## 5 Conclusions  
UGC and VGI online platforms have developed into a well-known phenomenon on the Internet in recent years. The review of previously conducted research in the realm of VGI in this article has clearly shown that the research community sees a lot of potential in the freely available data sources. OSM, with its exceptionally large community of registered and active contributors, demonstrates that collaboratively collected geographic information by volunteers around the world can lead to an impressive data source for multiple applications. In our study, we provided a comprehensive overview about the recent research projects in the field of VGI with a strong focus on the OSM project. The research efforts have been separated into two main domains: data quality and contributor analysis. A detailed discussion of the latest literature for each domain highlights the methodologies and findings for each study. VGI data quality analyses sparked the interest of the research community in the first few years after the platforms attracted more attention, resulting in a large body of studies in this domain. In more recent years, the analysis of contributor behavior, motivation and gender distribution in VGI projects has experienced more attention in the research community.  
Many VGI quality analyses have demonstrated in the past that the freely available data can be used for a variety of applications. However, it is still an important and major task to evaluate whether the data is acceptable for each use case. The quality of VGI datasets, as has been proven for OSM, can be heterogeneous when considering different countries or discrepancies between rural and urban areas. There is no reliable estimation if a certain object or other detailed attribute information is included in a VGI dataset, unless the potential user investigates the data in more detail and compares it to ground truth information or a reference dataset of choice. The long-term motivation of the volunteers that contribute to VGI projects, which has been questioned in recent years, has been proven, at least for the OSM project in this study.  
Based on the most recent developments in VGI research, we were able to discuss potential future trends in research and development, especially for the case of OSM. The intrinsic data assessment approach can be utilized for countries where no reference dataset is available or potential costs are too high to acquire the datasets needed. However, new methods need to be developed that utilize this approach and potentially include multiple VGI datasets. Similarly, conflation methods that utilize several VGI sources or combine VGI with other license conform datasets could be helpful in the near future. To make VGI more respected and eligible for these tasks, however, questions about credibility and trust in VGI datasets, with a focus on the contributor, need to be answered. Thus, the development of new methods that compute a trust factor, contributor reputation or individual contributor data quality is required. Finally, additional surveys are needed to gather more information about the differences in contributor motivation and behavior, especially when considering different continents and cultures.