----------------------------Original message----------------------------
 
The "T" on elevations written in long-hand on USGS 7.5-minute provisional
maps stands for a photogrammetric elevation measured with a stereo plotter.
 
The "AT" on elevations on provisional maps stands for Analytical elevation,
and was measured using an analytical stereo plotter.  AT elevations are
much more precise than "T" ones.  Analytical stereo plotters are used
to extend photogrammetric control from well established elevation points
to other points in the stereo model.
 
This answer came from a map compilation office at USGS, but was interpretted by
by me in the USGS Earth Science Information Center, so it may be somewhat
garbled via journalistic license.
 
Topographic Map Symbols brochures are available for free from USGS.  You may
request them by calling (800)USA-MAPS or by writing to the USGS Map
Distribution office, Box 25286, Building 810, Denver Federal Center,
Denver, CO  80225.