Abstract: The additional secondary phase factor (ASPF) plays an important role in propagation delay of the low-frequency ground-wave. A method of calculating attenuation for ground-wave propagating over irregular terrain called integral function has been developed recently. It is derived by means of a stationary-phase integration that reduces the dimensionality of the general version, but such an approximation is not valid for all terrain types. The terrain of the actual propagation path changes greatly, and the actual propagation path is corrected by the mathematical morphology method to keep the basic geometric contour and smoothness of the path. The calculation results of the integral equation method show that the actual propagation path can be effectively optimized through the mathematical morphology method.