A method of identifying errors in information describing terrain features is disclosed. According to the method, a first set of data is obtained which describes an area of terrain. The first set of data has a first type of error. A second set of data is obtained which describes the area of terrain. The second set of data has a second type of error. The second type of error is characteristically different from the first type of error. A subset of the first set of data corresponding to a first portion of the area of terrain is compared with a subset of the second set of data corresponding to the first portion to identify information not present in one of the first and second sets of data.