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I've read articles like these https://www.nationalgeographic.com/history/article/maya-laser-lidar-guatemala-pacunam and wanted to understand what's going on here. My understanding is the lidar would mostly bounce off of the canopy but to create maps like the ones in the article you would need to generate height maps from below the canopy.

Is there some sort of filtering of the point cloud that's going on? Do enough of the pulses reach the ground and back that meaningful data can be collected? I'm just wondering how surveyors able to generate meaningful maps of what what the canopy is hiding and make these amazing discoveries?

Here's an example image: enter image description here

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Google research indicates there that seem to be multiple techniques. Some involving multiple looks from different angles (see "High-resolution foliage penetration with gimbaled lidar"). Some involving multiple lasers where the interference between two beams cancels out the foliage return (see "Lidar system able to penetrate foliage and map obscured environments". This seems to also be called "gated digital holography" (see "Seeing the forest through the trees with a new LiDAR system"). Filtering for flecks of the ground as you suggested seems to be the basis for most of the techniques.

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I saw the article a couple of years ago and thought the same. Looks like filtering is the only option. If they flew the drone near the surface, few of the multi-million beams might have hit the ground and I guess they were extracting the lowest points to generate the ground mesh.

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