Loran-C receivers measure time delay differences in the signals they receive, compute differences of distance, and hence determine the user's position. The conversion from time to distance requires knowledge of the signals' velocities, which differ when propagating over sea or land. Precise positioning requires the delays of land paths to be accurately mapped, a procedure traditionally entailing expensive and time-consuming marine surveys. The resulting data, in the form of Additional Secondary Factors (ASFs), may be stored in Loran receivers. This paper introduces the concept of Loran-C Additional Secondary Factors, and shows how it is possible to map them efficiently. It focuses on recent advances, chiefly the development of powerful software for modelling the growth of ASFs over mountainous terrain. This employs an implementation of an algorithm proposed by Monteath. The modelling capability of the algorithm is demonstrated by examining propagation along a route that crosses mountains and deep fjords. In addition, the ASFs it predicts are compared with sample measurements made around the west coast of Scotland.