different views. Bird's Eye imagery is currently available for highly populated counties in the US and Europe and is being increased every month. As of November 2006, Virtual Earth features geospecific, photorealistic and ACCURATE 3D city models. Current coverage is approaching 100 cities, mostly in the US but with a handful of those in the UK. What surprises most people is that the 3D models are created from the orthophotos and not from the Bird's Eye imagery as you might expect, given the oblique views in the latter. Finally, for road map hill-shading and 3D terrain, Virtual Earth includes digital elevation models. DEMs for global coverage is at 90 meter (or better) resolution, for national at 30 meter (or better) and for local high interest areas 10 meter or better.