This article, written by 3Dsurvey’s Amy Nicol, explores the challenges of applying photogrammetry in on-site environments, such as dense vegetation, changing weather, and complex drone operations.
It offers practical advice on overcoming these obstacles through optimal data collection conditions, strategic drone flight planning, and using additional data sources like videogrammetry.
Photogrammetry, the process of creating accurate 3D models from 2D images, has revolutionised surveying and mapping in recent years. However, its application in challenging on-site environments present some unique hurdles. From dense vegetation, weather conditions to optimal drone flying, navigating these obstacles requires a nuanced understanding of both the technology and the environment in which it operates.
The challenges of photogrammetry on-site
Obstructed Views
One of the primary challenges faced in photogrammetry is dense vegetation. Photogrammetry relies on having clear and unobstructed images of the terrain or objects to create accurate 3D models. Trees, shrubs, tall grasses or vines can obstruct views, leading to incomplete or inaccurate reconstructions.
Photogrammetry also depends on identifying common features in multiple photographs to triangulate their positions in 3D space. The dynamic nature of vegetation, with swaying leaves and branches, complicates the process, creating a cluttered visual environment that challenges the matching of key features across images.
Shadows
Moreover, changing shadows cast by foliage throughout the day introduces inconsistencies in image capture, further challenging the accuracy of reconstructions. This is especially evident when doing long drone flights, or recording the same site at different times of the day or in different weather conditions.
These are important factors to consider when in the field. Therefore, when collecting your photogrammetric data on-site, opt for an overcast day with little to no wind. If that is not possible, consider early morning or afternoon when shadows are not too stark. If filming needs to take place over multiple days, film at the same time each day so that when it comes to inputting your data into processing software, you can recreate the environment with as little inconsistencies as possible.
Drone surveying: Flight recommendations
Careful planning
When using drones for photogrammetry, careful planning is essential to ensure optimal results. If you’re in need of capturing something more complicated than just a plane surface such as a road, then the flight path must be meticulously designed to maximise coverage while minimising the risk of collisions with obstacles. In these cases, altitude plays a crucial role in balancing detail and coverage, with higher altitudes offering broader views but potentially sacrificing resolution.
Various heights and angles
When dealing with dense vegetation and complex building forms, drone images should be taken at various heights and angles to provide both overall coverage and detail. If you fly too low over trees, you will most likely get holes in your 3D model of where the trees should be. This is why it’s recommended to fly at altitudes of 60 meters or higher. This not only ensures better coverage of forested areas but also ensures the overall coverage of the environment below, leading to best outcomes when creating a traditional orthophoto. Flying lower and closer to the object, however, will result in more detail and better texture in higher resolution. If you want to create a high fidelity representation of reality, it is very important to take images at varying angles. Remember: what you see is what you get.
When flying manually
Some of the most commonly used commercial drones on the market for professional surveying are made by DJI (Mavic 3 Enterprise, Phantom 4 RTK, Matrice 300 RTK). When flying manually or in ATTI mode (short for Attitude mode), a specific DJI flight mode where the GNSS and VPS (Vision Position Systems) are disabled, there is a greater risk of crashing your drone. One would switch to ATTI mode when wanting to fly closer to the ground or objects of interest.
When flying in this mode, it’s crucial to adjust sensitivity settings on the remote controller to match skill level, opting for lower rates for smoother control or higher rates for faster response. Lowering maximum horizontal speed increases reaction time and reduces risk, especially in windy conditions. Maintaining visual line of sight (VLOS) is essential, with the option to use a spotter for situational awareness and guidance. Remaining calm and focused while avoiding sudden, panic-induced movements on the controls ensures safe and controlled flight operations.
Incorporating additional data sources
Leveraging additional data sources, such as Videogrammetry, can complement drone-based photogrammetry in certain scenarios to overcome on-site challenges. Equipped with an external RTK antennae and video enabled scan app, a smartphone can be used to capture ground-based geotagged video footage to aid the 3D model construction. It is extremely useful when only a small area of detail needs to be documented, such as areas beneath the trees.
Photogrammetry offers a powerful tool for creating detailed 3D models of environments, but navigating the challenges inherent with the technology requires knowledge, precision, and careful planning. As technology continues to advance, the potential for photogrammetry to improve surveying and mapping practices remains vast.