A method of constructing a bounding box comprises: acquiring a set of sensed data points; adding, for each sensed data point, at least one calculated data point; and defining a bounding box containing the sensed and calculated data points. A method of identifying voxels in a voxel grid corresponding to a plurality of data points comprises: calculating, for each data point, a distance between it and each voxel; creating a subset of voxels comprising voxels having a distance from one data point that is less than a predetermined distance; creating another subset comprising those voxels that neighbor a voxel in the first subset; computing, for each voxel in the second subset, a distance between it and each voxel in the first subset; and identifying each voxel in the first subset that is a distance away from each voxel in the second subset that exceeds a predetermined distance.