Therese M. Shea 
Reading Map Keys [PDF ebook] 

Just as a car can’t be started without the right key, many maps would be indecipherable without their legend, or key. Map keys show what pictures and colors on a map represent and often offer clues as to the kind of map. Readers learn how map keys can illuminate all kinds of maps, from street and political maps to economic and physical maps. Common map key symbols are discussed alongside full-color examples of many kinds of maps and keys to guide readers through the use and importance of each.