LiDAR or photogrammetry ?

All you need to know about these two technologies

 

With the development of drones in recent years, especially for the world of geomatics, many myths and ideas are conveyed around these two technologies. Let’s take a look at LiDAR and photogrammetry, find out about their specificities and applications. Compared to traditional survey methods, these two technologies make it possible to obtain much faster results, and with a much higher data density.

 

 

How do these two technologies work?

 

LiDAR (or Light Detection and Ranging) is a technology based on the emission of a laser beam, which then receives an echo (like the radar), and can accurately determine distance from an object. LiDAR is considered an active sensor because it emits its energy source rather than detecting the energy emitted by the objects on the ground.

Photogrammetry is a passive technology. It consists of taking measurements in a scene, using the parallax obtained between images acquired from different points of view. This technique is based entirely on a rigorous modeling of image geometry and their acquisition. It allows to rebuilt an exact 3D copy of reality.

 

Functioning of photogrammetry

Functioning of photogrammetry

 

To summarize, LiDAR uses lasers to perform measurements, while photogrammetry is based on analysis of captured images. These two technologies can of course be combined to allow even more precise measurements.

 

What are the LiDAR and photogrammetric survey results like?

 

The main product of LiDAR reports is a 3D point cloud. Its density depends on the characteristics of the sensor (scanning frequency and repetition rate), as well as the flight parameters. Assuming that the scanner pulses and oscillates at a fixed rate, the density of the scatter plot will depend on the flight altitude and the speed of the vector.

It should also be noted that LiDAR captures only colorless dots, creating a monochrome data set that can be difficult to interpret. This is why data is often visualized using false colors based on reflectivity or elevation. For example, it is possible in post-processing to overlay color based on images or other data sources. The color can also be added according to the classification of a type or group of objects (trees, buildings, cars, electric wires …).

 

point clouds LiDAR

Example of a scatter plot obtained by the SuperFox6 LiDAR from Heliceo to the bridge of Mauves (44)

 

Regarding photogrammetry, it can generate 2D and 3D models in color, which is more visual and easier to interpret than LiDAR results. The main results of photogrammetric surveys can be :

  • Raw images
  • Orthophotography
  • Digital surface models
  • 3D point clouds

 

Aerotriangulated images and 3D point cloud

Aerotriangulated images and generated 3D point cloud – Heliceo

 

 

But then, what about their applications?

 

LiDAR laser beams are able to penetrate vegetation , unlike photogrammetry. Therefore, it is possible to cross the spaces between canopy, and reach objects below. That’s why LiDAR is ideal for generating digital models of terrain previously unreachable with photogrammetry.

LiDAR scheme

 

Aerotriangulated images and 3D point cloud

Coupe obtained by LiDAR with colors based on the elevation of the terrain – Mission Heliceo realized for © Drone Check (Belgium)

 

LiDAR is also very useful for modeling fine objects such as power lines or telecommunication towers. In addition, it can work with little light, or even at night.

For its part, photogrammetry is the technology of choice for use cases where a visual assessment is required. It is ideal for building inspections, asset management, or agriculture.

Unlike LiDAR, photogrammetry may not recognize objects that are narrow or hard to see. In contrast, point clouds generated by photogrammetric processes are more visual. Each pixel is defined in RGB colors, but often with generalized details. They may therefore be suitable for objects for which a lower level of geometric detail is acceptable, but for which visual interpretation is essential.

 

And what about the precision ?

 

Let’s start by recalling that in topography, precision always has two dimensions : relative and absolute. Relative Precision is the measure of the position of objects relative to each other. Absolute Precision, on the other hand, is the difference between the location of objects and their actual position on the Earth. This is why a survey can have a good relative precision, but a low absolute precision.

The precision of LiDAR and photogrammetry

Achieving a high level of accuracy with an aerial LiDAR is quite difficult as the sensor is moving. This is why airborne LiDAR sensors are always paired with an IMU (inertial unit) and a GNSS receiver. These provide information about the position, rotation, and motion of the scan platform. All these datas are combined and make possible to reach a high relative precision of the order of 1 to 3 cm. High absolute accuracies require the addition of 1 to 2 ground points ( GCP – Ground Control Point) as well as other control points for verification purposes. In some cases where additional GNSS positioning accuracy is required, advanced RTK / PPK positioning systems can be used. LiDAR is one of the most accurate topographic measurement technologies. This is particularly the case for terrestrial lasers where the sensor is positioned on the ground, and its exact location is measured using topographic methods. Such configuration allows to reach sub-centimeter level accuracies.

SuperFox6 LiDAR by Hélicéo

pod LiDAR by Hélicéo

The SuperFox6 by Heliceo et his LiDAR sensor

Photogrammetry can also achieve level accuracies of 1 to 3 cm. However, it requires significant experience in selecting appropriate equipment, defining flight parameters, and processing data appropriately. High absolute accuracies require the use of RTK / PPK and additional GCP technology or may be based on a large number of GCP only.

 

Fusion VTOL Heliceo

The Fusion (aircraft drone VTOL) RTK/PPK by Hélicéo

 

Data acquisition, treatment and efficiency

 

There are also significant differences in the speed of acquisition between these two technologies.

In photogrammetry , one of the critical parameter required to process data accurately is the image overlap that should be 60-90% (front and side). This is done according to the structure of the ground and the equipment used.

LiDAR surveys only require 20-30% overlap between flight lines. This makes the data acquisition operations much faster.

For absolute accuracy, photogrammetry requires more ground control points to achieve the same accuracy as LiDAR. The measurement of GCP usually requires traditional survey methods, which implies additional time and cost.

LiDAR data processing is very fast. The raw data requires only a few minutes of calibration, in the order of 5 to 30 minutes, to generate the final product.

In photogrammetry , data processing is the longest part of the overall process. It requires powerful computers capable of handling operations on gigabytes of images. On average, processing takes between 5 and 10 times more time than field data acquisition.

On the other hand, for many use cases such as power line inspections, LiDAR point clouds require an extra classification that can take a lot of work. It also requires expensive software.

 

 

In conclusion

 

If we compare LiDAR and photogrammetry, it is essential to understand that both technologies have their applications and limitations. The 2 techno are complementary in most cases of use. None of these technologies will cover all use cases.

LiDAR should certainly be used for reporting narrow structures such as power lines or telecom towers. It can also be useful for mapping areas under the canopy.

Photogrammetry will be the best option for projects that require visual data : construction inspection, asset management, agriculture for example.

For many projects, both technologies can provide valuable data (for example, mines or earthworks). The choice of the method will depend on the following parameters :

  • Use Case
  • Available time
  • Budget
  • Capture conditions

To sum up, LiDAR and photogrammetry are two powerful technologies if you use it the right way. Both technologies are still in their infancy for drone applications. In the years to come, we will undoubtedly see other developments.

Stay tuned, we’ll keep you informed.

 

 

Source : « Drone LiDAR or Photogrammetry ? Everything you need to know. »