Judging the existence of matching area, which is digital elevation basis diagram, in the map suitable for matching, is one of the key steps in cruise missile path planning. It is completed mainly by handicraft nowadays. According to the basic ideology in mathematical morphology, a new method was proposed in this passage that the cast shadow on the horizontal plane was produced by the basis diagram. Considered as the structuring element, it was used to probe into the cast shadow produced by matching area and considered as destination diagram. Then, the digital elevation basis diagram was settled down automatically. Experiments prove this is feasible. It also paves the way for improving the automatic degree of path planning.