1. A method of three-dimensionally transforming two-dimensional linear data in accordance with three-dimensional topography data in real time, comprising: searching triangles in which vertexes constituting the two-dimensional linear data are contained among triangles constituting the three-dimensional topography data; calculating heights of the vertexes by using three-dimensional coordinates of the searched triangles; calculating intersection points between the triangles constituting the three-dimensional topography data and straight lines constituting the two-dimensional linear data; and calculating heights of the calculated intersection points.