For 2021, the length of the Appalachian Trail from Maine to Georgia (or vice versa) is officially 2,193.1 miles, as documented in this classic work. Sometimes termed "the bible of A.T. hiking" because of its use in essential hike planning, each year's Data Book consolidates the most basic information from 11 detailed official guidebooks into a lightweight table of distances between major Appalachian Trail shelters, road-crossings, and features. It is divided according to the guidebook volumes 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.