Maximum number of returns per laser pulse in airborne LiDAR?

Question

What is the maximum amount of returns a discrete aerial laser scanner is capable of recording?

For example in a LAS 1.2 metadata under Point Data Records there is the category 'Number of Returns' with minimum and maximum values listed:

  number_of_returns   1          6
  edge_of_flight_line 0          1
  scan_direction_flag 1          1
  classification      1         12
  scan_angle_rank   -30         29
  user_data           0          0
  point_source_ID    45         80
  gps_time 176457.257923 348033.538859
number of first returns:        1906575
number of intermediate returns: 2157
number of last returns:         1906575
number of single returns:       1881894
WARNING: there is 1 point with return number 6
overview over number of returns of given pulse: 1881894 45403 5441 593 76 6 0

As I got a warning I wanted to check what was the maximum number of returns the laser software and/or the processing software was capable of recording. Would the warning indicate that a return number would be erroneous?

I also looked at an example of a laser scanner (Riegl LMS-Q1560) and could not find any reference to maximum possible returns.

Answer 1

There is a maximum of 5 and it depends on the software. There can be 3–5 returns possible per laser pulse.

Discrete return lidar can record multiple measurements within a single laser pulse. If the reflected signal strength exceeds a given threshold, then the sensor will record another measurement, up to the maximum number allowed by the sensor (laser pulses reflected where there is no canopy will result in a single ground return per laser pulse). Lidar sensors use variable gain to compensate for landscape-level variations in ground brightness and surface object reflectivity, resulting in non-calibrated intensity values. Most commonly, depending on the sensor, 3–5 returns are possible per laser pulse. It is recommended that a sensor capable of at least three returns be specified. Some sensors and post-processing software assign additional return values that are data flags and not new measurements. These additional returns reference whether a return is single or one of several: first, intermediate (2nd, 3rd, etc.) or last. For example, data collected by TerraPoint® for the Puget Sound Lidar Consortium collected up to four returns per pulse. Return levels were 1–4 for returns when there were not last returns and 5–7 when there were last returns

Source: https://www.mdpi.com/2072-4292/1/4/776/pdf

Answer 2

Found the reference below that some LiDAR instruments can record up to six returns depending on the discretization settings:

Modern instruments can process the energy-backscatter pertaining to a single beam and identify up to six returns, but the majority support only up to four.

A Guide to LIDAR Data Acquisition and Processing for the Forests of the Pacific Northwest

I will ask the vendor what discretization settings the software has.

Answer 3

"As I got a warning I wanted to check what was the maximum number of returns the laser software and/or the processing software was capable of recording. Would the warning indicate that a return number would be erroneous?"

The LAS 1.2 format has in its section 'Point Data Records' 3 bits for storing Return Number and 3 bits for Number of Returns (see in asprs.org, 'LAS 1.2 Format Specification September 2 2008 (PDF)', from page 7 to 10, point data record formats 0 to 3).

3 bits represents 8 combinations (so, 8 different numbers). This means it is possible to record more than 5 return types.

The warning WARNING: there is 1 point with return number 6 you got is from the lasinfo report from LAStools. Martin Isenburg (LAStools dev) explains it in this post:

... . The WARNINGs about return number 6 and 7 usually mean the data was acquired with RIEGL scanner that occasionally generate more than the usual 4 or 5 returns that you get from the common Leica and Optech systems. Originally the LAS format was only designed to have at most 5 returns per shot (Paul Galla from Leica Geosystems was part of the original LAS design team in 2004). However, enumerating returns from 0 to 7 is possible in the LAS format (there are 3 bits) and the 1 to 7 range is commonly used (the 0 is usually assigned only to either synthetic or non-LiDAR points such as those generated by dense-matching ). I should probably reformulate the WARNING to raise less concern.