LiDAR Demystified

For anyone commissioning a LiDAR project or using LiDAR data this is an important article to read. It is valuable to have some knowledge of the various system components and the effect of each variable on the accuracy of the final product.

Table 1 below lists these data capture variables and those covered in Part 1 of our series.
 
Essentially when undertaking a LiDAR survey, individual laser points are fired through at least one rapidly rotating mirror in a pattern left and right of an aircraft traveling at perhaps 220 km/ph. The aircraft speed, altitude and the mirror angle are recorded at the time each point is transmitted and received plus the transit time of each point.

The coordinates of each reflected point are computed, ground and non-ground points are separated and each swathe is adjusted to obtain a homogeneous data set. Finally, the points are transformed into the client’s nominated system.

Given all of these complex functions, data is routinely delivered with an absolute vertical accuracy of 0.1m at one standard deviation and a horizontal accuracy of 0.3m.

AAM has both Leica and Optech systems which can fly low level high precision LiDAR surveys ~10cm as well as broadacre regional surveys where +/- 25cm vertical accuracy may be requested.

Pulse Rates
One of the early points made in a recent article in Position magazine is that the pulse rate of an aerial laser scanner or LiDAR scanner is a “key differentiator”. This is certainly the case although most scanners available today have similar pulse rate capabilities. A high pulse rate (giving closer ground points) is a useful attribute when defining complex terrain or cityscapes, however, there are down sides which should be considered during project design. Perhaps the most significant of these down sides is the fact that, because the energy output for each LiDAR system is finite, the higher the pulse rate the less energy is emitted in each pulse.

This means that the chance of a pulse grazing between the leaves and limbs of vegetation, reaching the ground and reflecting back up to the system in the aircraft is reduced if the pulse has low energy when compared to a pulse with higher energy. Experience shows that the ground beneath dense vegetation is often modeled more reliably if the laser pulse rate is lower and each pulse is stronger.

Rather than emphasising a high pulse rate, the issue should be more about how many points are needed either on the ground or in the above ground data, to achieve an agreed outcome, rather than, the maximum number of points a system can theoretically emit.



Flying Height
Flying lower, whilst producing closely spaced points, requires more swathes to cover a given area hence leading to longer and more expensive data acquisition.

Multipulse Sensors
It is worth noting that state-of-the-art scanners now have the ability to emit and track more than one pulse at a time, known as “multipulse”. Older airborne laser scanners were only able to track one pulse at a time. They recorded each pulse emitted and did not emit the next pulse until the first had been reflected and received back at the aircraft. As the available pulse rate increased it became possible for more than one pulse to be in the air at once.

Laser Scanners with multipulse capability can fly higher (wider swathes, fewer swathes required, less building shadowing) and still produce closely spaced reflected points. But remember – higher flying heights mean lower vertical accuracy, larger footprints and increased positional uncertainty. The reduced vertical accuracy, perhaps 30 or 50mm, may be acceptable, particularly if the acquisition time and acquisition cost is reduced significantly!

Accuracy: Need versus Want
Whilst the accuracy of airborne LiDAR data is important, it is also important for potential users to carefully consider the use they wish to make of the data and the accuracy required. There is little use designing a survey to achieve a vertical accuracy of ground points of, say 0.1m, when the ground is covered with dense bushes or clumps of grass.

Remember that laser light passes between leaves, tree limbs etc. It does not pass through these objects! Over specifying a survey leads to significantly increased costs and may not achieve the desired result. High data accuracy, even in vegetated terrain, can be achieved using other measurement tools. The best solution depends on the size and location of your area and the task which requires the data.

Footnote on Eye Safety
Eye safety is an important consideration when planning a LiDAR survey. The LiDAR pulses emitted by some instruments will damage the eyes of people and animals that happen to look up as a survey aircraft passes directly overhead.

Many ‘new generation’ scanners are designed to be eye safe at low flying heights. This is done by using less powerful lasers and lasers which have a wider beam divergence, sometimes resulting in a larger pulse footprint on the ground. Other laser scanning systems achieve eye safety with automatic cutoff if the detected range approaches dangerous levels.

Whilst less powerful lasers are eye safe at lower altitudes it should be remembered that a larger footprint increases positional uncertainty. This is because individual pulses are reflected from that part of the footprint having the greatest reflectivity. They are not necessarily reflected from the centre of the footprint! A less powerful laser may also reduce the chances of LiDAR pulses passing between vegetation and being reflected from the ground.

For more information on LiDAR, click here.