This service uses Shuttle Radar Topography Mission, version 2, data from NASA. It has a resolution of either ~30m or ~90m (latitude dependent). Most of the world's landmass (99%) is covered between -60 and +60 degrees as standard. For high latitude regions such as Scandanavia and Iceland, extra tiles have been added courtesy of Jonathan de Ferranti at viewfinderpanoramas.org.
Global 90m/240ft coverage
30m/90ft coverage (USA)
To enhance the terrain data with man made obstacles such as buildings and wind turbines there are two clutter layer options available.
You can either add a layer of random clutter at a set height or upload your own defined clutter as a KML layer. Once loaded you can reuse the clutter in the future by selecting 'Own clutter' in the interface.