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In October last year, surveying company Upvision mapped the highest mountain in the Czech Republic, SnÄžka, which is 1.6km high. Situated on the border with Poland in the Krkonoše mountains, SnÄžka is one of the most visited natural places in the Czech Republic and Krkonoše National Park wanted to create a detailed 3D model and digital elevation model of the mountain, as well as determine an accurate height for it.
The area of four square kilometres around the mountain was large and bumpy, with poor access. Movement off marked paths is also banned in the park, so mapping the peak and surrounding areas required the use of a UAV. While it is possible to get to the peak by funicular, we would need to bring our UAV and equipment by car. That and the stoney and steep mountain meant we chose a plain with tall grass and small trees 200m below the peak for take-offs and landings.
Being situated so low below the peak, it was necessary to create a UAV flight plan for the individual strips of flying. This climbed simultaneously with the height of the scanning area and maintained the same image resolution for the aerial images. Due to the steep terrain, we chose an image resolution of 7cm/pixel and an overlap between aerial photographs of 80/70 to the top of the mountain.
Five ground control points were measured using RTK with a GPS TopCon Hiper GGD with TopSurv SW. This was very difficult, as the points were distributed differently over the top of the mountain with a variety of elevations, meaning a walk of several kilometres in total length, as well as in climbs and descents, to measure them. The points were measured in the evening during low temperatures and twilight in the mountains.
For our UAV, we picked a MaVinci Sirius with a calibrated Panasonic GX-1 camera, for several reasons: it could map a large area in one flight; it could take off from the hand; it could be manually landed;it was made of a strong material to withstand damage on landing; it could be repaired in the field; and it was registered with the Civil Aviation Authority. The UAV receives GPS and GLONASS information using an inertial measurement unit and logs the external orientation for all aerial photos it takes. However, there was also a GPS module in the body of the UAV, from which it is possible to send the position and use it in critical events, such as if we lose sight of the UAV.
Flying was carried out during a working day. Luckily, winds only reached speeds of up to 3m/s that day – wind speeds of 20m/s are common in the area, making it impossible to fly UAVs.
Mapping the peak and overall localities was carried out on one flight of the UAV of about 35 minutes. At the time of mapping, there were no visitors at the top of the mountain. Everything was controlled by two operators using a radio transmitter. Landing required about 30m of open area, with the UAV manually piloted into a spiral and then into the tall grass without any problems.
The results
The UAV acquired nearly 500 aerial photos, which were processed in SW Agisoft Photoscan Pro using autocorrelation at the highest quality in two days. The positions of the GCPs were determined to an accuracy of 5cm. From these inputs were generated an orthophotomap of SnÄžka in the Czech coordinate system with an image resolution of 7cm/pixel.
Digital surface models of the mountains were created using a point cloud of nearly 300 million placed points. The lowest point on the model was at an altitude of 927m in the valley and the highest point was 1.617km at the top of the mountain.
The 3D visualisation model will continue to serve as a basis for any other projects’ documentation, such as the reconstruction of the funicular. The orthophotomap will enable the accurate updating of digital maps of the access roads to the peak, on both the Czech and Polish sides. Outputs will also be integrated into Krkonoše Mountains National Park’s GIS.
The right choice of UAV can help, not only for detailed mapping and not only for smaller areas, but also in extreme conditions and surroundings, such as in mountain areas.
Jakub Karas is the co-owner of Upvision (www.upvision.cz)
