It’s hard to explain, but basically Carlson P3D Topo removes any points a designated deviation higher than what it considers low (ground) points across another designated horizontal measurement. A smaller cell across a larger window.
Native Point Cloud (8.2M points)
This is about a 10m x 10m window with 0.3m cells.
This is the point cloud after running a 0.3m cell and 3m window with a height tolerance of 0.3m and is the normal filter that I run on a construction site that usually gets rid of small groups of trees, cars and machines. Problem is that with a small 0.3m cell it is still has too many tops of lower canopies. It appears to have removed allot of point with 1.7M remaining, but the canopy was so dense that it is really not allot removed with how few ground points there are.
Here is an aggressive 1m x 5m x 0.3m which still leaves enough points to stitch a decent DTM, but I don’t think you could go much further without having to stitch too far across. What I would do with this result is go ahead and make a triangulated surface and turn on contours so I can easily see the remaining spikes. Unfortunately it is the opposite and the ground points are the minority so it would be allot of trimming and not enough points left to stitch to where I would be comfortable presenting.
The ground points are indicated with the magenta stakes.