Can someone help with an explanation and some references, in order to understand the nature of return pulses within a LiDAR system.
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2 Answers
ESRI has a pretty good help section on LiDAR (below). For more formal details on LiDAR, I would recommend the following books:
- Topographic Laser Ranging and Scanning: Principles and Processing
- Airborne and Terrestrial Laser Scanning
- Remote Sensing and Image Interpretation
LiDAR Laser Returns
Laser pulses emitted from a lidar system reflect from objects both on and above the ground surface: vegetation, buildings, bridges, and so on. One emitted laser pulse can return to the lidar sensor as one or many returns. Any emitted laser pulse that encounters multiple reflection surfaces as it travels toward the ground is split into as many returns as there are reflective surfaces.
The first returned laser pulse is the most significant return and will be associated with the highest feature in the landscape like a treetop or the top of a building. The first return can also represent the ground, in which case only one return will be detected by the lidar system.
Multiple returns are capable of detecting the elevations of several objects within the laser footprint of an outgoing laser pulse. The intermediate returns, in general, are used for vegetation structure, and the last return for bare-earth terrain models.
The last return will not always be from a ground return. For example, consider a case where a pulse hits a thick branch on its way to the ground and the pulse does not actually reach the ground. In this case, the last return is not from the ground but from the branch that reflected the entire laser pulse.
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1Also, for more in-depth information of the LAS format specification (there is a table in both with the supported class codes) asprs.org/a/society/committees/standards/… and (yet to be fully adopted) asprs.org/a/society/committees/lidar/LAS_1-4_R6.pdf. The 1.4 spec has changed some of the 1.2 class codes (applicable 1.1 and 1.3) with respect to 'model key points' and new class codes added. Commented Apr 13, 2015 at 22:48
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@JeffreyEvans Yes, it is from the link in the first sentence.– Aaron ♦Commented May 10, 2016 at 23:22
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2That is identical to a lecturer I gave several years ago at a lidar symposium in salt lake, UT. ESRI, is always eager to rip off materials. Commented May 11, 2016 at 4:11
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@JeffreyEvans This answer recently popped up on my radar. Do you have a reference to your materials that I can provide in the answer?– Aaron ♦Commented Feb 2, 2023 at 20:14
The picture below from Fernandez-Diaz (2011) might help complementing Aaron's answer.
Lidar returns are discrete observations* recorded when a laser pulse is intercepted and reflected by targets. Multiple returns derive from one laser pulse intercepting multiple targets (e.g. a top of a tree, its branches, and the ground).
*such as coordinates x, y and z; intensity of laser reflectance; scanning angle, among others possible .
As stated by Jeffrey Evans, it is important to keep in mind that:
discrete return lidar returns multiple measurements from a pulse, but are not tied to the specific pulse. Whereas, waveform lidar represents multiple binned measurements associated with each pulse. The waveform data allows one to derive a continuous distribution of the laser energy for each pulse where discrete return does not.
Reference:
Fernandez-Diaz, J. C. (2011). Lifting the Canopy Veil - Airborne LiDAR for Archeology of Forested Areas. Imaging Notes, 26(2).
answered Apr 17, 2015 at 16:54
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4One thing to keep in mind here is that discrete return lidar returns multiple measurements from a pulse, but are not tied to the specific pulse. Whereas, waveform lidar represents multiple binned measurements associated with each pulse. The waveform data allows one to derive a continuous distribution of the laser energy for each pulse where discrete return does not. Commented Apr 17, 2015 at 18:41
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@andre-silva, can you, please, explain the thing about "discrete return lidar". I had a feeling that LIDAR data always have the return number and the total number of returns, i.e. for each emitted pulse they know which returns belong this pulse. is that not correct? Commented Jan 16, 2018 at 11:25
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1@JohnSmith; the answer is JeffreyEvan’s comment. Discrete return has for each pulse a total number of returns and the return number, but in the final data itself (for example, in the .las file) each return is not tied to its respective pulse (the pulse which originated the return). On the other hand in full waveform data returns are tied to their respective pulse. Commented Jan 16, 2018 at 13:41
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@AndreSilva, ah, finally got it. Thanks a lot! But my understanding is that when the measurements are performed even with discrete returns "they" know which returns belong to which pulse. Which means that when the data is stored in a LAS-file, this information is basically lost. Is that right? Commented Jan 17, 2018 at 10:02
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2@JohnSmith, either that or the scanner doesn't record it (the pulse id, return id, etc) in the raw data (or maybe something else, probably due to technical limits?). I don't know the answer; perhaps it might worth a follow up question in GIS SE. "Why do discrete LiDAR returns are not tied to their respective pulses?" Make some research first to discover the most you can, explain in the body, make a comparison with full waveform, provide some references. Let me know if you decide to do it, I will support the Q. Tks. Commented Jan 17, 2018 at 12:13