Light detection and ranging (lidar)
Learn about lidar, a technology that uses high-speed laser pulses to generate three-dimensional structural data about the terrain and landscape features.
4 September 2008 | Updated 14 October 2011
What is lidar?
Light detection and ranging (lidar) is a technology that uses laser pulses to generate large amounts of data about the physical layout of terrain and landscape features.
The data can be analysed and used in diverse applications such as:
mapping areas for building and structures in the construction industry
generation of digital terrain maps for use in geographic information systems
generation of digital vegetation maps for use in the forestry and land management industries.
How does it work?
All varieties of lidar operate using the same basic principle.
The lidar instrument fires rapid pulses of light (laser pulses) at the landscape and a sensor mounted on the instrument measures the amount of time taken for each light pulse to bounce back.
Because light moves at a constant and known speed, the lidar instrument can then calculate the distance between itself and the target with high accuracy.
By rapidly repeating the process, the lidar instrument builds up a complex ‘picture’ of the terrain it is measuring.
The lidar instrument fires rapid pulses of light and measures the amount of time taken for each light pulse to bounce back.
On the ground and in the air
There are two basic types of lidar sensors:
For aircraft-based lidar, the lidar instrument is typically mounted below the aircraft.
If the aircraft had zero velocity, then the instrument would collect a single line of data directly below the instrument (if level with the ground), and the width of the line (the scan width, or swath) would be determined by the scan angle.
Once the aircraft starts moving, the velocity and height of the aircraft, coupled with the scan and pulse rate of the sensor (along with the number of points recorded per pulse) determine the density of the dataset collected by the instrument.
To ensure data accuracy, the airborne lidar instrument needs to always know its precise location as it moves. This is usually achieved by providing the instrument with GPS positional information and aircraft pitch data.
Lidar for foresty applications
While it was originally used primarily for digital terrain mapping, more recently lidar has proved very useful for examining vegetation in native and plantation forests.
CSIRO’s ECHIDNA™ ground-based lidar sits on a tripod and scans a full hemisphere about its set-up point. It records waveforms for every direction in the hemisphere.
By knowing the direction of the beam and the distance between the instrument and the target, ECHIDNA™ builds a three-dimensional dataset representing the forest in the scanned hemisphere.
This enables forest managers to measure forest structure with unprecedented detail and to provide a permanent record of a forest’s three-dimensional structure at a given growth stage.
Airborne lidar has advantages over other airborne remote sensing, such as hyperspectral or multispectral imaging, in that it generates three-dimensional structural data because the laser pulses can penetrate the forest canopy to reach the ground.
The actual points collected are dependant on the hardware, but the most common are first hit (usually the canopy layers), last hit (usually the ground), and a secondary hit that lands somewhere in between. Some systems just have first and last hits, some have many more.
What does the lidar data reveal?
By analysing the data (Airborne lidar data processing services), researchers can determine:
- basic vegetation height – this is a common property derived from the lidar data, and for a given location is roughly the lowest (ground) point subtracted from the highest point (the canopy). Due to the low probability of hitting the absolute highest point in the canopy for a given pulse (and a given species of tree), canopy height surfaces derived from lidar are consistently lower than field measurements, but this effect can be accounted for with empirical corrections.
- vegetation strata – these can be inferred from the vertical distribution of points, taking into account the probability of higher rates of interception in denser vegetation.
- biomass (the volume of vegetation in a given region) – this can be related to the number of vegetation hits per unit volume (and possibly per vegetation strata), which can in turn be used to calculate important parameters such as carbon stock estimates and fuel loading for fire threat classification
- vegetation cover – this can be determined by the ratio of vegetation hits to ground hits.
Learn more about Forests.
ECHIDNA™ trademark is owned by CSIRO Australia.