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I do not fully understand LiDAR Return Values, as opposed to Class Codes.

Both Arc and LAStools have options for selecting returns based these "values". For example, here is the Arc 10.7.1 Make LAS Dataset Layer tool, which has a "Return Values" section (LAStools has similar Return Values):

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Furthermore, ESRI documentation describes Class Codes, but I can't find anything in their documentation about Return Values.

Importantly, these "values" are not mentioned in the ASPRS LAS Specifications, which I assume (perhaps incorrectly) is the official repository of all things LAS.

I would like to learn more about these Return Values: Are there any published standards? How are they derived? Are their values consistent between different software packages?

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    What do you mean they're not mentioned? The LAS spec you linked has 88 hits when searching for "return". Briefly, a given laser pulse can reflect off of multiple features and return to the sensor multiple times. The first return would generally be nearest the sensor, so higher in elevation for aerial collects. Each point record can contain which return that point was derived from, the return value.
    – mikewatt
    Mar 27, 2020 at 18:53
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    What you named Return Value is the Return Number. Search for that in the specs (page 10)
    – JRR
    Mar 27, 2020 at 19:00
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    @JRR Thanks for your measured response. As you probably surmised, I was thrown off by ESRI's use of the phrase "Return Values". A search of that phrase through the ASPRS document returned no hits, and I therefore incorrectly concluded that ASPRS had not covered that topic. With such published standards, I will assume that software developers rely on them. In the end you helped me learn, for which I am grateful. If you'd like to convert your comment to an answer, I'd be happy to give you credit.
    – Stu Smith
    Mar 27, 2020 at 20:09
  • ESRI likes to invent their own terms to make sure their vendor lock-in works as intended. Always assume truth and standards to be the other way around.
    – inc42
    Mar 27, 2020 at 21:40

1 Answer 1

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What you named "Return Value" is the "Return Number". Search for that term in the specs. The first interesting occurrence appears page 10 of the linked document. It correspond to the fact that a single emitted pulse can return several points. Those points are numbered from 1 to 5 sometime up to 7. In ALS data we often process first returns only. This is why you can filter first returns only, but also first of many for example meaning the first points of pulses that return more than one point.

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