Quote - ...I am very interested in how you get real world elevation data into a 3d application.  I use Rhinoceros and Bryce.

Bryce can import USGS DEM files as terrain objects, upto 4096x4096 resolution, I believe.
.DDF is listed in the file types for import, but I can't ever get them to work. SDTS has replaced DEM at the USGS as the format of choice; and most of the downloads I've seen are in SDTS format, but there is a translator from SDTS to DEM:
http://data.geocomm.com/dem/sdts2dem.html

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