The aerial survey
Terra Drone Europe (still called Skeye at the time) carried out a 3D aerial survey of the longest dam in the Netherlands by the name of Afsluitdijk using a combination of an RTK drone and a manned fixed wing aircraft. This dam closes off the IJsselmeer from the North Sea and runs over a length of 32 km from the Province of North Holland to Friesland. The request for the survey came from our client Levvel (a combination of Van Oord, BAM and Rebel) who have been commissioned for the renovation of this dam. The height of the dam will be increased and newly cladded with concrete elements. Prior to starting the works Levvel asked Terra Drone Europe to create a 3D terrain model and orthophoto mosaic at high resolution and accuracy.
Drone or manned fixed wing survey?
Typically, a project of this size would lend itself better to the use of a manned fixed wing aircraft solution. The main reason for it being quicker and thus cheaper. Flying of drone must be done in line of sight with a maximum of 500 meters. This means effectively for this project that the team has to reposition 32 times. In addition, drone regulations in the Netherlands are very tough, for this project the best example is the fact that a major road with a speed limit > 80 km/h cannot be overflown with a drone. A downside to the use of a manned aircraft is the higher altitude from which images can be taken. This has a direct influence on the image resolution and thus the accuracy of the 3D model.
For this project Terra Drone Europe adopted a dual approach. For most part a manned fixed wing was used equipped with a 100 Mp Hasselblad camera and high accuracy GNSS positioning. Other parts were surveyed using the Phantom 4 RTK drone. Part of the project was completely detached from the mainland and it was impossible to place any ground control points. The Phantom 4 RTK drone was the ideal solution for this.
Ground Control Points and RTK?
Normally at Terra Drone Europe we always measure both ground control points and verification points using RTK GNSS. The ground control points are used in the photogrammetric process to better estimate the position of the photograph and calibrate the camera. The verification points are not used in the photogrammetric processing but used solely for quality control on the end products.
Many manufacturers claim that ground control points are not required when using their RTK solution. At Terra Drone Europe we firmly believe that these ground control points are still required for an accurate camera calibration and to further increase accuracy. Typically, accuracies between 2 and 3 cm are obtained using a combination between RTK and Ground Control points versus 5 cm when only RTK is used.
Since we always measure verification points to be able to guarantee and prove that accuracies are met, we might as well take sufficient (and sometimes too many) ground control points.
Data Processing
The final deliverables were a coloured point cloud, a 3D elevation model, a 3D terrain model, an orthophoto mosaic (one seamless image corrected for heights and colour differences) and last but not least the results of the quality control. The data from RTK drone and the manned fixed wing was merged seamlessly in the same data processing software.
