LiDAR vs Photogrammetry For Drone Surveys
In-depth comparison between drone LiDAR and drone photogrammetry: Which is best, pros and cons, and how the DJI Zenmuse L1 and P1 have revolutionised aerial surveying. ... Read More
- In-depth look at drone LiDAR and drone photogrammetry;
- Generally speaking, drone LiDAR is better in areas of high vegetation while drone photogrammetry is better for creating highly-visual digital assets;
- New DJI payloads - the P1 and L1 - have been released to aid drone photogrammetry and LiDAR missions.
What Is LiDAR?
LiDAR stands for light detection and ranging.So, how does LiDAR work? LiDAR - sometimes known as 3D laser scanning - works by sending pulses of light to the earth's surface or a feature on it and measuring the time it takes to reflect back.The amount of time it takes to return to the LiDAR source indicates the exact distance of the object or feature. This is known as the Time of Flight (ToF) principle.This gives an accurate positioning point of where on the earth's surface the laser hit. Depending on the sensor used, LiDAR units can blast hundreds of thousands of pulses per second, helping to build a detailed visualisation of what the sensor is focusing on.This can then be used to create 3D point clouds; an accurate visualisation representing the scanned area's terrain, topography, and features.What Is Photogrammetry?
Photogrammetry is the art of capturing high-resolution photographs to recreate a survey area.These images are processed and stitched together using sophisticated software to create realistic, geo-referenced, and measurable 3D models of the real world.LiDAR vs Photogrammetry - Pros And Cons
Like anything, LiDAR and photogrammetry have their upsides and downsides.These factors, coupled with the mission brief, may well determine the preference for either application.Mission Versatility
One of the major advantages of using LiDAR over photogrammetry is the enhanced performance during certain missions.For instance, LiDAR is more effective - and thus, more accurate - in survey areas with high vegetation cover.Why? Because LiDAR pulses are able to penetrate gaps between leaves and branches to reach ground level.In contrast, in traditional photogrammetry, vegetation can stop you from acquiring an accurate representation of what the ground topography looks like.For this reason, LiDAR is a more accurate solution than photogrammetry for building a Digital Terrain Model (DTM); a model of the bare earth (without any objects/buildings in it) containing elevation data of the terrain.This dataset, shared with heliguy™ by Ben Bishop, Technical Director at , shows the differences between LiDAR and photogrammetry for capturing ground levels for DTM generation.Ease Of Use
Accessibility is one of the biggest differences between LiDAR and photogrammetry.LiDAR is fairly complex and requires a high level of understanding, which can widen the room for error and increases the demand on an experienced professional.However, the DJI L1 is helping to bridge that gap by simplifying LiDAR workflows and making them more accessible to more people.That said, generally speaking, photogrammetry is a more straightforward surveying method.Yes, there are things to think about, such as what flight height will provide the best ground sample distance (more of that shortly), and what image overlap rate is needed to ensure the most accurate model.But, generally, advancements in drone photogrammetry have enabled simplified workflows, helping to bring accurate maps and models within the grasp of any organisation with a decent camera drone.Cost
LiDAR uses lasers, whereas photogrammetry relies on images collected from a camera.And traditionally, LiDAR sensors were large, so a bigger aircraft was needed.Therefore, photogrammetry has become known as a more affordable solution than LiDAR.However, this gap is closing. Take DJI's L1 LiDAR payload, for instance, which represents a more cost-effective and compact solution than other LiDAR sensors on the market.LiDAR v Photogrammetry - Which Is More Accurate?
When it comes to surveying, one of the most important considerations is accuracy.Done correctly, LiDAR and photogrammetry are reliable and accurate methods.But, before digging deeper into this, here's the definition of two terms which are integral to this section:- Relative (Vertical) Accuracy: Relative accuracy refers to the accuracy of a dataset’s data points between one another. If the distance between point A and point B is 62.128m on the ground then with an overall relative accuracy of +/- 20mm you would expect the model distance to be between 62.126m and 62.130m. Data with relative accuracy WILL be collected with reference to a coordinate reference system, but not to a high accuracy.
- Absolute (Vertical) Accuracy: Absolute accuracy refers to the accuracy of the dataset in relation to a coordinate system or datum. For example, with a 20mm absolute accuracy, there would be a 40mm margin for error (20mm each way).
