1. A method for semi-automatically reconstructing a 3-D building model, comprising. using a plurality of building outline segments to construct a plurality of closed polygonal building boundaries, wherein each the neighboring building outline segments are either connected or disconnected; creating an initial building model; performing a pre-processing process on the building outline segments; successively splitting the initial building model and generated 3-D building models by the building outline segments to generated a plurality of 3-D building models; removing the 3-D building models that connected to the initial building model; successively analyzing the shared boundaries of two connected building models whether or not having a corresponding one of the building outline segments; if no corresponding building outline segment exists, then merging the two building models into one single building model; if one corresponding building outline segment exists, then keeping the two building models as the 3-D building models; assigning height to every one of the building's boundaries according to the height of the corresponding one of the building outline segments, wherein the corresponding one of the building outline segments is one of the building outline segments with a height information, and successively analyzing whether each of the building model's boundaries is an independent-edge or a shared-edge, wherein the independent-edge means that it does not overlapped with the other building model's boundaries on the 2-D horizontal plane, and the height information for the independent-edge can then be assigned and fixed from the corresponding one of the building outline segments; wherein if only one the independent-edge is found, it is necessary to check whether its surrounding rooftop is fixed or not; wherein if all of its surrounding rooftop have already been fixed, then its rooftop can be inferred from the independent-edge; and if at least two the independent-edges exist and are sufficient to fit into a planar face, then a least-squares coplanar fitting can be applied, otherwise, the system provides a most possible solution by a consecutive-coplanar analysis.