## 3 Aerial Survey by UAS
### Mission Planning  
Mission planning in UAV photogrammetry applications is the first and essential step to ensure the success of a survey mission. Photogrammetric mission planning can be defined as the process of planning the places to be flown over (waypoints, i.e., a reference point in space) and the actions to be performed, such as taking the photo [53]. The main steps to perform a survey by UAS for photogrammetry purposes are [54]:  
(i) selection of a sensor and platform suitable for the purpose;  
(ii) flight plan design;  
(iii) assessment of factors to be controlled during the flight mission. The sensors, initially designed and adapted for installation aboard aircraft (single- or twin-engine) or helicopters, are adapted over the years (in terms of size and weight) to UAV platforms due to technological development. Today, many UAS solutions are present on the market for photogrammetry purposes, as described in Section 3.1. In the photogrammetric environment, an important parameter to take into account is the ground sample distance (GSD) which is the distance between the centre points of two consecutive pixels of an image, measured on the ground (Figure 4a). Using a pinhole camera model (or, sometimes called projective camera model), i.e., a model that relates the physical image space to the physical workspace using the projective transformation [55], it is possible to calculate the GSD [56,57]:  
\[GSD=\frac{H}{f}s_{x}, \tag{1}\]  
where \(H\) is the normal distance from the ground to the perspective centre, \(f\) is the focal length of the camera, and \(s_{x}\) is the pixel size. Graphically, these parameters can be sketched in the following picture (Figure 4b), where the focal length is exaggerated in order to identify the key elements of the photogrammetric approach; details of the focal length relative to the sensor are sketched in Figure 4c. In fact, from the point of view of flight operations, it is necessary to calculate the absolute height above sea level (ASL) of the flight from the knowledge of the mean height above ground (AGL). This means adding the ground elevation to the height determined  
Figure 4: GSD calculation: pixel size and its relation to the GSD (**a**), geometric acquisition (**b**), and detail of main elements of the camera (**c**). in relation to the GSD, which can be determined using regional and global digital terrain models. The more accurate the digital terrain model, the more the GSD is respected. Once the sensor and platform suitable for the type of survey or investigation have been identified, flight planning can be designed. To cover the entire area, it is necessary to produce more images. In fact, by acquiring the same scene from two different viewpoints, i.e., from two metric photographs, it is possible to obtain a stereoscopic 3D view where the distance between the two perspective centres of two consecutive photos (in the direction of the UAV movement) is defined as the \"baseline\". If the length of the baseline varies, then several longitudinal overlaps (i.e., along the direction of FL) can be obtained. Generally, traditional longitudinal overlap values range from 70% to 90%. The overlap between adjacent parallel strips, called sidelaps, takes on values ranging from 50% to 80%. One of the effects that influence the correct planning of a UAV survey is the so-called \"Motion Blur\" phenomenon, i.e., the effect of motion blur in a photograph or sequence of frames. This effect occurs when the drone flies too fast compared to the shutter speed, causing the scene to change substantially during a single camera exposure. In general, we can define the \"Motion Blur\" (\(MB\)) effect by the following relationship:  
\[MB=CSI\ v_{UAV}, \tag{2}\]  
where:  
\(CSI\) camera shutter interval;  
\(v_{UAV}\) UAV speed. Although it is difficult to completely eliminate motion blur during a 3D mapping/modelling mission, it is possible to reduce it considerably to obtain better results in post-processing by setting a correct drone speed beforehand to minimise this effect.