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Maybe I do not fully understand, what is requested but I think am missing something.

I thought one would like to have a ground profile, _without_ the trees. If one laser penetrates the leaves (and not the ground), why the complicated setup with to laser signals and calculating the difference. The second penetrating would be sufficient.

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My bad with explaining. The two laser example was to try to give an easy example and I ended up confusing everyone I guess. You have a single laser (for each wavelength usually). The laser shoots and is being returned (leaf). The same impulse is being returned again but from another surface at a marginally later point in time (the ground). If you would visualise that with common LIDAR software, you would have two points on the same trajectory: one at e.g. 20 m height above zero (ground) and one at 30 m above zero (leaf). You only have a single laser available that penetrates leaves but not ground. I hope that makes it more understandable. English is not my first language as well, so in case you want me to explain something again, I can try again or look up resources in English!

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