----------------------------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.