Aerial Surveying: LiDAR versus Photogrammetry
LiDAR and photogrammetry have long been indispensable tools for surveying and mapping professionals. However, new advancements in drone technology have improved the way data is collected.
In comparison to previous aerial surveying, which depended nearly entirely on human aircraft, drones provide a more secure, precise, and inexpensive alternative. As a result, surveying solutions have become more accessible to the general public. Agriculture, building, conservation, mining, and accident scene reconstruction, among other applications, may now benefit from comprehensive point clouds, precise maps, and 3D models.
For both experienced surveyors and those new to the industry, the decision to use LiDAR or Photogrammetry will be familiar. We're going to go through the advantages and disadvantages of both strategies in this post. It is not that one is inherently superior or should be favoured over the other. Rather than that, the appropriate choice is determined by the nature of the task at hand, the operator's abilities, and, as always, the available money.
What is Light Detection and Ranging (LiDAR)?
The term 'light detection and ranging' is an acronym for 'light detection and range. LiDAR sensors function by producing pulses of light and measuring the time required for them to reflect back off the ground, as well as their intensity.
Although LiDAR technology has been known for decades, it has only recently gotten tiny enough to be integrated into a payload that a drone can carry.
The LiDAR sensor is only one component of a complex process. To collect the data necessary to create a precise point cloud that properly represents the terrain and its topography, LiDAR utilises two other high-precision systems: satellite location (GNSS data) and an inertial measurement unit (IMU).
LiDAR aircraft may be used to create 3D point clouds and intensity maps, which require considerable skill to analyse but give important data for mining, forestry, agricultural, and construction operations.
The Advantages of LiDAR
The most frequently touted advantage of employing LiDAR for mapping is its accuracy. However, as a stand-alone statement, this leaves us with nothing to work with.
To begin, it's critical to understand the implications of accuracy for you and your project. Are you more concerned about relative or absolute accuracy? In other words, are you concerned with the accuracy of your final product's characteristics in relation to one another or with their location in the world?
LiDAR is the most accurate method and is often the best choice when the goal is to create a realistic bare ground model. That is because it is the most accurate way to account for elevation, vegetation, and the current circumstances.
The combination of LiDAR and GNSS data, as well as the fact that it is a direct measurement - shooting hundreds of laser pulses from above - ensures that your final digital terrain map has great vertical precision.
The Drawbacks of LiDAR
The most obvious disadvantage of using LiDAR is its expense. Due to the increased operational complexity (and the requirement for more sophisticated components and sensors), a comprehensive survey system can easily cost hundreds of thousands of dollars.
Additionally, its intricacy increases your margin for mistake and your reliance on an experienced specialist. With several sensors and data that is not easily available without considerable processing, getting the data you want is not easy.
It's also worth noting that LiDAR sensors have historically been bigger than ordinary cameras. With the increased use of drones for aerial surveying, the requirement for a larger drone capable of carrying a higher payload can add to an already hefty expenditure.
The last disadvantage of LiDAR is, perhaps, its greatest strength: It is the ideal instrument for the job in extremely particular scenarios. For the majority of applications, standard photogrammetry will suffice. This is a practise that is gaining traction as image processing software becomes more advanced.
Photogrammetry is a term that refers to the process of capturing images.
Simply said, photogrammetry is a technique for measuring distances through the use of images. Specialist software is used to process these pictures in order to create accurate and realistic representations of the world.
Orthomosaic maps and three-dimensional models may be used for a range of purposes, ranging from construction planning and current project management to marketing materials.
Hundreds to thousands of photos are required for good photogrammetry. It all depends on the size of the location in issue and the level of detail and precision desired.
Drone pilots must identify the appropriate flight height to get the required ground sample distance. Additionally, you'll need to specify an overlap on each image to guarantee that your programme can sew your photographs together flawlessly.
Photogrammetry's Advantages
The primary advantage of using photogrammetry is its accessibility. Drone technology and mapping software advancements have streamlined operations and made realistic maps and 3D models accessible to any business with a capable camera drone.
Apart from camera calibration, basic flight planning, and drawing your ground control points, carrying out a mapping operation and converting the data is pretty simple. There are several instances when this technique yields concrete effects in areas as diverse as building, conservation, mining, and agriculture.
Notably, the results are also accessible. Maps and models with easily identifiable features and colours are instantly intuitive, making them an excellent collaborative tool and a platform from which stakeholders may work without spending excessive time altering the data.
Another significant component of Photogrammetry's attractiveness is its affordability. As said previously, getting started requires spending a few thousand dollars on a good camera drone and far less on the software required to interpret your data.
