A method of generating a map for image aided navigation comprises detecting one or more edges in one or more terrain images, and obtaining a compressed edge representation of the one or more edges. The compressed edge representation is obtained by a method comprising fitting a geometric form or other mathematical model to the one or more edges, calculating one or more coefficients based on the fitted geometric form or other mathematical model, and storing the one or more coefficients in a database for use in generating the map for image aided navigation.