A vector generation unit selects, for each of three or more reference points set at locations spaced apart from each other on a sphere, a line segment to be drawn from each of the three or more reference points without intersecting another line segment, from among one or more line segments forming an airspace defined by a closed curve on the sphere, and generates, for each of the three or more reference points, a vector from the selected line segment to each of the reference points. An airspace recognition unit recognizes one of two regions on a true sphere as an outside of the airspace and recognizes the other region as the airspace, the two regions being separated by the closed curve, the one of the two regions including the vectors of more than half of the three or more reference points.
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