Combining UAS and historical aerial imagery SfM photogrammetry for cultural heritage documentation and research: the case of Torre de Modorra (Vinhais, Northern Portugal) João Fonte1,2, José Manuel Costa-García3,4, Óscar Moutinho5, Ana Rodrigues5, Lia Duarte6, José Alberto Gonçalves6 ¹Institute of Heritage Sciences (Incipit), Spanish National Research Council (CSIC) 2Department of Archaeology, University of Exeter 3Universidade de Santiago de Compostela 4Vrije Universiteit Amsterdam 5Eye2Map Lda. 6Department of Geosciences, Environment and Land Planning, Faculty of Sciences, University of Porto Structure from Motion (SfM) photogrammetry has been frequently used to document and study cultural heritage. This is due to its straightforward and nearly automatic use, which conjugates an easy workflow with accurate results. When dealing with architecture, Unmanned Aerial Systems (UAS) photogrammetry is capable of providing the aerial perspective of the top of a building, but also to obtain an accurate information of its façades, in order to fully capture 3D data from an historical building (Campana 2017). In this study, we have used UAS photogrammetry to survey a Roman military tower, providing fast and reliable data collection and detailed results. Case Study Torre de Modorra (Vinhais, Northern Portugal) has been catalogued as Roman military tower. This stone building suffered great damage in the last decades, being now at risk of collapse. It is imperative to document its present state of preservation and delve into its historical characterisation Material and methods In addition to the UAS survey, we incorporate a diachronic perspective through the use of historical aerial imagery (SPLAL, 1950). To photogrammetrically process these images, a GIS open-source application, MicMacGIS, was used (Duarte et al. 2016) and improved with new pre-processing functionalities: The automatic creation of shapefiles showing the projection centre of the camera. This process allows to check if one photograph is missing in any of the strips, and to analyse the position of each photo; The creation of shapefiles revealing the area covered by each photograph. It is possible to verify if the area of interest is correctly covered by checking the photo overlap; To account the number of images that overlap in each location and to assign the resulting number to raster surfaces. This process help us to analyse if there is enough overlap for a multiview stereo and to check the areas with greater density of photos. Hardware and flight plan: 3DR Solo UAV; Canon SX230HS camera (triggering at every 2 seconds); Mission Planner software for flight plan (overlap and sidelap: 80%); Altitude above ground level (AGL): 40 m; Ground sample distance (GSD): 1.2 cm; 10 ground control points (GCPs) for absolute orientation: GPS Trimble R6 (RMSE 3.35 cm). Results This methodology allowed us to carry out a rigorous survey of this historic building: a 3D model was obtained, as well as an orthomosaic and digital surface model. Through the combination with other information sources, we were also able to get some insights regarding the original structure and its historical context, taking into account that a good survey engages with the history of the building (Bertocci & Bini 2012). The tower suffered great damage in the last decades. The historical aerial photos reveal the existence of a more complex structure. The topographic map commissioned by the Vinhais Municipality in the 1970s also shows a similar situation, being the tower part of a much larger building complex. A geophysical survey could help us to better document these nowadays razed structures. Bertocci, S. & Bini, M. (2012). Manuale di rilievo architettonico e urbano. Novara, CittàStudi. Campana, S. (2017). Drones in Archaeology. State‐of‐the‐art and Future Perspectives. Archaeological Prospection. doi:10.1002/arp.1569 Duarte, L., Teodoro, A. C., Moutinho, O., & Gonçalves, J. A. (2017). Open-source GIS application for UAV photogrammetry based on MicMac. International Journal of Remote Sensing, 38(8-10): 3181-3202. doi:10.1080/01431161.2016.1259685 Video about the Torre de Modorra produced by Eye2Map, Lda.: https://youtu.be/uTeXLfgBhck