## 4 Critical Literature Review  
The areas of application identified in the scientometric analysis include different techniques related to the generation of 3D models, land mapping, construction site monitoring, building surveying to detect structural damage and energy losses and urban remote sensing. These techniques based on UAVs can be applied to buildings or public spaces from the design and construction processes when the project is initiated to the later stages of maintenance and inspection of the project during its life cycle. In the initial process of designing a project, UAVs can be used to carry out remote sensing tasks to assess the feasibility of the project on an urban scale. Aerial images of the site can be combined with geographic information systems (GIS) to estimate parcel dimensions, accesses or altitude limitations. They can also be a useful source of information on the surrounding urban infrastructure, road inspections and water and electricity facilities [51]. The use of UAVs at this stage of the project gives the option to visualise the design, planning and organisation of the site before construction. Figure 8: Fields of application and techniques based in clusters. Figure 9: Number of documents per cluster. At the beginning of the construction, UAVs as a data acquisition platform and as a measuring instrument are very attractive for topographical and terrain mapping applications [1; 4; 52; 53]. Their geometric accuracy is better than traditional methods that usually incur a large error as demonstrated by [PERSON] et al. (2017) who compared classical surveying, satellite images and aerial images from an unmanned aerial vehicle in an archaeological zone. They concluded that 3D models created by UAV imagery can achieve great accuracy and can replace reliable the in situ topographic survey [54]. The evaluation of aerial images is performed faster compared to images obtained from land [41]. They also provide more information than satellite images operating from a single vertical perspective [55]. Using photogrammetry techniques it is possible to acquire and visualise large quantities of images and videos which, if processed later by software, can provide orthophotos and digital surface models (DSM) [22], [56]. [PERSON] et al. (2014) converted aerial images captured by an UAV into a three-dimensional model of a terrain using topography software [57]. In photogrammetry tasks are necessary to ensure a good overlap of images to capture the same points through several shots and to use a suitable visual detection device to capture high precision and quality photos covering the whole area to be examined. In this field, the most optimal payloads are vision stereo cameras and RGB cameras with depth sensors which are able to augment the conventional image with depth information (the distance to the sensor) necessary to generate 3D models [58]. These photogrammetry techniques are also used for site monitoring activities to increase the productivity of the construction process [59]. These inspections, which can be daily or weekly, make it possible to monitor progress, safety, quality and efficiency in the construction process [60]. Today, on-site data collection is traditionally more costly, time consuming and error prone (because it is a manual task) compared to the automated process of an aircraft [61]. One of the challenges of monitoring a construction site on a daily or weekly basis is the large amount of visual data that must be captured by the UAV and the computational load for further processing [12]. UAV images are taken by seconds at constant intervals which is a problem for the storage system if monitored on a daily or weekly basis. [PERSON] et al. (2017) estimated a visual data size that could reach 6 Terabytes if a construction site was inspected weekly for 2 years. They noted that to monitor a 70,000 m\({}^{2}\) site, over 400,000 images would be needed over the lifetime of the project [46].