Using Aerial Documentation for Underwater Georeferencing at UNESCO Pile Dwellings “See am Mondsee” in Austria

Keywords: 3D reconstruction, UUV, SFM, Mondsee

Abstract:
Georeferencing underwater 3D models is an ongoing challenge. Localization technologies such as GPS cannot be used as it’s short-frequency waves only travel a very short distance under water. A common workaround is to use GPS to determine the position at the water surface, in addition to measuring the distance to the water’s bottom [1]. The Archaeonautic Group at the HTW Dresden and the Kuratorium Pfahlbauten are actively developing a semi-autonomous system for documenting the remains of the pile dwellings, which are located in shallow water in the lake. The pile dwellings are over 6000 years old, and are a UNESCO World Heritage Site [2]. For this purpose, it is necessary to document the site, “See am Mondsee” regularly. This results in numerous underwater 3D models that need to be georeferenced. In April 2018, an aerial survey of the terrain was used for the first time prior to the dive campaign as the basis for planning and georeferencing (fig. 1). For this purpose, a georeferenced 2.5D terrain model from a flight with 849 individual images was produced on the first documentation day (using an image resolution of 4864×3648 pixels). The water surface is flat, with the underlying pile structures clearly visible, giving us the top-down coordinates of various underwater structures. Furthermore, a few measuring points can provide the missing depth coordinate. In parallel with the aerial survey, a total of 7 dives with the mini submarine “Manio” were completed over two days, resulting in a 3D underwater model of approx. 50×60 meters. Manio has a module for real-time 3D reconstruction and can perform semi-autonomous documentation [3]. The transfer of the coordinates from the georeferenced terrain model results in a measured underwater model. The results are promising and can be a fast and accurate alternative to comparable documentation situations. Furthermore, best-practice solutions and processes for successful underwater documentation with UUVs will be presented.