How LiDAR Surveying Works
Lidar technology has been available since the late 1960’s and for commercial purposes for around two decades. As it becomes more common for a wide range of uses it has proved to be a valuable asset in the surveying field.
Historically, lidar has been a more costly option for surveying worksites compared to photogrammetry, but its benefits are significant, especially when it comes to surveying, tracking progress, and modeling vegetation-heavy areas.
With the continuous drop in the cost of lidar sensors, more and more sites are beginning to adopt lidar technology to great success. While it doesn’t actually ‘see through’ trees like some people may believe, it has capabilities and benefits unrivaled by other methods of surveying
In this article, we’ll dive into how lidar drone surveying operates, what it entails, and how it measures up against photogrammetry.
What is LiDAR?
LiDAR (Light Detection and Ranging) works by emitting laser light pulses toward objects like terrain, trees, or cliff faces and measures the time it takes for the reflected pulses to return back to a sensor. By calculating the time delay between when the laser is emitted and when it is received back, lidar systems can determine the distance between the sensor and the target with high precision. Tens or hundreds of thousands of laser pulses can be emitted per second in different directions. Each time it hits an object, it creates a point and measures a distance which as the sensor moves through an area will create a precise 3D point cloud of the environment. This means is is possible to capture very thin objects like power lines as well as fit between the gaps in leaves and foliage and reach bare earth (if the area isn’t overly vegetated).
Professionals have been using lidar for years to accurately calculate distances using trigonometry. Thanks to advances in drone technology processing software, LiDAR now allows users to capture millions of data points to form detailed models of the bare earth in vegetated landscapes.
Lidar hardware can be mounted on different platforms, from drones to tripods, cars, trains or even planes.
Lidar Drone Surveying
The process for lidar drone surveying mirrors drone photogrammetry, especially when utilising sensors such as DJI’s Zenmuse L1/L2 on drones like the M300 or M350 RTK.
A typical lidar drone surveying process includes:
- Planning the flight and setting up the base station
- Piloting the drone through the automated or manually flown mission
- Landing the drone and uploading the data for processing
Lidar vs. Photogrammetry for 3D Mapping
With the basics covered, the key question is: how do lidar and photogrammetry differ, and which is more suitable for your site? Each technology has its strengths, and one isn’t necessarily better than the other. Your needs will ultimately determine which is the best fit, and in some cases, you may use both technologies during different stages of a project.
The choice depends on your objectives—whether it’s penetrating dense vegetation, creating a photorealistic 3D map, or surveying in low-light conditions.
Let’s explore the differences between lidar and photogrammetry and when each method excels.
Weather and Lighting
Both lidar and photogrammetry rely on favorable weather conditions for drone flights, meaning minimal wind, moisture, or extreme temperatures. Photogrammetry, however, requires sunlight to capture high-quality images, whereas lidar is less affected by lighting conditions.
For night-time drone flights, special permits are required regardless of technology. Using a tripod-mounted lidar system can sometimes bypass the need for these permits, particularly in low-light environments.
Output Comparison
The most striking difference between lidar and photogrammetry lies in the final output. Photogrammetry produces photorealistic 3D models using actual images, much like what you see in Google Earth. In contrast, lidar generates a 3D point cloud, which lacks visual detail and realism.
Vegetation and Canopy Penetration
Vegetation presents one of the biggest challenges when comparing the two technologies. Lidar is far better at surveying areas with dense vegetation, while photogrammetry excels on cleared sites.
Lidar’s ability to send millions of light pulses into the environment allows it to penetrate leaves and foliage, capturing data on the terrain beneath. Under optimal conditions, lidar can measure through up to 90% vegetation cover, whereas photogrammetry tops out around 60%. Photogrammetry tools like Propeller can filter some vegetation, but lidar is the superior choice for densely vegetated areas.
Can Lidar See Through Vegetation?
Lidar doesn’t “see through” objects but instead measures the spaces between them. For areas with dense vegetation, lidar systems need to have a high number of points per second (PPS) to ensure accurate data. Some lidar systems can achieve as many as three million points per second, making them ideal for dense environments.
However, for extremely dense areas, manual surveys might still be necessary to capture ground-level points.
When to Use Lidar
Lidar is especially useful for:
- Measuring features through dense vegetation
- Surveying tall structures or structures with intricate features (such as powerlines)
- Surveying the entire structural composition of an asset accurately and quickly
- Operating in low-light or night-time conditions (where permitted)
When to Use Photogrammetry
Photogrammetry is ideal for:
- Creating highly detailed, photorealistic maps and 3D models
- Surveying cleared sites
- Daytime operations
- When the budget doesn’t allow for LiDAR and will provide the same or better result
Survey Accuracy and Data Processing
Both lidar and photogrammetry can offer high accuracy in drone surveys, but the precision ultimately depends on drone stability rather than the technology itself.
For both techniques, using a ground control base station is essential. With tools like RTK and PPK, photogrammetry can achieve accuracies up to approx 3cm, while lidar can reach vertical accuracy around 5cm.
Processing lidar data is similar to photogrammetry and will need to uploaded to software to develop the 3D pointcloud. From here, features can be identified such as the ground which can then be removed, providing a bare earth model (digital terrain model or DTM) which is not possible with photogrammetry on vegetated areas.
Conclusion
Lidar is a powerful tool for surveying densely vegetated areas and delivering highly accurate measurements, while photogrammetry is the go-to for creating realistic 3D maps or models on cleared sites or structures. The two technologies complement each other, and both are valuable for various surveying and project management needs across industries.
