In this article, we will discuss 3D modeling of heritage sites on the example of Chillon Castle, located in one of the most beautiful corners of the Swiss Riviera on the shores of the Geneva Lake, surrounded by the deep blue water surface on one side and the Alpine Mountain area on the other.
Due to the special location of this castle on the coast and the impossibility to perform ground laser scanning along most of the castle walls from the lake side, we decided to use drone technology and photogrammetry to carry out 3D modeling job.
We used the AUTEL EVO II PRO drone with additional PPK hardware on board as the main working tool for data collection.
Fig. 1. TOPODRONE AUTEL EVO II PRO PPK.
You may reasonably ask why the choice was not made in favor of other types of survey drones, such as the DJI Phantom 4 RTK? In order to answer this question, you only need to study the specifications and features of this drone:
-
Flight time. TOPODRONE AUTEL EVO II PRO PPK is able to stay in the air and perform a survey mission for 30 minutes, against 20 minutes for DJI drones. This characteristic was important for us when capturing images over water, where battery life is critical.
-
Image quality. Based on the results of testing and visual analysis of photos, we concluded that the AUTEL EVO II PRO delivers more details and provides high sharpness of images, which subsequently affects the detail and quality of the 3D model. You may rightfully claim, well how come, but the DJI Phantom 4 RTK camera has Global Shutter and the EVO II Rolling Shutter? As you probably already know, most photogrammetric software, e.g. Agisoft Metashape, Pix4Dmapper, 3Dsurvey - provide rolling shutter distortion correction without any difficulty when using high precision photo locations, so Rolling Shutter is not a limitation for photogrammetry.
-
RTK or PPK. The geographic location of the site resulted in the following challenges for surveying: the inability of the total station to determine control points on the walls facing the lake, the close proximity of mountains covering half of the sky, and the possible unstable operation of RTK on the drone would result in poor accuracy of the image coordinates. To solve these problems, we have installed on the AUTEL EVO II PRO a module from TOPODRONE, which is synchronized with the camera, allowing after post-processing of the GNSS measurements to identify the coordinates of the photo centers and use them for further photogrammetric processing of images. At the same time PPK equipment does not require real-time communication with the base station and provides more stable and accurate determination of image coordinates and drone movements as compared to RTK technology. We used EMLID REACH RS2 GNSS receiver as a base station.
-
Cost of equipment. AUTEL EVO II PRO + TOPODRONE PPK kit has two/three times lower cost compared to DJI P4 RTK.
We used the built-in AUTEL application to plan aerial survey trajectories and performed a mission with overlap of 70% and 80% at 60 meters in double grid mode, as well as oblique flight.
Fig. 2. Survey flight control in Autel App.
After performing fieldwork, we loaded the images into TOPODRONE Post Processing software, calculated high-precision coordinates of the images, and recorded them in the EXIF tags of the images.
Fig. 3. Autel EVO II Pro post processing.
The next step was to create a project in 3Dsurvey software, start the block alignment and generating a dense point cloud.
Fig. 4. Photo’s location.
Fig. 5. 3D model of Chillion castle.
Fig. 6. 3D model of Chillion castle.
Fig. 7. 3D model of Chillion castle.