Drone LIDAR: Many Happy "Returns"
Author: Lewis Graham
Published On: September 24, 2020
Evaluating General Purpose Sensors
Drone LIDAR systems with pricing below around $150,000 USD generally use a laser scanner designed for general purpose work (such as autonomous driving) as opposed to being purpose-built for survey grade drone mapping, such as RIEGL and Optech. This means that system manufacturers (and sometimes you, the end user) have to be very careful in evaluating the characteristics of these general purpose (GP) sensors. Of course, the most obvious parameter is the range of the sensor when judged using 10% reflective targets. However, another important technical feature that might be missing from GP scanners is support for multiple returns.
Time Of Flight Laser Scanners
Time of flight (TOF) laser scanners (most kinematic laser scanners use the TOF principle) send out a very short (i.e. a meter or two) pulse of light and "listen" for return energy reflected from objects. Using the fact that the speed of light is more or less constant (it is affected by the medium through which it travels, such as the atmosphere), a simple computation of distance is possible.
Laser scanners with very fast time recovery electronics (or alternatively, wave form processors) can "listen" for multiple reflections from the same outgoing pulse. This is illustrated in Figure 1.
Figure 1: Multiple Returns from LIDARThe ability to collect and process multiple returns has been a fundamental attribute of mapping LIDAR systems for quite a long time. The very first version of the ubiquitous American Society for Photogrammetry and Remote Sensing (ASPRS) LAS point cloud file format included detailed tracking of multiple returns (up to five returns per outgoing pulse, later increased to 15). Formally, Multiple returns are encoded as "Return 2 of 3", "Return 1 of 1" or, in general, "Return M of N." Knowing which return of an outgoing pulse a return represents is very valuable in both visualization and automated algorithms. For example, the ground has to be a Last return (since the laser is not ground penetrating). On the other hand, an overhead wire is quite often a First return.
True View EVO Tools
In our own True View EVO software, we will shortly be releasing an automatic wire extraction algorithm. This algorithm is "seeded" by sketching a guideline with a vertex at each power pylon. Using the multiple return attributes of the laser scanner makes it easy to pick out the wires and towers when sketching a guide. For example, Figure 2 shows an area over a transmission corridor colorized by elevation. If you look closely, you can see both the wires and the tower, but it is a bit of an eyestrain.
Figure 2: LIDAR data colorized by elevation In Figure 3, we are still coloring by elevation, but I have turned off all but "First of many" returns. Printing to a document does not do the image justice, but in exploitation software such as True View EVO (which is bundled with every True View sensor), you can clearly see the wires and tower.
Figure 3: Powerline corridor, color by elevation, first of many returns only
Multiple Returns in Automatic Ground Detection
Multiple returns are also invaluable in automatic ground detection. Here you would set the return "filter" to "Last Returns Only." These would be 1 of 1, 2 of 2, and so forth. Using Last returns in ground classification can diminish the occurrence of false ground points close to the true ground.
One of the criteria we used when selecting the Quanergy M8 Ultra laser scanner for our True View 410 system was how well the scanner supports multiple returns. We have seen many "utility grade" scanners advertised as multi-return but, when we evaluated the system, the count of "First of many returns" in canopy areas was disappointingly low. Some scanners (e.g. the Ouster OS2) that are promoted as good scanners for drone mapping do not support Multiple returns at all!
If you are dealing with a knowledgeable airborne mapping technology company, you should not have to worry about these details; we are ensuring the selected scanners have the attributes you will need to perform high quality mapping. If, however, you are shopping solely on price or are considering building your own scanner, be aware of the importance of features such as Multi-return. These features often mean the difference between success and failure in generating deliverable, high quality products.