## 2 Materials and Methods
### Traditional Parameters for the Implementation of the Flight Project
However, it provides a higher quality and, above all, metric accuracy to the final model, especially when flying with low-cost sensors. The Ground Control Points (GCPs) are a set of points that area easily identifiable in the images. They are generally materialized with photogrammetric targets whose coordinates with respect to the selected reference system are defined by means of surveying methods. They are usually evenly distributed along the area that is modeled, and they allow the orientation and scaling of the model. The technology of most photogrammetric software programs designed for processing images acquired with drones is based on Structure from Motion (SfM), which allows obtaining 3D models from 2D images taken from different points of view. This methodology does not require GCPs, as the scale, georeferencing, and orientation of the images for the generation of the models are developed by automatically relating the images from a set of points, which allow the reconstruction of the location of the cameras in the space to generate the point-cloud [10]. However, their use while processing clearly improves the geometry and accuracy of the final model. Platforms that include GPS within the sensor market, which do not require photogrammetric support [11], are available in the market, but they are not low-cost devices. * Speed of flight. This parameter alludes to the UAV momentum during the itinerary that must follow. It is usually set at about 4 m/s. The definition of a higher or lower speed of flight conditions the total time of flight, the time between shots, and even the shuttering speed. All these parameters must be taken into account due to the limitations of both the UAV and the nonmetric cameras. Generally, it can be stated that the speed of flight is relevant, as it greatly conditions the duration of the flight, but it has little affection with respect to the geometry of the flight, and even less in terms of the accuracy of the subsequent models. * Height of flight. It is a parameter of great relevance, as it conditions most of the final parameters and specially the number of images and resolution. The resolution is usually expressed as the Ground Sample Distance (GSD), which is defined as the distance between the centers of two consecutive pixels. It is directly dependent on the height of flight, i.e., a lower height implies a lower GSD and, therefore, a higher resolution. A limit or level of acceptable resolution must be established to set the maximum height of flight. On the basis of the GSD concept, the height of flight can be defined by applying Equation (1) [12]. \[\text{F}_{\text{H}}=\text{GSD}\cdot\text{F}_{\text{L}}\cdot\text{P}_{\text{N}} /\text{S}_{\text{W}},\] (1) where \(\text{F}_{\text{H}}\) is the height of flight (m), GSD is the Ground Sample Distance (m), \(\text{F}_{\text{L}}\) is the focal length (mm), \(\text{P}_{\text{N}}\) is the number of pixels per image width, and \(\text{S}_{\text{W}}\) is the sensor width (mm). After setting the GSD that is desired to obtain, Equation (1) determines the height of flight that guarantees that resolution. This procedure is constrained when low-cost sensors are applied, as the focal length may suffer variations that alter the metric desired. However, this parameter can be determined with relatively good precision during calibration. Another disadvantage lies on the lack of defined GSD values for the different applications of the virtualization. This is due to the difficulties to establish a required level of detail for an archaeological site, in which the asset or the land to model can have very different dimensions. Hence, it can be stated that this is one of the main factors to define in a flight project [13], and there is a great controversy about fixing its value. At best, the authors of several works focused on this field offer an approximate recommendation of the values to apply. * Forward overlap. It is defined as the overlapping between two consecutive frames in the same pass. It is generally expressed as a percentage of the surface covered.