## 3 Results
### Authors  
The following section analyze the authors of the literature database according to the co-authorship, citation and bibliographic coupling networks. The analysis of the authors, their number of publications, co-authorship relations and number of citations, allows a  
Figure 2: Number of articles and reviews for each year in the period 2011–October 2020.  
deeper knowledge of the researchers that have most experienced the application of the UAVs in architecture and urban planning. The three analyses were performed using the authors unit of the downloaded data to create a map by selecting a threshold with a minimum of two documents per author and a minimum of 20 citations per author, which led to the 52 most published and cited authors in the database (initially, 863 authors were identified). In the analysis, shown in Figure 3, each author is represented by a node. The colour of the nodes represents the average year of publication of an author. Table 1 shows the main authors with the most publications and citations, as well as additional information about their affiliation and average year of publication. The total link strength of each author is also shown, indicating the total strength of their links to other authors. It has to be noted that these networks are limited to the top 52 of productive researchers to facilitate the interpretation of the analysis, thus the link strength only reflects their connections (the rest of the researchers are not included). Figure 3: _Cont._  
In the co-authorship network (Figure 3A), the size of the nodes indicates the number of publications of each researcher. According to the number of publications, the authors with the most articles published are: [PERSON], [PERSON] (five publications), followed by [PERSON], [PERSON], [PERSON], [PERSON] and [PERSON] with four publications each one (Table 1). The links in Figure 3A reveal a strong evidence between the co-authorship relation of these authors together with [PERSON], [PERSON], showing the highest total link strength in terms of \begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Author** & **Organization** & **Publications** & **Citations** & **Year** & \begin{tabular}{c} **Co-** \\ **Authorship** \\ **1** \\ \end{tabular} & \begin{tabular}{c} **Citation** 1** \\ \end{tabular} & \begin{tabular}{c} **Bibliographic** \\ **Coupling** 1** \\ \end{tabular} \\ \hline [PERSON] & University of Twente & 5 & 226 & 2017 & 13 & 61 & 1379 \\ \hline [PERSON], [PERSON] & \begin{tabular}{c} University of \\ Braunschweig \\ \end{tabular} & 4 & 256 & 2016 & 10 & 46 & 1386 \\ \hline [PERSON] & University of Twente & 4 & 184 & 2016 & 8 & 29 & 1086 \\ \hline [PERSON] & University of Twente & 4 & 102 & 2018 & 11 & 39 & 1590 \\ \hline [PERSON] & Aarhus University & 3 & 463 & 2015 & 1 & 26 & 360 \\ \hline [PERSON] & \
\begin{tabular}{c} Georgia Institute of \\ Technology \\ \end{tabular} & 3 & 54 & 2017 & 2 & 3 & 228 \\ \hline [PERSON] & University of Coimbra & 3 & 39 & 2018 & 9 & 37 & 1076 \\ \hline [PERSON] & University of Illinois & 2 & 253 & 2016 & 3 & 13 & 260 \\ \hline [PERSON] & NC State University & 2 & 253 & 2016 & 3 & 13 & 260 \\ \hline [PERSON] &
\begin{tabular}{c} Qatar Mobility \\ Innovations Center \\ \end{tabular} & 2 & 225 & 2018 & 0 & 1 & 7 \\ \hline [PERSON] & Texas A\&M University & 2 & 179 & 2017 & 2 & 13 & 173 \\ \hline \hline \end{tabular} \({}^{1}\) Total Link Strength.  
\end{table} 
Table 1: Summary of network analyses. Authors with the highest number of publications and citations. Figure 3: Authors network analysis. **(A)** Co-authorship network, **(B)** Citation network, **(C)** Bibliographic coupling network. scientific collaboration (between 8-13). It can be seen that the greatest number of the most prolific authors is working in the same university (University of Twente in The Netherlands). The identified research group has collaborated with other European researchers from the Technical University of Braunschweig and the University of Coimbra.