Wednesday, 9 August 2017

The structure of the LAS1.3 file format used to store full-waveform LiDAR data

There are a few LiDAR file formats but the LAS1.3 was the first format to contain FW data and it is the file format supported by DASOS ( According to the LAS1.3 file specifications, a .LAS file contains information about both discrete and FW LiDAR data, with the waveform packets attached to discrete returns and saved either internally at the end of the .LAS file or externally in a .WVS file.

As shown at the following Figure, the .LAS file is divided into four sections.

 A brief explanation of each section is given here:
  • The Header contains general information about the entire flightline. For example, it includes the maximum scan angle used during the flight, whether the waveform packets are recorded internally or externally  and the number of Variable Length Records (VLR).
  •  Regarding the VLR, which contain arbitrary "extension" data blocks, the most important information given is the waveform packet descriptors that contain essential information on how to read the waveform packets (i.e. an ID, the number of wave samples and the size of each intensity in bits).
  •  The Point Data Records are the discrete points and the waveforms are associated with first return discrete points. Each Point Data Record has a spatial location, an intensity and optionally a pointer to a waveform packet as well as the ID of the corresponding waveform packet descriptor.
  •  The waveform packets is a list of intensities and they are either saved internally into the Extended Variable Length Records section of the .LAS file or inside an external .WVS file. Starting from the associated first return point, the spatial locations of the waveform packet (wave sample intensity) are calculated by adding an offset defined in the associated Point Data Record.
The full LAS1.3 file specifications are available here:

Please note that this text was taken from the EngD thesis of Milto Miltiadou, which was submitted to University of Bath in 2017.

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