3.3. Manual modeling in BIM software
The main goal in 3D as-built BIM creation is to improve the productivity of component modeling while reducing human input. Conventional approaches either extract highly qualitative components only under
ideal conditions [13] or directly produce 3D BIM while incurring errors
[1]. We suggest an alternative, compromise approach that serves both
productivity and quality. First, as already explained, 3D lines defining
the boundaries of 3D plane objects are obtained. Then, the traced
boundaries are merged into a single polygon. At this stage, line generalization is not applied because any shape-change operation would cause
loss of information. Additionally, the remaining points not included in
any plane segments are imported as the input for the detail-modeling
process. Typically, such remaining points are considered as noise, as
they are probably small and tiny structural interior details. During the
detail-modeling process, the data is drastically reduced, down to 5% of
the original size or less, without loss of detail. The 3D boundary lines
of 3D plane features and the remaining points of un-segmented features
are used for manual modeling input. In this process, the model objects
are semantically annotated according to object categories, and their topological relationships are established by the modeler. Compared with
conventional direct modeling from original point clouds, each object is
more easily identified and modeled using automatically extracted
lines as guides. At the same time, system failure due to the huge data izes of point clouds is not an issue, because the qualitative modeling
is conducted with only about 5% of the original point cloud.