1. A method of online building-model reconstruction using a photogrammetric mapping system, comprising steps of: (a) connecting a photogrammetric mapping system to access three-dimensional (3D) structural lines of a stereo scene in an image space of said photogrammetric mapping system at real time, wherein said 3D structural lines are polylines digitized into continuous 3D segments; and (b) reconstructing building models by using said 3D structural lines.