However, for now we must remove unwanted data and this requires filtering to identify, classify and separate the wanted and unwanted data. Although there are a myriad of filtering methods, the following paragraphs will detail three, relatively simple methods of filtering data from terrestrial laser scanner point clouds.
Many of the debris basins have significant vegetation re-growth, which necessitates manual vegetation filtering. Without a limit box, the resulting mesh (from which volume measurements are made) is less accurate because clipping vegetation would remove bare surface points which lie behind vegetation (regardless of your perspective) requiring either leaving the vegetation untouched or deleting the vegetation along with bare earth points. Both these options were not suitable for the debris basin scans because the volumes derived needed to be quite accurate as they factor into evacuation plans for neighborhoods down slope from the debris basins.
Just as a proof of concept for the debris basin methodology, shown below are the volume measurements (100 measurements per square meter) made from a common plane to the two meshed surfaces. The vegetated mesh (left) resulted in a volume of 52.62 meters^2, while the filtered mesh volume was estimated to be 54.67 meters^2. As expected the filtered mesh volume is greater.
One recent and interesting development for filtering point clouds was the release of edgewise filtering software by clearedge3d. This software is the first of its kind and can automatically detect objects (pipes, walls) and ground surfaces within a point cloud. The advanced ground finding function in particular seems unbelievable, but technically it only uses the same methods that I have detailed above. After-all, a point cloud is really only x,y,z coordinates.
This material is based upon work supported by the National Science Foundation (NSF) under Grant No. 0934131, entitled “Geomorphic Connectivity in a Recently Burned Watershed” - Thad Wasklewicz, PI
This work is also based upon work supported by a Cooperative Agreement with U.S. Geological Survey Landslide Hazards Program entitled "Close-Range Photogrammetry and Terrestrial Laser Scanning Data and Analysis" - Thad Wasklewicz, PI