The present invention relates to a system for use in generating a 3D model of a terrain comprising an elevation data generation apparatus (100) for derivation of elevation data based on a series of circles of varying radius generated on the terrain by the apparatus (100) and a reference concentric circles data. Said elevation data generation apparatus (100) comprises an image-capturing device (202) coupled with a wide-angle lens (214), a rotary light-reflecting device (206) having a plurality of reflective surfaces (208) disposed thereon with different reflective angles relative to a fixed position of laser beam irradiating from a laser beam source, and a dual rotation mechanism configured to facilitate laser beams to be reflected on the terrain with different reflective angles and thereby producing the series of circles of varying radius based on the actual topographical characteristic of the terrain.