key map meaning in Information Science terminology / glossary / dictionary is:
A map showing the coverage of a set or series of more detailed maps, usually distinguished on the larger map by number, letter, or some other system of symbols. The keyed maps may or may not be overlapping (see this clickable keymap of Christchurch, New Zealand). Also refers to a small diagram located in the margin of a sheet map or chart showing the outlines of the sheet and all adjacent sheets in the series. Also used synonymously with index map.