LiDAR Takes Its Place on Drones
While drone deployment is still in early stages for many companies, an important trend to watch is the falling price of laser imaging, detection and ranging gear (LiDAR).
The drop in LiDAR prices is suddenly making LiDAR deployment on drones a more affordable alternative that is likely to compete with photogrammetry as a choice for capturing visual imagery from drones.
Why would companies using drones prefer LiDAR over more traditional photogrammetry? Much of the decision rests on how much imagery detail a drone mission needs to capture.
With photogrammetry, the video imagery is in photographs. In order to stitch together a 2D or 3D rendering of a targeted terrain, a drone using photogrammetry must take a large number of snapshots in order to render that 2D or 3D image. What photogrammetry produces is precision color images of the actual ground cover. This can be especially helpful when surveying the health of crops in a field.
LiDAR’s approach and advantages are different. With LiDAR, light-based sensors bounce waves across the surface of a targeted terrain. Unlike photogrammetry, the LiDAR sensor waves actually have the ability to penetrate small gaps and get past objects such as brush or trees — enough to provide a more accurate view of the terrain. If a construction company needs to see the contour of the land beneath ground cover, it can.
While it isn’t likely that LiDAR will supplant photogrammetry in all drone missions, it is likely that more companies will use it. This comes at a time when many companies are relatively unfamiliar with LiDAR as the technology of choice on drones — or with how to plan missions using LiDAR and drones.
Planning for LiDAR
“When planning drone LiDAR missions, firstly companies need to be familiar with the area they are going to fly over. Secondly, they need to be familiar with the requirements of the outcome,” says Kristaps Brass, SPH Engineering at UgCS, which provides drone mapping software. “A LiDAR drone operator needs to be able to generate data based on client requirements and take into account factors such as exact borders of the area, topography and vegetation. Abilities of the drone cannot be overlooked either. The operator needs to know what area can the drone cover in a single flight to gather data with the required accuracy and how many flights need to be made.”
Since drones have limited flight time, knowing how much area you have to cover in a single flight and to what degree of detail are central considerations for any drone LiDAR mission plan.
“The most important tradeoff you must consider in planning is the one between point density and flight time,” says Brass. “Flying closer to the ground and at a lower speed will give a greater point density per square meter or per square mile, and the LiDAR beam will be able to go through vegetation. However, since drones have limited flight time, this also will mean that the area the drones will be able to scan will be smaller. The higher and faster you go, the less accurate of a model you can make from the gathered data, so you need to find a good middle ground based on the requirements.”
A good example of LiDAR and drone mission planning can be found in the construction industry.
Let’s say a construction company wants to know the terrain elevation data of a proposed building site before excavating the site, but it realizes that whatever elevation data is on file is out of date. The company’s first objective is to perform a general survey of the site by using LiDAR-equipped drones that fly at a high altitude and can capture the entire site without any obstacles.
The data from this high-level flight can be converted into a 3D site elevation model that gets imported into the company’s mission planning system. The company then follows up with a second round of lower altitude LiDAR/drone flights that can more accurately record current conditions and obstacles specific to the site that the initial, higher altitude survey could not.
Most companies will undoubtedly use a combination of both photogrammetry and LiDAR on drone flights, with LiDAR being able to solve sticky problems like being able to see beneath a tree canopy or detecting unique site features, such as a field of rocks. LiDAR also has the ability to eliminate the time, expense and risk of sending out field crews to do the same.
“In my mind, the most important skill of all is the ability to fly a drone in a safe manner in both manual and automatic modes,” says Brass. “LiDAR equipment is by no means cheap, so the operator needs to be confident in his or her piloting as well as in mission planning skills. The best suggestion for inexperienced drone operators is to sign up for a training workshop in which they can gain these skills in a safe environment and later apply them in the field with confidence.”