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I'm working on an ASCII LiDAR file that has the first and last pulse returns included.

Can anyone tell me why a first pulse return may come back with more blank spaces in the data than a last pulse return? I just think logically it should be the other way around, but maybe I'm missing something?

Andre Silva
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Kim
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    Is this raw data? If it is not, perhaps someone already did some kind of processing with it like thinning. Did you analyse first and last returns by elevation? Perhaps someone set direct hits in the ground as last returns, instead of first. This is an interesting question, but you could provide more details about your data: what is the context, how is the surveyed area, how did you get such data, types of pre-processing accomplished on it, some screenshots showing the blank spaces, etc. – Andre Silva May 10 '16 at 13:20
  • I have seen this before with raw data when the returns are on bare ground such as highly reflective pavement. There are numerous explanations for this phenomenon according to some of the experts I have talked with, and each one of the reasons makes sense, but one that makes the most sense was, with matching first and last return readings for the bare ground, the last return arrives at the same time as the first, and there is no difference in the reading to accumulate. I have been told this does not happen often, but when it does, it is a good thing to check corresponding imagery. – jbgramm May 10 '16 at 14:28

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