1. A method for searching a triangle on which an object lies while the object moves on a trigonometric grid, wherein the triangle is one of the constituents of the trigonometric grid approximately representing a terrain, the method comprising the steps of: (a) retrieving information on a present triangle, the present triangle referring to a triangle on which the object lies at a present time; (b) predicting a next location after a predetermined time interval from the present time; (c) finding a nearest vertex, the nearest vertex referring to a vertex of the present triangle being nearest to a present location of the object; (d) selecting candidate triangles sharing the nearest vertex; and (e) deciding whether or not the next location lies on said each candidate triangle.