## 5 Conclusions
Certain gaps in the literature were also observed such as the lack of collaboration between the most prolific authors from different continents. It should be mentioned that the lack of collaboration between authors from different universities and different countries affects negatively the dissemination of published articles. The different application areas and techniques most frequently used in this research field were identified based on the most frequently cited journals and documents and the most frequent keywords in the literature database. The analysis of the most frequently cited journals and documents was useful to identify the fields of study in a general area that was delimited through the analysis of the most frequent keywords in: generation of 3D models, land mapping, construction site monitoring, building surveying to detect structural damage and energy losses and urban remote sensing. The large body of literature was synthesised and the documents were filtered according to the area of application and the techniques found in order to optimise the literature review process that followed the lines drawn by the keywords. The literature review showed that UAVs provide a useful multi-tasking tool at any stage of a construction project. First, the different techniques applied in the design and construction process of a project were analysed, such as the use of photogrammetry in urban spaces, land and construction sites to study the viability of a project or to monitor a construction site. Then, the tasks corresponding to the maintenance and inspection stages of a building or public space during its useful life were studied using other techniques. Methods based on laser scanning, infrared and post-processing techniques were used to generate three-dimensional models of cities or heritage monuments, and to carry out energy analysis or damage assessment on an urban and building scale. In each of the application areas, research trends and future perspectives were discussed. Regarding the limitations of the research, it is important to highlight that any review based on a data-driven approach is highly dependent on the quality of the data collected [129]. Although a thorough article filtering process was carried out, it cannot be assured that all relevant articles were collected because: (i) other interesting non-English language publications were not taken into account, (ii) the literature searches were organised by logical combinations of words related to the topic, but not all synonyms or alternative terms that other researchers may have used in their work could be collected, (iii) the ScopusTM database finds the searched terms in the title, abstract and keywords, but does not analyse the full text of the publication. For the definition of the application fields, VOSviewerTM grouped the keywords according to their co-occurrence by clusters and then they were manually labelled according to the authors' criteria. These could be called by another name by other experts. In addition, some keywords are likely to belong to more than one cluster, despite being marked in a single colour, especially when the topics are related to each other. For example, the word Defects is grouped in the Energy efficiency prospection cluster but would also be linked to the Structural damage detection cluster. Further studies are needed to define best practices for the current and future integration of UAVs in architecture and urban planning. Some of the research areas identified present certain challenges that, if solved in the coming years, could have great potential. UAVs have demonstrated certain limitations in their positional inaccuracy despite the use of GPS and IMU components. Improved aircraft control, the use of anti-collision sensors and new GPS replacement devices will offer a solution to the problem of data acquisition inside buildings with energy or structural pathologies.