Number of Returns
Imagine you’re hiking in a forest. You look up. If you can see light, this means that LiDAR pulses can go through too. Also, this means that LiDAR can also hit the bare Earth or short vegetation. A significant amount of the LIDAR energy can penetrate the forest canopy just like sunlight.
But LiDAR won’t necessarily only hit the bare ground. In a forested area, it can reflect off different parts of the forest until the pulse finally hits the ground:
Using a LiDAR to get bare ground points, you’re not x-raying through vegetation. You’re really peering through the gaps in the leaves. LiDAR collects a massive number of points.
These multiple hits of the branches is the number of returns.
In a forest, the laser pulse goes down. We get reflections from different parts of the forest – 1st, 2nd, 3rd returns until it finally hits the bare ground. If there’s no forest in the way, it will just hit the surface.
Sometimes a pulse of light doesn’t reflect off one thing. As with the case of trees, one light pulse could have multiple returns. LiDAR systems can record information starting from the top of the canopy through the canopy all the way to the ground. This makes LiDAR highly valuable for understanding forest structure and shape of the trees.