This paper tests the applicability of an unmanned aerial system (UAS) in obtaining spatial data from archaeological sites in forested areas. Our case study discusses the remains of the anti‐Turkish fortifications called ‘Turške Šance’—the defensive ditches that are preserved in the area of the Preški vrh (Ravne na Koroškem, Slovenia). Up to 10 fortifications of various shapes and sizes were built in the last quarter of the 15th century. Today, they represent a unique archaeological site. A detailed geodetic survey of the site was performed and compared with results of photogrammetry using the DJI Phantom 4 PRO UAS drone and DJI Phantom camera. Furthermore, official Slovenian LiDAR data of the area were obtained, and archaeologic field surveys, in situ inspections and metal detector scanning were made. The points obtained from the total station were used for 3D modelling and taken as a reference. Drone photogrammetry was performed once per week to compare each point cloud with another using CloudCompare and to the reference one depending on the volume of the tree leaves which was estimated with green leaf index (GLI) and leaf area index (LAI). The increase of the tree leaves volume deteriorates the point cloud obtained with a UAS. The paper analyses how significant this influence is. As expected, the impact grows more severe with the increase in leaf mass from week to week.