You are right, most of the measurements correspond to the forest canopy. However, the latest lidar sensors use very high frequencies and shifting angles, so some laser beams do get to the ground. The resulting point clouds are then classified by applying different algorithms (High vegetation, low vegetation, ground, etc.) and you can apply filters to create a surface from assumed ground points only. It's not perfect, but still very very good.
mistoplus t1_iuempwf wrote
Reply to comment by hughk in Lidar technology unearthed tropical megapolis beneath forest canopy of the Calakmul Biosphere | Ancient Maya by marketrent
You are right, most of the measurements correspond to the forest canopy. However, the latest lidar sensors use very high frequencies and shifting angles, so some laser beams do get to the ground. The resulting point clouds are then classified by applying different algorithms (High vegetation, low vegetation, ground, etc.) and you can apply filters to create a surface from assumed ground points only. It's not perfect, but still very very good.