difference between "Return Number" and "Number of Returns (given pulse)" (ASPRS documentation)

[Alessandro Montaghi]

Dear members group.

Today I was reading the ASPRS *.las file documentation in order to extract some info for my work. I have found these two definition: 

Return Number: The return number is the pulse return number for a given output pulse.  A given output laser pulse can have many

returns, and they must be marked in sequence of return. the first return will have a return number of one, 

the second a return number of two, and so on up to five

returns.

Number of Returns (given pulse):  The number of returns is the total number of returns for a given pulse.  So a laser data point may be

return two (return number) with a total number of five returns.

I am sorry for the simple question, but I have this doubt:

for a given LiDAR data set the "Number of Returns (given pulse)" is always the same for all laser resturns (ex: 2,3,4, or 5)

Thanks in advance

Alessandro  

[Jason Stoker]

Alessandro, not necessarily. The number of returns per pulse is variable per pulse. The number of returns you receive back is dependent on several factors- such as instrument, flying height, scan angle, vegetation height and density, etc. While an instrument may say it resolves 2,3,4,5 returns, it doesn't resolve them no matter what. It may return four returns per pulse over vegetation, but only one return per pulse over bare ground. In the technical sense a detector triggers a recording of a return based on the amount and distribution of energy returned- there are not fixed gates for each return number. We used to use the terms 'first and only', 'first of many', 'last of many', and 'intermediate' to explain returns. That seemed to  be easy to understand. That way if we were wanting bare earth points we could be pretty sure that they were not 'first of many' or 'intermediate' ones.

Jason

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[Alessandro Montaghi]

Thanks Jason,

If I understand (buy maybe I am wrong and correct me please) when we use terms of  'first and only', 'first of many', 'last of many', and 'intermediate' to explain returns we refer on about Return Number. While if I read at example 2 in Number of Returns that means I have 2 returns (and First and Last).

Really Thanks in Advance

Alessandro

[Jason Stoker]

Yes, but it actually kind of combines both together.  For example, 'first of many' means it is the first return back of several returns per that one pulse.  So that means it is return #1 of n (let's say 4) total returns.  In the 'last of many' case, that means it is return #4 out of 4 total returns per pulse. 'intermediate' would mean that it could be return # 2 or #3 of four total returns back from that pulse.  'First and only' means that you had only one return come back from that pulse. So that means it is return #1 of 1 total returns coming back from that pulse. 

The system could fire one pulse out and get four returns back, and then in the next pulse it fires it could get only one return back. 

 Here's a link to an old presentation of mine that might help: http://www.magicgis.org/magic/projects/lidar/lidar_101.zip  I tried to animate the returns to show how they work.  It is a big file, so sorry about that.

[Volker Wichmann]

Hi Alessandro,

just to clarify, each pulse can have several returns, with the number of
returns for each pulse stored in the 'number of returns (given pulse)'
attribute.

So for bare earth you will only have one return, resulting in

return number / number of returns (given pulse)
1/1

With vegetation, a single pulse will most likely result in several
returns, let's say three. This results in three coordinates (returns),
with one being the first, one the intermediate and one the last return:

return number / number of returns (given pulse)
1/3
2/3
3/3

Hope that helps,
Volker

> *
> *
> *Return Number:* The return number is the pulse return number

> for a given output pulse. A given output laser pulse can have many
> returns, and they must be marked in sequence of return. the
> first return will have a return number of one,
> the second a return number of two, and so on up to five
> returns.
>

> *Number of Returns (given pulse):* The number of returns is the

> total number of returns for a given pulse. So a laser data
> point may be
> return two (return number) with a total number of five returns.
>
> I am sorry for the simple question, but I have this doubt:
>

> for a given LiDAR data set the "*Number of Returns (given
> pulse)" *is always the same for all laser resturns (ex: 2,3,4, or 5)

[Hans Karl Heidemann]

FO:        NR=RN=1
FM:        NR>RN=1
IM:        NR>RN>1
LM:        NR=RN>1
 

H Karl Heidemann, GISP
Physical Scientist, Lidar Science
U.S. Geological Survey
Earth Resources Observation and Science Center
47914 252nd Street, Sioux Falls, SD 57198
Phn: 605-594-2861
Fax: 605-594-6529
kheid...@usgs.gov

 "Nothing matters very much, and few things ... matter at all."
                                                                -- Arthur James Balfour