Sometimes termed 'the bible of A.T. hiking,' each year's Data Book consolidates the most basic information from 11 detailed guidebooks into a lightweight table of distances between major Appalachian Trail shelters, road-crossings, and features. It is divided according to the guidebook volumes (one state or two or, for Virginia, parts of a state) and updated each December to account for Trail relocations, new (or removed) shelters, and other changes. In addition to codes for lodging, food, water, and other essentials, the Data Book is keyed to both the individual guidebook sections and to the separate maps. Day-hikers and long-distance hikers alike rely on this volume for armchair planning as well as on-the-trail orientation.This also is the source for the ever-changing official answer to, How long is the trail? For 2009: 2,178.3 miles.