Drone LiDAR Accuracy
When accuracy is king, LiDAR is an extremely reliable solution.Remember, LiDAR works by hitting the earth's surface, or a feature on it, by firing out laser pulses from above and measuring the reflection. It, therefore, represents a direct measurement.This, coupled with its integration with GNSS data, helps to give your digital terrain map high levels of vertical accuracy.LiDAR is also an ideal solution to achieve absolute accuracy, especially if the intention is to create a realistic bare earth model, because of its ability to account for elevation and vegetation.In fact, thanks to LiDAR's high data point density (with sensors emitting hundreds of points per square metre), surveyors can create a really robust data set.As a side note, when looking at different LiDAR solutions, the laser pulse rate is a key specification. A higher pulse rate collects more measurements per second, creating more detailed models and reduces the number of flights needed to cover a specific area.But, LiDAR is a complex system, and many factors determine accuracy.It's not just about the LiDAR sensor: Other high accuracy systems such as satellite positioning (GNSS data) and an inertial measurement unit (IMU) need to be of high quality to build an accurate point cloud that accurately reflects the survey area, its terrain, topography, and features.But, overall, LiDAR tends to produce scans with greater detail and accuracy compared to photogrammetry.Drone Photogrammetry Accuracy
However, that is not to say that photogrammetry is not an accurate survey solution.To the contrary. In fact, some drone solutions can yield centimetre-level accuracies.But there are some important factors to consider which can impact accuracy.For instance, the quality of the drone and the camera.Sensor size, its resolution, and focal length all affect ground sample distance (GSD) and the required flight altitude.For the record, GSD is defined as the length (in inches, centimetres, or millimetres) between the centres of two consecutive pixels on your map.So, if a drone achieves a GSD of 5 cm/px, that equates to one pixel on your digital map corresponding to 5 cm in reality.- Ensure the correct overlap between each image to improve the accuracy and the quality of the model when it is being stitched by photogrammetry software;
- Ensure the image quality is high;
- In most cases, ground control points (GCPs) need to be used.
What is a Ground Control Point?
GCPs are marked points on the ground that have a known geographic location.For aerial survey applications, GCPs are typically required as they can enhance the positioning and accuracy of the mapping outputs. Indeed, GCPs allow the survey drone to obtain fairly accurate data.What Is RTK?
RTK (real-time kinematic) is a GPS correction-technology technique that provides real-time corrections to location data when the survey drone is capturing photos of a site. Real-time correction is a major industry advantage.What Is PPK?
PPK (post-processed kinematic) is another GPS correction-technology technique that works to correct location data, except in the cloud after the drone data has been captured and uploaded.When Is Best To Use Drone LiDAR?
LiDAR can be a key tool in a surveyor's arsenal.While it can be used for a range of applications, there are perhaps specific mission types where it really comes into its own.These include:- Mapping complex terrain with vegetation cover
- During low-light conditions or during the night
- When you need to detect small/narrow objects like cables or pipelines, which may be missed or not recognised by alternative methods.
When Is Best To Use Drone Photogrammetry?
More and more businesses, organisations, and departments across a growing range of industry verticals are embracing drone photogrammetry.And there are certain scenarios which are perfectly suited to this aerial surveying technique. These include:- Data sets that need visual assessment
- Projects which need easy-to-understand maps and models
- Context-rich scans that are accessible and require minimal post-processing and expertise
Drone LiDAR And Photogrammetry: What Solutions Are Available?
A number of solutions are available across the DJI ecosystem to help collect LiDAR and photogrammetry data.Click for more information on .LiDAR Drone
DJI's flagship commercial platform, the , can be used as a LiDAR drone thanks to the release of the .The L1 integrates a Lidar module, a 20MP RGB camera, and a high-accuracy IMU.Its key features include:- Highly efficient, can cover up to 2 km² in a single flight
- IP54 weather-protection rating
- Supports three returns
- Point Rate of 240,000 pts/s
- 5cm vertical accuracy / 10cm horizontal accuracy
- 450m Detection Range
- Point Cloud LiveView
Photogrammetry Drone
M300 RTK And Zenmuse P1The can be integrated with the . This payload has a 45MP full-frame sensor, making it a highly-capable solution for photogrammetry.Other key features include:- Capable of achieving 3cm horizontal and 5cm vertical accuracies, without any GCPs
- Highly efficient, cover 3km² in a single flight
- 3-axis stabilised gimbal with Smart Oblique Capture, ideal for large surveying sites
- Global mechanical shutter with a shutter speed of 1/2000 seconds
- TimeSync 2.0, enables microsecond-level synchronisation
- Interchangeable fixed-focus lenses (24/35/50mm)
- RTK module
- 20MP 1" CMOS sensor
- Mechanical shutter reduces risk of rolling shutter blur
- TimeSync System, which aligns the flight controller, camera, and RTK module
LiDAR v Photogrammetry - Which Is Best?
There's a good reason why LiDAR and photogrammetry have become popular drone surveying techniques: They can capture quality data and provide valuable insights.But which is best?In truth, it is a difficult question to answer, because both have their values.In many ways, it boils down to the job at hand.LiDAR is extremely powerful for specific missions, such as challenging projects where elevation accuracy, complex structures, or areas involving high vegetation are key.written by
James Willoughby
James joined heliguy™ in 2018 following a 13-year stint in print and online journalism, having worked on regional and weekly newspaper titles. He is responsible for spearheading heliguy™'s content strategy and social media delivery. James collaborates with DJI Enterprise's European marketing team to coordinate and produce case studies and helps organise events and webinars.