Finally, photogrammetry provides a more adaptable method. You may exert greater control over the trade-off between mission speed, altitude, and accuracy depending on the job at hand.
The Disadvantages of Photogrammetry
There are various disadvantages to photogrammetric surveying technologies.
The first is that the accuracy of your maps and models is heavily dependent on the camera and flight characteristics of your drone.
The sensor's size, aperture, resolution, and focal length all have an effect on the ground sample distance (GSD), as does the height at which you fly. Additionally, you may struggle to achieve 100% precision without several ground control points or an RTK or PPK-enabled drone.
Choose LiDAR for the following reasons:
Mapping inaccessible, complicated, and overgrown terrain
Detailed photography of thin structures such as electricity wires or roof margins
Projects that place a premium on accuracy and precision
When is Photogrammetry Appropriate?
Photogrammetry is a more affordable solution for people new to drone surveying. Although being less expensive than LiDAR is not its sole advantage.
Indeed, several applications would be better served by continuing to use Photogrammetry. This is especially true if you wish to collaborate on plans using orthomosaic maps, cooperate on projects using 3D models, or give accessible project progress updates at a low cost.
Consider photogrammetry for the following purposes:
Accessible, context-rich scans that need minimum post-processing and skill
Simple-to-understand maps and models
Visualization of data sets
Which is more accurate, LiDAR or photogrammetry?
As with the DJI L1's true-color point clouds, the answer is not black and white.
In comparison to photogrammetry, LiDAR often produces more detailed and accurate scans. Additionally, because it performs effectively in the face of environmental obstacles - think poor light or dense foliage - it is suitable for situations when precision is paramount.
LiDAR point clouds may be extremely coarse, with up to 500 points per square metre and an accuracy of less than three centimetres in the vertical direction. A dense amount of data points results in a more robust dataset, which provides you with greater flexibility when it comes to processing your findings.
That is not to imply, however, that photogrammetry is intrinsically inexact. If your landscape is generally flat and devoid of thick vegetation, you can still create extremely accurate maps and models – even more so if you use an RTK positioning module.
Photogrammetry vs. LiDAR: The Data
LiDAR and photogrammetry are fundamentally dissimilar data collection techniques.
With LiDAR, you end up with thousands of data points that combine to generate a three-dimensional point cloud delineating the landscape. To make it visually accessible, you'll need to include colour from various databases.
With photogrammetry, hundreds or thousands of photos must be analysed and stitched together to create something useful: a 3D point cloud, a map, or a navigable model.
LiDAR processing in the cloud is not as widespread or as accessible as photogrammetry processing in the cloud. Which means you'll need an on-site professional capable of transforming raw data into usable information, in addition to the appropriate software.
Phantom 4 RTK Photogrammetry Solutions by DJI
The Phantom 4 RTK strikes a great blend of pricing, accuracy, and accessibility for professional surveyors.
With the onboard real-time kinetic (RTK) sensor, images are automatically geotagged and adjusted to centimeter-level precision against ground control points.
The Phantom 4 RTK is an excellent entry-level investment regardless of your surveying requirements.
300 RTK Matrice + P1
DJI's flagship photogrammetry solution is this combination. The P1 is a high-performance photogrammetry payload that features a full-frame sensor and interchangeable fixed-focus lenses. It is suited for large-scale photogrammetry flights because to its worldwide mechanical shutter and software capabilities like as Smart Oblique Capture. Utilizing the M300 RTK, the P1 enables surveyors to cover 3 km2 in a single trip and produce results that are 3 cm horizontally and 5 cm vertically accurate without the need of GCPs.
DJI's solutions for LiDAR
300 RTK Matrice + L1
The L1 is equipped with a Livox LiDAR module, a high-precision IMU, and a 1-inch CMOS camera mounted on a three-axis stabilised gimbal. With a vertical precision of 5 cm and a horizontal accuracy of 10 cm, the M300 RTK, the L1, and DJI Terra combined constitute a full surveying solution that provides 3D data, details of complicated structures, and exact visual reconstructions.
Finally, some reflections
Comparing LiDAR and photogrammetry as two adversarial data acquisition techniques is not the most enlightening approach. As previously said, neither is inherently superior to the other. Ultimately, the optimum answer will be determined by the work at hand.
If the contrast, lighting, subject, and settings are favourable, photogrammetry is likely to suffice. However, for difficult mapping tasks including elevation precision, complex buildings, or partially obscured terrain, LiDAR is generally the best option.
Naturally, the cost and experience of your staff will factor into any choice between the two. Although DJI's latest payloads, the P1 and L1, demonstrate surveying technology's increasing price and accessibility.
Professionals in the industry will eventually need to become competent at utilising both technologies as the drones carrying them advance in sophistication.