Google Earth Terrain

Just a quick question about how the terrain data is captured and what exactly it is the elevation of. What i have noticed happening in some situations is when i import a terrain that i believe to be one way (from being onsite) it is imported with different dimension than i expect.

What actually captures the terrain data?

When it is capturing the data does it read the actual ground (surface) or does it pick up the tree canopy and recognize this as terrain?

It would seem that Google Earth uses Shuttle Radar Topography Mission (SRTM) data as the source for its surface topography.