Lidar Return Number and Classification

Point clouds are collections of 3D points located randomly in space.

Map showing the spatial pattern of classifications.  In the upper left is a DSM constructed from the LIDAR shows data gaps primarily along the stream.  Water often has no returns unless it is directly under the sensor.

Water returns appear in the upper right.  If the water surface is flat, there will be few returns except directly under the aircraft.

Ground returns appear in the lower left.  This level of classification is almost always included in a LAS file, as it is needed to create a DTM.

Unclassified returns are in the lower right--they are mostly vegetation, but the data set was not classified, and there are also some buildings.
The same area shown above, with the return numbers shown.  There are first returns everywhere, and they can be any category, except where no energy returned due to specular reflections from the water surface. Second and third returns only occur in areas with vegetation, or more rarely power lines.  As point densities increase, so does the dectection of power lines.

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last revision 6/15/2020