There are tools (LAStools, for example; http://rapidlasso.com/lastools/) that are used to alter the classification of points based on the ASPRS classification system. USGS data sets usually have just the ground class (#2) and perhaps water (#9) pre-classified from the point cloud. I have used LAStools to convert non-classified points (class #1) into low, med and high vegetation classes (#'s 3, 4 and 5) as well as mdifiying points to the building class (#6). Also, Ihave been able to classify water areas quickly if only the ground was classfied using LAStools as long as there was polygon data for zones of water available (which is common). Building footprint data, road right of ways, railway right of ways, bridges, etc. (now in Infraworks) can be used to assist in classifying the associated classes.
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