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I have obtained LiDAR data from my environmental agency in order to obtain the relative heights of each watercourse station. A far as I know, LiDAR signals do not get returned over water bodies. However my data does in fact show many last returns. Would these returns be reflections from nearby objects (vegetation)? The height values do however show that there is a relative drop from the river bank returns to the points returns on the water surface.

The image shows the course of a river and the LiDAR surface returns.

enter image description here

Could anyone explain why I get returns over water bodies and whether these returns have any practical use?

  • I have had no experience in using raw lidar but I would have assumed that returns from water bodies is where it is shallow. A river can have shallow and deep sections, gravel side bars or mid-channels bars that can be exposed. May be over-hanging vegetation? – Hornbydd Aug 28 '18 at 10:56
  • "Bathymetric LiDAR systems are able to detect two returns, delineating both the water surface and seabed." – Mapperz Aug 28 '18 at 14:18
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A far as I know, LiDAR signals do not get returned over water bodies.

It depends which laser wavelength was used in the survey. If it was a wavelength near the infrared (> 700 nm) it should be partially reflected/absorbed by water, and they are usually considered to be noise (see JeffreyEvan's comment). On the other hand, there are laser wavelengths which are not absorbed like the ones used for bathymetry (~ 532 nm). For example, take a look at: What is NIR LiDAR?

Would these returns be reflections from nearby objects (vegetation)?

They could be. Compare those return's intensity values with other locations with known vegetation. It could also be returns from shallow parts, river sandbanks, debris on water, or other type of sediments. But we can't know for sure, your image does not show too much. You can try to get other type of auxiliary data (like high resolution satellite or aerial image to confirm some of the cited hypothesis).

Also, perform a quality check on your point cloud (in land areas) and see if you find any other odd pattern within it (it might say something about the overall quality of your data).

Finally, the best move you can do is to contact the environmental agency (I assume they are the data providers) and ask them.

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    There are returns over water but they suffer from notable scatter and are often cleaned up by the vendor. Historically, returns in shallow water (eg., streams, creeks) were considered reliable and were been used in limited analysis. Early work by Jim McKean explored the use of shallow water NIR lidar for mapping sub-straight type but, abandoned it when EAARL became available. – Jeffrey Evans Aug 30 '18 at 18:21

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