Accuracy of UAV-based DEMs without ground control points

GeoInformatica https://doi.org/10.1007/s10707-023-00498-1 RESEARCH Accuracy of UAV‑based DEMs without ground control points Bartłomiej Szypuła1 Received: 11 October 2022 / Revised: 22 February 2023 / Accepted: 28 March 2023 © The Author(s) 2023 Abstract Unmanned aerial vehicles (UAVs) are increasingly used in various environmental research projects and other activities that require accurate topography images. The quality of elevation models derived from UAV measurements varies depending on many variables (e.g. UAV equipment used, terrain conditions, etc.). In order to improve the quality of digital models based on UAV image data, additional GNSS-RTK measurements are usually made at ground control points. The aim of this article is to evaluate the mathematical accuracy of terrain models created without ground control points. The accuracy of the models is considered in two directions: vertical and horizontal. Vertical (elevation) accuracy is calculated based on airborne laser scanning (ALS) data and horizontal (location) accuracy is calculated through comparison with high-resolution orthophotomaps. The average elevation accuracy of all created UAV-based DEMs is found to be 2.7–2.8 m (MAE), 3.1–3.3 m (RMSE), and the average horizontal accuracy is 2.1 m. Despite the low accuracy of the UAV models, the topography is reflected very well in the spatial images. This may be related to the regular and symmetrical distribution of height errors. To improve the accuracy parameters of UAV-based DEMs, it is proposed that they be rapidly georeferenced based on orthophotomaps. Keywords Unmanned aerial vehicle (UAV) · Digital surface model (DSM) · Digital elevation model (DEM) · Orthomosaics and orthophotomaps · Horizontal and vertical accuracy · Result conformity index 1 Introduction Nowadays, it is increasingly common to use unmanned aerial vehicles (UAVs) or, more broadly, unmanned aerial systems (UAS) for missions that result in the acquisition of high-resolution imagery. These images offer great material from which a digital surface model (DSM), digital elevation model (DEM) [1, 2] or orthomosaic [3, 4] can be created relatively quickly and easily. UAS systems enable us to obtain spatial information from * Bartłomiej Szypuła 1 Institute of Earth Sciences, Faculty of Natural Sciences, University of Silesia in Katowice, Będzińska 60, Sosnowiec 41‑200, Poland 13 Vol.:(0123456789) GeoInformatica a wide area of interest, and the obtained data are detailed and of high resolution. UAS systems can thus be ranked between space-based remote sensing and detailed in situ measurements in environmental monitoring systems [5]. The widespread availability and decreasing cost of such flying equipment has led to its increased use in scientific research, to, for example monitor soil erosion [6], detect and conduct spatial analyses of periglacial landforms [7], perform detailed geomorphological mapping [8], monitor coastal dune change [9, 10], analys karst landforms [11] or conduct a multi-faceted analysis of the Antarctic ecosystem [12]. This technique is also often used in mining [13–15], monitoring various natural geohazards and phenomena [16, 17], detecting of agricultural crops or trees [18, 19] and, dam and riverbed erosion [20], modelling topographic features [21], updating cadastral data [22], estimating solar irradiation [23], researching cultural heritage and archaeology [24, 25] and monitoring traffic [26, 27]. All of these activities require specific data of adequate accuracy. The quality of the final product (UAV-DEM/DSM, orthomosaic) depends on the initial image acquisition, and ground sampling distance (GSD, i.e. spatial resolution of the UAV images) is a fundamental element of successful image processing. GSD is affected by many factors, including the flight height and path, the employed sensor (RGB, multispectral/hyperspectral) [28], lighting conditions (sun angle and cloud cover) [29] and, the terrain surface cover. Furthermore, the resulting model accuracy also depends on camera pitch [30, 31], image overlap [32], flight trajectory [33], camera calibration method [34, 35] and the software used for reconstruction [36]. Besides the above technical aspects related to designing and executing a UAV mission, additional measurements can be applied to improve the quality of UAV data. Standard methods to increase the accuracy of DEMs/DSMs derived from UAV measurements consist of using ground control points (GCPs), which are points measured in the field, during pre-processing and independently using high precision geodetic GNSS RTK [37–41]. UAV methods are sometimes combined with terrestrial laser scanning (TLS) systems [42–44]. Some authors apply multi-view stereopsis (MVS) techniques combining photogrammetry and computer vision with ground control points collected using a Differential Global Positioning System (DGPS) [45]. However, UAV-mounted (on-board) GNSS RTK are utilised in an increasing number of assignments [32, 35]. Unfortunately, UAV cameras equipped with a GNSS RTK or LiDAR receiver are still expensive (prices start at $25,000—$30,000). There is also a more advanced approach to measuring the same area from different altitudes and angles, which leads to much better results [46]. Although the highest possible accuracy is required from models created on the basis of UAV measurements, it often occurs that high precision (centimetre/decimetre) is not absolutely necessary. Similar insights emerge in the relevant literature [see 47]. Based on such an assumption, the aim of this paper is to vailidate the final accuracy of UAV-DEM and UAV-DSM, which were created based on image data from UAV flights. These models were created using the simplest method, without ground control points (GCPs) measured earlier in the field with GNSS RTK equipment. Accuracy is considered here in two directions: vertical accuracy (calculated on the basis of airborne laser scanning [ALS] data) and horizontal accuracy (calculated on the basis of high-resolution orthophotomaps). The author is interested in mathematical and statistical accuracy, specificallythe extent to which the model obtained from UAV flights differs from reality. The article focuses on two issues: 1) the extent to which the heights of digital models created from UAV data differ from ALS reference models (with an elevation accuracy of ± 0.1 m) and 2) the extent to which the locations of objects are shifted horizontally compared to orthophotomaps acquired from the 13 GeoInformatica national geoportal (with a resolution of 5 cm × 5 cm). The results of the study make it possible to determine the order of magnitude of accuracy errors to be expected when using this type of data in environmental studies. 2 Study area The study area is located in southern Poland, in the Katowice Upland (mesoregion) and the Silesian-Cracow Upland (subprovince), which belong to the strip of Polish Uplands [48]. The midpoint of the research area is situate (...truncated)