# Estimation of the Block Adjustment Error in UAV Photogrammetric Flights in Flat Areas
Valencia km 7, 28031 Madrid, Spain; [EMAIL_ADDRESS]  
[PERSON]  
4 Geo-Environmental Cartography and Remote Sensing Group (CGAT), Universidad Politecnica de Valencia, Camino de Vera s/n, 46022 Valencia, Spain; [EMAIL_ADDRESS]  
[PERSON]  
5 Institute of Geosciences (IGEO), CSIC-UCM, [PERSON], 7, Ciudad Universitaria, 28040 Madrid, Spain; [EMAIL_ADDRESS]
UAV-DAP is one of the most widely used RS techniques in small extension studies, due to its flexibility in data acquisition, low operating costs, and high spatial and temporal resolution [3; 4]. UAV-DAP is based on structure from motion (SfM) algorithms, and uses the high spatial resolution of photographs to recognise textures [5] and orient the photographs by identifying common points, without the need to know the camera parameters or the grid geometry [6]. It produces results such as digital surface models (DSM), contour lines and orthomosaics, which are used in a range of studies, such as the inspection and surveillance of natural risks [7], infrastructures or civil engineering works [8], calculating earth movements in open mines or quarries, levelling beaches [9], coastal monitoring [10], landslides monitoring [11], subsidence analysis [12], fire surveillance [13], precision agriculture [14], forest inventories [15], and vegetation monitoring [16]. Among the main advantages of UAV-DAP is its lower application cost compared to lidar (light detection and ranging) technology [17], while its results are similar in terms of precision and spatial resolution [18]. Compared to classic topography (GNSS, total station, tachymetry), one of the most important benefits is the generation of a dense cloud with less fieldwork. The UAV-DAP technique has a higher spatial resolution [19] than the satellite segment and allows data to be obtained under cloud cover. There are currently a range of applications for planning and automating flights with UAV and photo processing programmes. During the flight planning phase, it is necessary to know the legal aspects governing the flight, and to verify whether the study zone is within an urban or rural area and whether it is affected by any restrictions from the civil aviation authority. The regulations in each country (Ref. [20], for the case of Spain) and the general specifications of the equipment, contained in the manufacturer's manual, must be followed to operate the UAV. There is usually an obligation to obtain an operating licence in order to avoid accidents and prevent causing harm to third parties. In regard to technical aspects, the correct location of the ground control points (GCPs), flight time, maximum sensor speed, and the orientation and position of the photographs to be captured must be established, and the programmed flight must be uploaded to the application to be used. It is important to consider the solar height and the GNSS satellite constellation, and to review any meteorological phenomena that directly impact the flight (Regulation (EU) 2019/947). There are also some general recommendations, such as determining the topography of the area using a digital elevation model (DEM), in the case of steep terrain, as this produces a better image cover and a more homogeneous ground sampling distance (GSD) [21]. On this type of terrain, or when there are occlusions, it is recommended to take oblique photographs to improve the orientation process and obtain more orthogonal shots of the subject. The aspects that affect the accuracy of the camera orientation, and hence the photogrammetric outputs, include the loss of the GNSS signal and the transformations in the system of coordinates (image-terrain). The GCPs allow the absolute orientation, transforming the solution to its position on the ground. In absolute orientation, the cartographic product is oriented, levelled, and scaled. In photogrammetric flights, the GCPs must therefore be taken strategically, preferably on the periphery or on the corners of the block for the planimetric component, so that the transmission error is minimal inside the block, in the case of regular surveys.