ASTER ("Advanced Spaceborne Thermal Emission and Reflection Radiometer")
is an imaging instrument that is flying on the TERRA satellite launched
in December 1999 as part of NASA's Earth Observing System (EOS).
The quality is highly variable. There are gaps, and the data
is relative not absolute, so it can require extensive adjustment.
However, if all that is done, the results can be excellent, and similar
in resolution to USGS 30m DEM.
Although TIFF is an image format, there are people who abuse the
format (er, extend it in a non-standard way) to represent
elevation, not imagery.
Generally, this is done with TIFF's support for 16-bit integer
values, for each "pixel". For better precision, some people abuse
the format even further by using floats (floating point
numbers) instead of ints.