Did you know there are multiple types of drone surveying? Drones can be outfitted with very different kinds of technology. Some of the most common types are lidar and photogrammetric drone surveying, which we’ll be going over here.
If your work requires getting a bare earth survey on land cluttered with vegetation, you might have heard that lidar is the best option, but with new advances in software photogrammetric surveying can achieve the same results using more affordable equipment.
Further, the operational ease-of-use for photogrammetric surveying is much improved, and you don’t need special training to survey with a drone. (Depending on where you’re located, you just need to get certified by your country’s aviation authority. For the US, you need a Part 107 from the FAA. For Australia, you need to get an ARN from CASA.)
What is lidar?
Invented in the 1960s, lidar stands for “light detection and ranging,” and it measures distances by sending laser pulses at a feature (a tree, the ground itself, a cliff face) and measures the reflected pulses with a sensor. With some trigonometry, you can calculate distances very accurately, and with many, many points of reflection (think millions) you can reconstruct a point-based model of terrain.
Lidar can be likened to sonar or radar, which use sound and radio waves respectively to map surfaces and detect objects. In most cases, lidar uses infrared light. Its hardware can be mounted on a plane, tripod, or automobile, as well as a drone. It’s sometimes called 3D laser scanning.
How photogrammetric surveying differs from lidar
Lidar is a direct measurement—you’re physically hitting a feature with light and measuring the reflection. Drone photogrammetry is when you use images captured via drone to reconstruct the terrain in 3D using image overlap and sufficient ground control.
We won’t go into much detail here, but you can learn more about how drone photos turn into 3D surveys in this blog post.
The main difference between these to surveying methods is, of course, how they capture their data. But many people are under the false impression that photogrammetric surveying can’t get good bare earth accuracy.
With the right filtering, like the bare earth filtering in the Propeller Platform, you can get the data you need while retaining the useful visuals of a photogrammetric survey if your vegetation cover isn’t extremely dense.
Lidar produces a point cloud-like image, whereas photogrammetry uses photos of the real site and can then be used to make a fully 3D, visual model of the real world.
Lidar’s ideal uses
Like all technology, lidar has use cases that it’s especially well-suited for. When it comes to surveying land with dense vegetation, where the light pluses can still penetrate between branches and leaves, lidar gets highly detailed information.
This is a big advantage over photogrammetry, as photogrammetry can only produce accurate surveys when there is sparser vegetation on site. While lidar does have similar limitations, under good conditions it can penetrate areas with up to 90% vegetation. (Photogrammetry sits closer to the 60% mark.)
This is thanks to the nature of direct measurement we discussed above—because lidar is firing many, many pulses at its target terrain, they can more easily get between the foliage and make it to ground level.
If you’re working on a site with vegetation denser than 60% all the time, and have your own in-house processing software with someone to run it, lidar may be worth it for your business.
As you would imagine, all these capabilities make lidar a fantastic solution for the forestry industry. There, it has the dual advantage of mapping denser vegetation and very large tracts of trees. It’s also great for vast, watershed-scale surveys via plane.
But when it comes to the broader surveying needs in the construction, mining, and aggregates industries, photogrammetric drone surveying is the more affordable solution, with great accuracy and ease-of-use, in most cases.
Debunking common lidar myths
After talking to many customers who were looking into lidar surveying for their own sites, we realized there are a couple misconceptions popping up all the time.
Lidar can see through vegetation.
False. Lidar isn’t the same as X-ray vision. Lidar penetrates the spaces between the foliage, trees, or other flora. It can map the ground covered with denser foliage, which is a point over photogrammetry. But note that should your site be extremely dense with vegetation, you might have to send a surveyor out to shoot a few points under the canopy.
Lidar is the most accurate surveying solution.
False. Photogrammetric surveying is just as accurate as lidar. With workflows like the Propeller PPK solution, you can get accuracy of 1/10ft with a few simple tools. (See our whitepaper for more information.) For both types of drone surveying, accuracy in the air is affected by the movement of the drone, not just the sensor itself. So, no matter what tech you’re using, you’re still running the same data correction.
Benefits of photogrammetry in surveying
When you build a site survey with photogrammetry, you get a model as true to the real world as possible. With processing platforms like Propeller, you also get a 3D model of your site that’s fully measurable and easily navigable.
Lidar gives you a point cloud, but because photogrammetry is stitching photos together to create your model, you get the visual details of every feature on your site, taking abstraction out of the conversation. Think of it like this: lidar produces an elevation set, while photogrammetry gets you that plus visuals.
Photo-based models have the additional benefit of being able to use them to verify what feature you’re seeing. When it comes to fine-detailed surfaces and features, lidar can be unspecific enough to be unsure about what exactly it is you’re seeing.
For example, if you’re surveying a cliff face to evaluate the safety risk, it might be easy to see a broadly articulated tree on the model, but smaller flora can be hard to detect and easily mistaken for rock (as in the image below).
Affordability and lower replacement cost
One of the biggest differences between the two systems is price. A drone-based lidar system can set you back $60,000 or more for the hardware alone. Whereas a drone from DJI’s Phantom series, like the Phantom 4 Pro or Phantom 4 RTK, cost a few thousand dollars and are generally widely available.
Lidar’s upfront cost also translates down the line. It’s more financially risky to put a five-figure drone into the air than a four-figure drone. If you damage or crash a lidar drone, you need to replace it at full cost.
And due to the specialty manufacturing and involved setup process, you likely won’t be back in the air immediately. With a DJI drone, you can pay for the repair package for a few hundred bucks and have a replacement drone on its way to you the day of.
Drone setup and calibration
When it comes to user-friendliness, both drone surveying methods have their pros and cons.
Lidar doesn’t use a camera, so you don’t need to calibrate camera settings for environmental conditions like you do on a photogrammetric drone.
But lidar’s hardware can be difficult to set up—calibrating for the center of gravity and measuring the lever arms both require a huge amount of precision.
Ground control is a necessary part of surveying no matter which technology you’re using. In the past, lidar had the advantage of not requiring laying out GCPs, saving people a lot of legwork and time. However, with Propeller PPK, it’s essentially the same setup workflow as lidar: set up your base and fly.
Data capture and processing
The time expended on drone-based lidar and PPK drone surveying is on par. Neither system requires traditional ground control setup, just a base station on the ground to for data correction.
In some ways, lidar is easier to process simply because it is less detailed than photo-based datasets. However, there is no cloud-based lidar processing on the market at present, you have to do it yourself. That takes in-house software and computing hardware, as well as a specialist who can properly render and correct that raw data.
Cloud-based data processing saves you and your team time, money, and headaches
With Propeller, all you need to do is upload your drone photos, you base station data, and tell us what coordinate system you’re working in—we do the rest, and give you a 3D site survey 24 hours later.
When it comes to matters of cost, user friendliness, and visuals in your survey, we recommend photogrammetric drone surveying for 95% of cases.
Want to see photogrammetry in action? Check out a demo of the Propeller Platform.