According to the UVS International definition, a UAV is a generic aircraft design to operate with no human pilot onboard. Unmanned Aerial Vehicles (UAV) has recently become commercially available at very reasonable cost for civil applications. The advantages linked to their small mass (typically around 500 grams) are that they do not represent a real threat for third parties in case of malfunctioning. In addition, they are very easy and quick to deploy and retrieve. The term UAV is used commonly in the geomatics community, but also terms like Remotely Piloted Vehicle (RPV), Remotely Operated Aircraft (ROA), Remote Controlled (RC) Helicopter, Unmanned Vehicle Systems (UVS) and Model Helicopter are often used.
In the past the development of UAV systems and platforms was primarily motivated by military goals and applications. Unmanned inspection, surveillance, reconnaissance and mapping of inimical areas were the primary military aims. For geomatics applications, the first experiences were carried out by Przybilla and Wester-Ebbinghaus (1979). In the last years, more and more applications of UAVs in the geomatics field became common. UAV photogrammetry indeed opens various, new applications in the close-range aerial domain and introduces also a low-cost alternatives to the classical manned aerial photogrammetry (Colomina et al., 2008; Eisenbeiss, 2009). This development can be explained by the spreading of low-cost platforms combined with amateur or SRL digital cameras and GNSS/INS systems, necessary to navigate the UAV with high precision to the predefined acquisition points. The small size and the reduced pay-load of some UAV platforms limit the transportation of high quality IMU devices like those coupled to airborne cameras or LiDAR sensors used for mapping. GNSS is mainly used in code-based positioning mode and thus it is not sufficient for accurate direct sensor orientation. The use of RTK techniques would improve the quality of positioning to a decimeter level, but the system would become too complex, expensive and heavy.
Based on size, weight, endurance, range and flying altitude, UVS International defines three main categories of UAVs:
- Tactical UAVs: which include micro, mini, close-, short-, medium-range, medium-range endurance, low altitude deep penetration, low altitude long endurance, medium altitude long endurance systems. The mass ranges from few kilograms up to 1,000 kg, the range from few kilometers up to 500 km, the flight altitude from few hundred meters to 5 km, and the endurance from some minutes to 2-3 days;
- Strategical UAVs, including high altitude long endurance, stratospheric and exo-stratospheric systems which fly higher than 20,000 m altitude and have an endurance of 2-4 days; and
- Special tasks UAVs like unmanned combat autonomous vehicles, lethal and decoys systems.
The typical domains were UAVs images and photogrammetrically derived 3D data or orthoimagess are generally employed include:
- Forestry and agriculture: producers can take reliable decisions to save money and time (e.g. precision farming), get quick and accurate record of damages or identify potential problems in the field (Newcombe, 2007). Assessments of woodlots, fires surveillance, species identification, volume computation as well as silviculture can also be accurately performed (Restas, 2006; Grenzdörffer et al., 2008);
- Archaeology and cultural heritage: 3D documentation and mapping of sites and structures are easily achieved with a low-altitude image-based survey (Cabuk et al., 2007; Lambers et al., 2007; Sauerbier et al., 2010);
- Environmental surveying: quick and cheap regular flights allow land and water monitoring at multiple epochs (Thamm and Judex, 2006). Post-disaster response and mapping, excavation volume computation and natural resources documentations are also feasible;
- Traffic monitoring: surveillance, travel time estimation, trajectories, lane occupancies and incidence response are the most required information (Haarbrink and Koers, 2006; Puri et al., 2007).
- 3D reconstruction: unmanned aerial vehicles are a valuable source of image data for general 3D reconstruction purposes of man-made structures (Wang and Li, 2007; Irschara et al., 2010). A typical image-based field surveying with UAV systems require a flight or mission planning, GCPs measurement (if not available and required for geo-referencing), image acquisition, camera calibration and image orientation, image processing for 3D information extraction.