aCerro Tololo Inter-American Observatory, National Optical Astronomy Observatory, and AURA New Initiatives Office bGemini Observatory

The Giant Segmented Mirror Telescope (GSMT), along with other proposed Extremely Large Telescopes (ELT’s) with apertures over 20-m, is likely to impose rather different site selection criteria than those for existing large telescopes. Advantageously, remote-sensing techniques allow rather more objective comparisons than was possible in the past, and the general task is aided by numerical modeling and new ground-based measurement techniques. In recognition of the difficulty of the site-selection process, co-operation between the several ELT projects is the norm. A description i