If you have an area of several 25's of acres (several 100,000 m²) and your only source of GNSS Information is that of the drone, can there be distortion of the 3D Model / orthomosaic that are so large that calculations based on this model cannot be trusted?
In other words: Do GCPs not only add global georeferenced accuracy, but also decrease the error of the scale of the result (for example if you want to measure landfill, the surface area or the volume of some rocks or debris) ?
Yes, without GCPs and RTK/PPK, it is highly possible to obtain wrong or inaccurate geometric information in UAV-based photogrammetric mapping. When dealing with large areas, relying solely on the drone's GNSS information can lead to distortions in the 3D model or orthomosaic. GCPs not only add global georeferenced accuracy but also help decrease errors in the scale of the results, making them essential for reliable measurements.
The accuracy of UAS-based photogrammetric mapping depends on several factors, including Ground Control Points (GCPs), flight height, camera resolution, GNSS accuracy of the device, weather conditions, processing software, user experience, etc. While it is possible to obtain satisfactory 3D models without GCPs or RTK/PPK, for precise measurements such as surface area or volume calculations, GCPs are essential. For more in-depth information, you can refer to my MSc thesis and the following articles. In the following studies, you can find comparisons of processing with and without GCPs as well.
Article Accuracy evaluation of UAS photogrammetry and structure from...
Article Accuracy of Unmanned Aerial Systems Photogrammetry and Struc...
MSc Thesis:
https://www.researchgate.net/publication/367257672_Accuracy_analysis_and_evaluation_of_UAS_photogrammetry_and_structure_from_motion_in_engineering_surveying
Good luck on your journey of exploration and innovation!
Sven Szardien
Hi,
Yes, UAV aerial photogrammetric mapping without Ground Control Points (GCPs) and Real-Time Kinematic (RTK) or Post-Processed Kinematic (PPK) data can lead to wrong or inaccurate geometric information in the resulting maps and models. Here's why:
Georeferencing Errors: GCPs serve as reference points with known coordinates on the ground, which help in accurately georeferencing the aerial imagery. Without GCPs, the UAV-derived data may lack accurate spatial referencing, leading to misalignment and errors in the map.
Scale and Distortion Issues: UAV images suffer from various forms of distortion, including radial and tangential distortion. The use of GCPs helps in correcting these distortions and ensures the correct scale of the map. Without GCPs, the images may have scale issues and introduce inaccuracies in measurements.
Elevation Accuracy: RTK or PPK data provides accurate elevation information, which is crucial for generating Digital Elevation Models (DEMs) and Digital Surface Models (DSMs). Without accurate elevation data, the generated models may have significant errors, affecting slope calculations, volume estimations, and other terrain-related analyses.
Absolute vs. Relative Accuracy: GCPs and RTK/PPK data provide absolute accuracy, which means the coordinates are referenced to a global coordinate system (e.g., WGS84). Without them, the accuracy of the map is limited to relative accuracy based on tie points within the imagery. While relative accuracy is useful for some applications, it is insufficient for tasks that require precise geospatial information.
Effects of Environmental Factors: UAV flights are subject to environmental factors such as wind, temperature, and GPS signal quality. RTK/PPK helps compensate for these factors, improving the accuracy of the flight path and subsequently the mapping results. Without RTK/PPK, these factors can introduce errors in the data.
Positional Shifts: A lack of GCPs and RTK/PPK can lead to positional shifts and mismatches between overlapping images used for photogrammetry, resulting in distorted 3D models and orthomosaics.
In summary, using UAV aerial photogrammetric mapping without GCPs and RTK/PPK can cause significant inaccuracies and reduce the overall reliability of the generated geospatial products. Therefore, for applications requiring high accuracy and precision, it is essential to use GCPs and RTK/PPK data to improve the quality and reliability of the photogrammetric outputs.
Please recommend my reply if it is useful .Thanks.
Thanks for your answers! Sayed Ishaq Deliry, Aahed Alhamamy, Jens Kleb Very good information so far.
Can you say something about flying a site on several/different days (because of the size) without the use of RTK?
Due to barometric dependency of the drone's understanding of altitude this can cause issues when trying to make one single 3D Model from all days, if you do not use RTK. Do you agree? And if yes, do you know a way to solve this?
Regards, Sven
A simple way to connect scenes without ground control points is to identify natural features between two overlapping scenes and use the individually derived DEM to coordinate them from both scene processing. The coordinate difference provides an estimate of the error involved. If the error is within bounds, you take the average of such point coordinates and use them as control points to create the block.
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