Header information given by the lasinfo

[Sanka Perera]

Hello,

I am using lasinfo tool to extract header information....

when I use the tool to read header information, I got following information (only showed this part) in addition to long list..

I cannot clearly imaging what the exact meaning given by the below yellow highlighted terms are.

....

.......

reporting minimum and maximum for all LAS point record entries ...

  x 8565 355318

  y 6107 789975

  z 60906 73176

  intensity 1 445

  edge_of_flight_line 0 0

  scan_direction_flag 0 0

  number_of_returns_of_given_pulse 1 4

  return_number 1 4

  classification 6 6

  scan_angle_rank -16 -4

  user_data 2 2

  point_source_ID 3408 3408

  gps_time 269659.046017 269673.420564

........

.........

Therefore, can anyone explain me the exact meaning given by those terms are.

Thank you in advanced.

[Martin Isenburg]

Hello Sanka.

Here is what the latest LAS specification says about those fields:

http://www.asprs.org/wp-content/uploads/2010/12/LAS_1_4_r13.pdf

Edge of Flight Line:  The Edge of Flight Line data bit has a value of 1 only when the point is at the end of a scan.  It is the last point on a given scan line before it changes direction or the mirror facet changes.  Note that this field has no meaning for 360° Field of View scanners (such as Mobile LIDAR scanners) and should not be set.

Your Edge of Flight Line bits are not set.

There has been a bit private discussion within the LAS Working Group (LWG) a couple of weeks ago how to interpret this exactly in case of multiple returns for polygon versus oscillating mirror systems and I am not quite how it was resolved. I seem to recall that it was agreed upon that the exact definition is not clear. The discussion ended with the chair stating that he was not using this bit in his own software as it seemed rarely set “correctly” or not set. [Neverending Lament On] Due to the continued lack of transparency of the ASPRS LAS Working Group the only way to find out about such discussions is to ask the right question to someone who is part of that group and is getting cc-ed on those emails. (-: [Neverending Lament Off].

Scan Direction Flag: The Scan Direction Flag denotes the direction at which the scanner mirror was traveling at the time of the output pulse. A bit value of 1 is a positive scan direction, and a bit value of 0 is a negative scan direction (where positive scan direction is a scan moving from the left side of the in-track direction to the right side and negative the opposite).

Your Scan Direction Flag bits are either not set or you have a non-oscillating scanning system that only scans in one direction (e.g. rotating polygon mirror).

Scan Angle Rank: The Scan Angle Rank is a signed one-byte number with a valid range from -90 to +90. The Scan Angle Rank is the angle (rounded to the nearest integer in the absolute value sense) at which the laser point was output from the laser system including the roll of the aircraft. The scan angle is within 1 degree of accuracy from +90 to –90 degrees. The scan angle is an angle based on 0 degrees being nadir, and –90 degrees to the left side of the aircraft in the direction of flight.

Your Scan Angle Rank ranges from -16 -4. Either you are looking at only a very small subset of the data or the laser system was not scanning in nadir direction (or the Scan Angle Rank was not populated correctly).

User Data: This field may be used at the user’s discretion.

Regards,

Martin @rapidlasso

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