## 3 Results
### Countries  
In this section, countries were analysed according to three criteria: co-authorship, citation and bibliographic coupling networks. These networks between regions make possible to identify the places where this research area has been more investigated.  
The three analysis were performed using the countries unit of the downloaded data to create a map. Fifty-Two countries were initially identified and no limits or thresholds were set. Each country is represented by a node whose colour depends on the average year of publication by country (Figure 4). The main countries with the highest number of publications and citations (five European countries, three Asian countries, two American countries and one from Oceania) are shown in Table 2 along with more information from the analysis (like the region, the average year of publication and the total link strength). In this case, the link strength indicates the connections between all the countries because no previous thresholds were established.  
Figure 4: _Cont._  
In the co-authorship network (Figure 4A), the size of the nodes indicates the number of publications of each country. According to the number of publications, the countries with more articles published are: the USA (69 publications), followed by China (39), Italy (36), Spain (18) and the Netherlands (15), as can be seen in Table 2. The links in Figure 4A show co-authoring relationships between all countries. The nodes of the USA and China have a strong collaborative relationship with the rest of countries as their total  
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline
**Country** & **Region** & **Publications** & **Citations** & **Year** & **Co-Authorship 1** & **Citation 1** & **Bibliographic Coupling 1** \\ \hline USA & America & 69 & 1902 & 2018 & 43 & 141 & 6629 \\ \hline China & Asia & 39 & 527 & 2017 & 20 & 42 & 3078 \\ \hline Italy & Europe & 36 & 437 & 2018 & 9 & 31 & 1331 \\ \hline Spain & Europe & 18 & 330 & 2017 & 9 & 30 & 861 \\ \hline Germany & Europe & 15 & 566 & 2017 & 13 & 55 & 1588 \\ \hline Netherlands & Europe & 10 & 361 & 2017 & 7 & 40 & 961 \\ \hline Australia & Oceania & 10 & 199 & 2018 & 11 & 23 & 1318 \\ \hline South Korea & Asia & 10 & 108 & 2018 & 4 & 9 & 507 \\ \hline UK & Europe & 10 & 64 & 2019 & 9 & 21 & 962 \\ \hline Canada & America & 8 & 169 & 2018 & 5 & 13 & 627 \\ \hline Qatar & Asia & 3 & 346 & 2018 & 7 & 21 & 1910 \\ \hline \end{tabular} \({}^{1}\) Total Link Strength.  
\end{table}
Table 2: Summary of network analyses. Countries with the highest number of publications and citations.  
Figure 4: Countries network analysis. **(A)** Co-authorship network, **(B)** Citation network, **(C)** Bibliographic coupling network.  
link strength indicates (43 and 20). In Europe, Germany stands out as the country with the highest rate of collaboration (13), despite not having as many articles published as other European countries.  
In the citation network (Figure 4B) and in the bibliographic coupling network (Figure 4C), the size of the nodes indicates the number of citations of each country.  
According to the number of citations, the most frequently cited countries are: the USA (1902 citations), followed by Germany (566), China (527) and Italy with 437 citations (Table 2). As in the co-authored map, the USA and China are positioned as the countries with the highest total link strength ratios in terms of citations in both maps (Figure 4B,C). The same pattern is repeated in terms of co-authorship and citation: both nodes are positioned centrally and some Middle East countries such as Saudi Arabia, Qatar and Turkey are grouped together and tend to move away from the rest of the countries. It is remarkable that Qatar has a high citation rate (346 citations) despite having published only three articles and the UK on the opposite side with only 64 citations in 10 publications.  
It can be seen that researchers in Europe were the pioneers in researching the applications of UAVs in architecture and urbanism with an average publication year set at 2017.