Remote Sensing in Mongolia -An Opportunity for archaeological UAV-Prospection
Peter Heimermann, Matthias Lang, Till Sonnemann, Lkh. Munkhbayar and Jan Bemmann
The central Mongolian steppe offers highly favorable conditions for applying remote sensing techniques to document archaeological structures. The lack of forestation makes it ideal for the use of sensors specifically developed for generating digital surface models (DSM) using structure from motion (SFM) principles. Moreover, the short steppe grassland raises hopes for detecting detailed vegetation features, making the generation of high-resolution aerial images promising. Additionally, the low population density in Mongolia, and soils unsuitable for agriculture, means many archaeological structures have remained well-preserved, undisturbed by modern farming or settlement activities.
The rapid advances in drone, battery, and sensor technology facilitate archaeological research in remote environments. Fixed-wing drones achieve flight times of up to 90 minutes, enable UAV surveys to map hundreds of hectares within a few hours. This is particularly valuable in Mongolia, where archaeological sites in the sparsely populated landscape are often several hours away from the nearest power source. The flight times of fixed-wing drones now make it possible to fully map extensive settlement areas in the remotest parts of the Mongolian hinterland with various sensors in high resolution in a single day.
Since 2023, the Bonn Center for Digital Humanities (BCDH) participates in the DFG-funded research project "Urban Influence on the Mongolian Plateau: Interdependencies of Urbanism, Economy, and Environment", which aims to explore urban structures from the Mongol Empire era (1206-1368) and their impact on the surrounding environment. The subproject "Virtual Landscape and Multispectral Survey" concentrates on the applicability of drone-based remote sensing methods in Mongolia. This approach aims to produce detailed plans of city ruins and uncover previously unknown structures within and around the sites. Multispectral orthophotos and high-resolution DSMs help to digitally reconstruct and investigate these archaeological landscapes.
UAV surveys in both the central Mongolian forest steppe and the Gobi Desert allowed to assess the applicability of various methods across different climate and vegetation zones. In 2023 and 2024, the project surveyed a total area of 5,958.6 hectares, successfully documenting 21 settlement sites from different epochs. These settlements include Karakorum, capital of the Mongol Empire, and the Spring Palace of Ögedei Khan (1229–1241), successor of the famous Chinggis Khan. Both the DSMs and multispectral images yield impressive results, which also include previously unknown settlement sites.
Minimal invasive re-evaluation of a palimpsest landscape – Combined remote sensing analysis of the oppida Závist and Stradonice in Bohemia
Till F. Sonnemann, Daniel Bursák and Katja Kothieringer
Oppidum Závist south of Prague is a unique example of urban land use in Iron Age Central Europe. Today, its central part is a heavily forested heritage site, while the periphery is used for agricultural purposes. The summit of one of the longest-settled hilltop sites in Bohemia has already been intensively studied by the Czech Academy of Sciences. From the 1960s to the 1990s, an ensemble of geometric man-made stone structures was revealed by excavation, a cult place with supra-regional function today known as the “Acropolis.” However, large parts of the archaeological site that towers over the Vltava river still remain unexplored.
With “Oppidum Závist in the 21st Century – New questions and methods for research and education”, its prehistoric occupation has become focus of a comprehensive surveying campaign, by combining remote sensing and geophysical mapping techniques with minimally invasive test coring and small excavations. The Bavarian Czech Higher Education Agency (BTHA)-funded project investigates the multi-walled enclosure extending 150ha over two hills for its use of space through time. GPR surveys revealed on “Balda”, another extensive man-made platform south of the acropolis, massive structural support and potential building foundations, most recently verified by excavation. At the baily on the foot of the eastern slope, magnetic surveys had previously revealed pits and linear features. Here, GPR results provide additional insights and a depth component. At the slopes, erosion has left culturally rich deposits for soil coring. Uncovered material is being 14C and OSL dated, as well as geochemically and micromorphologically analysed to differentiate the areas by their time of use. Systematic metal detector surveys display a spatial distribution of artefacts.
The aerial remote sensing focus lies on the less forested space within the outer walls of Závist, which was identified by magnetometry as a living and farming space. In consequence, any woodless area within the enclosure was covered by a DJI Mavic 3M multispectral drone survey to analyse potential cultural features through vegetation growth differences. Previous flights at other locations in Europe have shown great potential but also the limits of this method, as its success is very dependent on preceding weather conditions, similar to traditional aerial photography. Pre-flight area coverage planning and rapid and fully autonomous, GPS-guided data recording have simplified data collection and processing immensely. Using Structure from Motion (SfM), digital surface models of 10 cm and orthophotos of 3 cm resolution were created for the complete areas. If available on site, the usage of drones allows regular systematic coverage of a predefined area. Due to different tasks to be completed, flights at Závist were currently limited to one full area coverage in May 2024. Bohemia had suffered a dry spring, which prevented crops to grow and prevented better survey results in some areas. Image enhancements and vegetation indices have improved the visualisation of results. For comparison purposes, aerial data was also collected at Stradonice, another large oppidum to the south-west of Prague. This location is mostly covered by agricultural fields, theoretically better supporting multispectral remote sensing for archaeological purposes.