Fundamentals of accuracy for 3D Digital Twins from drone images

Drones have revolutionized the way we gather data about the world around us. We can now create high resolution copies of the physical world at scale and a portion of the cost, but it requires the use of proper tools for accurate photorealistic 3D digital twins.

In this article, we will explore how such unmanned aerial vehicles (UAVs) help producing Digital Twins with photogrammetry and at which accuracy level.

But first, some terminology.

What is photogrammetry?

Photogrammetry is the science of making measurements and 3D models from photographs. It involves the use of specialized software to process multiple images of the same object or scene, taken from different angles, in order to create a 3D model or map.

Drones are the most agile tool for gathering images of the real world. They can capture aerial images from a variety of angles and altitudes and are able to access areas that are difficult or dangerous to reach.

What does accuracy mean in drone mapping?

When it comes to reconstructing the real-world from images, there are two ways to think about accuracy: relative accuracy and absolute accuracy.

• Relative (or local) accuracy
  Relates to the position of elements within the reconstruction. A good relative accuracy is for example when the distance measured between two windows on the model of a building is the same as in the real world.
• Absolute (or global) accuracy
  Relates to the accuracy of the reconstruction in relation to its true position on earth. A good absolute accuracy is when the latitude and longitude of a point on the building model corresponds well with the actual GPS coordinates of this point in the real-world.

A reconstruction can have a good relative accuracy while having a poor absolute accuracy.

What is ground sampling distance?

Ground sampling distance (GSD) is the distance between the center of two consecutive pixels. In other words, it is the resolution of the reconstruction.

The GSD depends on the drone flight height and on the camera specifications. The same flight height with a higher resolution camera will generate a lower GSD and more details in the reconstruction.

Which drones are used for 3D digital twin creation?

Two main types of drones are used for photogrammetry purposes: multi-rotor and fixed-wing.

Multi-rotor drones

Multi-rotor drones can operate in a wide variety of environments as they can take off and land anywhere. On the downside, they require a lot of power to maintain lift, which results in a limited flight time. On average, a multi-rotor drone can only fly between 20 to 30 minutes before the battery runs out.

At Nomoko, we work with the DJI Matrice 300 RTK.

The DJI Matrice 300 is a versatile drone as it can be equipped with up to different payloads configured to any mission need. We currently use an oblique camera system with 5 sensors. The goal is to obtain the fine details of a building’s facade, something that can’t be achieved with regular cameras in such high quality.

It is a very reliable industrial drone platform with redundant safety systems such as GPS, IMUs, batteries, compass, barometer, and vision sensors. This allows us to perform safe operations over urban areas and to avoid single failure accidents. Together with the DJI M300 RTK, we use drone-independent and certified parachute systems that guarantee that no people are injured under any circumstances.

The GSD of projects captured with this system is 1.5 cm/px, which means that each reconstructed pixel corresponds to 1.5 cm in reality.

Fixed-wing drones

Fixed-wing drones take advantage of their aerodynamics allowing them to fly for extended periods of time and making them ideal for surveying large areas. The typical flight time for a fixed wing drone is between 50 to 90 minutes.

However, fixed-wing drones can only fly forward and cannot hover in place, so they are less maneuverable than multi-rotor drones. Additionally, they are similar in design with aircrafts, which means that they require a large, clear space to take off and land. This is why at Nomoko, we work with the WingtraOne Gen II, a VTOL drone (vertical take-off and landing) combining the benefits of fixed-wing and multi-rotor drones.

The expected GSD for projects captured with this system is 3 to 4.0 cm/pixel.

What accuracy level do I need for my Digital Twin project?

To determine the accuracy level required for a particular project, it is important to consider the intended use of the 3D model or map. For example, a 3D model of a building for architectural design purposes may require higher accuracy than a 3D model of a large area for contextualization purposes.

The level of accuracy also impacts the cost of the 3D model as it influences the required time to fly over an area, capture the images and process them.

Recreate the world in 3D with drones

To create an accurate 3D model, the images need to be processed with high-quality software that can accurately match features in the images and create the final 3D model.

Through our custom-built 3D modelling process, we convert 2D drone images into high-resolution Digital Twins at a rapid rate and in a scalable way. We are able to capture entire cities in extreme detail and faster than ever before.

Any project in mind? Get in touch with us!