1. A method for registering a set of frames of 3-dimensional digital elevation model data points which include both ground points representing a 3-dimensional surface and obstruction points which represent an obstruction lying above the 3-dimensional surface and are positionally interspersed among the ground points, said method comprising the steps of: (a) extracting a ground surface shell from the set based on a plurality of ground points from which the obstruction points have been isolated; (b) finding a plurality of pairs of corresponding closest points in overlapping frames of said ground surface shell such that a plurality of said pairs of said corresponding closest points are found for each one of a plurality of pairs of said overlapping frames; (c) determining a plurality of 3-dimensional transformations based on said corresponding closest points found in step (a), each respective one of said transformations defining a 3-dimensional rotation and translation representing an alignment of a first frame and a second frame of a respective one of said overlapping pairs of frames, said transformations being determinable without reliance on ego motion information, and (d) substantially simultaneously registering all frames included in said plurality of pairs of said overlapping frames by performing a rigid transform according to a matrix in which all of said transformations are represented.