## 2 Materials and Methods
### Background
#### 2.2.2 Data Production Technology  
Photogrammetry is the art, science, and technology used to obtain reliable information about physical objects and the environment through the process of engraving, measuring, and interpreting photographic images, energy patterns, and other phenomena [32]. The most widespread of its possible branches are aerial photogrammetry, which, from aerial photographs taken under meteorological and geometric conditions, allows us to draw plans and maps of large areas with precision and agility. In the case of urban areas, the limit drops considerably. However, it all depends on the accuracy and characteristics of the terrain in question [33]. The technique is based on the principle of stereoscopy to ensure that by viewing the terrain through photographs, the relief of the field can be reproduced. Two mathematical conditions must be met to make a 3D reconstruction of objects obtained in 2D images: collinearity and coplanarity. (1) The first condition refers to the linear relationship between any point in the actual space, the perspective of the center of the camera, and the same point (now a pixel) recreated in the image space. (2) The second condition refers to when any point (pixel) in the image space is combined with the same point (pixel) of other images that were collected considering slope; it defines that the points belong to the same plane [34].  
One of the most commonly used techniques for 3D reconstruction from image processing is Structure from Motion (SfM). A SfM reconstruction uses images taken from different approaches or viewing angles [35]. In addition, lighting conditions, shadow changes, and missing data areas have been shown to impact the quality of 3D reconstruction negatively. Therefore, image acquisition planning is relevant and must be acquired in such a way that the conditions are constant, and the presence of moving shadows is minimized. The processing of the images begins with the detection of points which are characteristic of each image; these points are grouped by identifying common points in several different photographs. However, there are geometric distortions because images are taken from different perspectives, and it is necessary to identify points called key points. Since there is no guarantee that a key point will have a correspondence in another photograph, the next step is to apply algorithms to discard key points without a good match between them and images [36].  
The scaling and georeferencing requirements for the target surface will vary depending on the used 3D model and should be considered before image acquisition. An object or surface can be completely 3D reconstructed without any scale or position of information, but ground control points (GPPs) must be provided to extract oriented and scaled data. The scale can be adjusted in the model by knowing the distance between two points in an image or the model. GPPs are transportable or stationary devices located in different locations within the area to be surveyed. They must be visible in the recorded images as they contain coordinate information to scale, rotate, triangular, and georeferenced the digital surface model (DSM) [32]. Theoretically, it is enough to place three GPPs in the area surveyed; however, it is recommended to position a greater amount to obtain more accurate results [37]. In recent years with the implementation of flights with GPS and inertial sensors, the number of GPPs needed to adjust a photogrammetric block has been greatly reduced correctly. GPS records the projection center coordinates of each photo and, together with the camera, works independently by obtaining data at different time intervals [33].