## 1 Introduction  
Since the discontinuation of the selective availability of the Global Positioning System (GPS) in 2000, allowing users to receive a non-degraded signal, the increase of GPS-enabled devices and new technological developments allowed more and more people to use their location information for designated location-based services (LBS), to position their photographs or other information on a world map or to use it for spare time activities, such as geocaching [1, 2, 3, 4]. In a similar timeframe and as a consequence of the Web 2.0 phenomenon [5], Internet users began not only to passively consume information, but also started to create or edit web content based on their individual requirements, preferences or interests. [PERSON] [6] described these participants as \"prosumers\", a portmanteau of producer and consumer, whereas [PERSON] [7] titled them \"producers\". As a result of this development, projects, such as Wikipedia, or image and video sharing platforms, such as Flickr or YouTube, experienced a significant increase in user numbers and contributors over the past few years.  
Several terms were introduced in mainstream media and research alike to describe this new pattern in data development. General data contributions, such as Wikipedia entries or blog posts, are oftentimes summarized as user-generated content (UGC; [8]) or user-created content [9]. The additional geographic component, usually represented by latitude and longitude values, separates a special data type from these contributions, oftentimes termed crowd-sourced geodata [10, 11], collaborative GI [12, 13] or more commonly known as volunteered geographic information (VGI; [1]). Related concepts that do not solely focus on the collection of information have also been termed in many different ways, such as collaborative mapping [14], wikification of geographic information systems (GIS) [15], participatory GIS [16], public participation GIS [17] or web mapping 2.0 [18].  
VGI has become a widespread phenomenon in media and academia alike. A number of research projects in recent years have analyzed the advantages and disadvantages related to VGI. In many cases, the researchers investigated the data and contributor information of the OpenStreetMap (OSM) project, one of the most successful VGI projects in recent years [19, 20, 21, 22], which has also been frequently cited in the GIS community [1, 23].  
The objective of this paper is to provide an overview of current developments in VGI research, focusing on the different methods that were applied to analyze the members and their corresponding data contributions. After discussing the most essential results from the selected studies, different lessons that can be drawn from the presented research and potential future trends that lie in the empirical analysis of VGI datasets will be discussed.  
The remainder of this article is structured as follows: The next section briefly introduces VGI, followed by a comparison of different VGI projects. Section 3 provides an overview and summarizes the findings of previously conducted OSM research. Future trends and questions are discussed in Section 4, followed by a conclusion in Section 5.