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Global Positioning System (GPS) on Unmanned Aerial Vehicles (UAV) platform relies on Micro Electro Mechanical Systems (MEMS) technology with a precision of 10 m at shooting time at UAV camera stations positions. Nonetheless, obstacles to the GPS signal at the finest flight altitude can prevent accurate camera stations positions retrieval. In this research, three different georeferencing techniques were compared with geometric precision. The first is Direct Georeferencing (DG), which mainly depends on using Navigation GPS onboard without using any Ground Control Points (GCPs). The second is Indirect Georeferencing (IG), which mainly depends on three GCPs used to assist Aero-Triangulation (AT). The third is Modified technique depends on the same three GCPs used in the second method and enhanced location of camera stations usage of the Linear Relation Model (LR Model). The study area was in the south of the Moscow Region, Russia. Threeimaging strips have been taken using the DJI PHANTOM 4 PRO UAV. The accuracy assessment was carried out using image-derived coordinates and checkpoints (CPs) residuals. This study emphasizes that the Modified methodology using enhanced camera stations positions gave better accuracy than using the drone GPS camera stations positions.

The practical aspect of utilization of digital photogrammetry methods and their products in Slovak forestry is presented. We deal with the comparison of various data obtained by digital cameras, their accuracy, utilization and effectiveness for mapping. Based on presented results we summarize the main conclusions in the field of forest mapping. We also deal with the process of pan-sharpening for the preparation of image data for the interpretation and classification of forestry features. Also, the statistical characteristics of two photogrammetric projects with different geometric resolution photos (Ultracam D digital camera with ground sample distance of 10 cm and UltracamX digital camera with ground sample distance of 20 cm) from the same experimental area of the University Forest Enterprise Zvolen are described with the aim of finding an appropriate solution suitable for forest mapping. In both cases, the horizontal accuracy of the photogrammetric projects is presented. Finally, the process of true orthophoto generation and its utilization in forestry is the focus. Both experiments in this paper presented results which fulfil the accuracy standards defined by the state cadastre within the mapping of large scale maps. All projects, apart from the project of direct georeferencing, fulfilled the third accuracy class of mapping, so they can be used within cadastral mapping, land consolidations and, of course, within forested land mapping. Still, all the projects meet the accuracy requirements within the forest mapping standard, where the forestry features and forest spatial distribution units are mapped.