# Review of Photogrammetric and Lidar Applications of UAV
## 1 Introduction  
In recent years, mapping and surveying the landscape, terrain, and the Earth's surface have been the most dynamic, efficient, and progressive surveyor tasks. They made significant progress thanks to the development of technology and software equipment. Non-contact methods are preferably used to map the landscape, including radar scanning, laser scanning, or aerial and terrestrial photogrammetry. Non-contact methods provide a large amount of spatial data with high accuracy and detail, which capture parts, elements, and components of the landscape or the Earth's surface. The most common outputs from data collection using non-contact methods are aerial images, digital terrain and surface models, point clouds, vector maps, and orthophoto maps representing real-world objects [1, 2]. Among non-contact methods, laser scanning is increasingly used for landscape and terrain mapping. Laser scanning is an active method of remote exploration of the Earth, which uses electromagnetic radiation as a source. Laser scanning enables the recording of the 3D geometry of objects, surfaces, and landscape relief with high accuracy and detail in a relatively short time [3, 4, 5, 6]. Another non-contact method of data collection in the field is photogrammetry. Structure from Motion (SfM) reconstruction is a modern and advanced method in the photogrammetric processing of images [7, 8, 9]. The main principle of the SfM method is to estimate the three-dimensional structure of a spatial object using two-dimensional images resulting from a moving recording media carrier (digital camera, video camera). The main advantage of the SfM method is its availability and deployment in practice with precise and detailed results. Using non-contact methods based on piloted or the more available remotely piloted (unmanned) aerial vehicles is currently very popular. We distinguish between piloted(planes, helicopters, balloons, airships) and remotely piloted (UAV, UAS, RPAS) airborne carriers. Today, UAVs are preferred because they are affordable and easy to use. Their advantage is in minimizing fieldwork, applying the high speed of photogrammetric data collection, and processing with a high level of detail and accuracy of results. As a result, UAVs are currently used in almost every scientific, technical, and non-technical discipline. A global navigation satellite system (GNSS) defines the sensor's orientation and position. It locates the geographical position of the sensor, along with the Inertial Measurement Unit (IMU), which measures the aircraft's speed, pitch, orientation, and exact position [10]. When choosing a data-collection method, area size, required density of points, positional and height accuracy, the time needed for fieldwork, and economic aspects must be considered. The accuracy of the resulting model depends on camera parameters, the accuracy of measurement of image coordinates, image configuration, defining the coordinate system, the accuracy of the scale and ground control points (GCP), quality of camera calibration, the stability of internal orientation elements, and other parameters. In surveying applications, the use of GCP points is required to verify the accuracy of measurements. GCPs are used for georeferencing photogrammetric data. With the help of GCPs, the internal and external orientation elements on the images are correctly determined. Determining GCP coordinates requires the use of classic geodetic methods with subsequent transformation into positional and height coordinate systems. The second option is to use the RTK/PPK approach position when imaging. Then, each image is assigned a precise position. In this case, GCPs are not necessary but are recommended. This alternative is mainly used when stabilizing GCP points in the field is difficult. Many authors [11; 12; 13; 14; 15] have investigated and compared the accuracy of UAVs with onboard RTK/PPK GNSS versus UAV measurement using GCP points with relevant and interesting results.