## 2 Volunteered Geographic Information
### The OSM Project
The availability of both imagery platforms had a large impact on the collection of new objects in OSM. Particularly the release of the Bing imagery datasets resulted in a strong increase in building information [55]. Additionally, several countries achieved a large data collection in OSM by importing commercial or governmental road network datasets that comply with the OSM license. Examples can be found in the Netherlands, Austria and the United States. For Spain and France, cadastral building information was successfully imported to the OSM database. OSM contributors can use multiple ways to communicate with each other. Most of the project-related information is collected and shared in the official OSM wiki, which covers a plethora of subjects, such as detailed information about tutorials for beginners, usable software or documentation about how objects should be mapped. Additionally, contributors use a variety of Internet relay chats (IRCs) [56] or mailing lists [57] to ask questions regarding tagging conventions in OSM, software development, data imports or other topics. Figure 1 illustrates the digital infrastructure of the OSM project and its community. Many active OSM contributors also participate in so-called \"Mapping Parties\", during which the contributors meet at a certain location, get to know each other, share experiences about OSM and spend some time exploring and mapping the community [23]. Sometimes, these events can also take place at previously unmapped areas to improve the data collection efforts in those regions [49]. The main events in the OSM community that attract most participants are the yearly \"State of the Map\" conferences, which are held at different locations in the world. To make modifications to the OSM database, the contributors can use multiple editor types. The newly developed iD editor makes it easier for new contributors to add information to the map, while the long-established editors Potlatch or JOSM (Java OpenStreetMap Editor) are preferably used by more advanced members [58]. There are also a large number of different editors available for multiple mobile devices, such as smartphones, and different operating systems. The data contribution patterns of the active OSM community have changed over the past few years for different world regions. During the first few years of the project, most volunteers focused their data collection efforts on road network data. Nowadays, other real world features, such as buildings, land use or public transportation information are being added in many regions to provide more details to the users. When a volunteer creates an object in the OSM database, representing a real-world feature, she/he can use three different object types [59]. Point information is represented by a Node object in OSM, whereas a Way object is utilized when mapping lines or polygons (latter, in the form of a closed line feature). If a number of Node and/or Way objects are related to each other, the Relation object can be utilized to map this particular information (e.g., turn restrictions of the street network or tram/bus lines for public transportation). Any modifications or contributions made to the OSM database by a single member are stored in a changeset, and its extent covers the entire area within which a contributor made her or his changes. Each object in OSM can be annotated by a variety of attribute information, also referred to as tags, which consist of a key-value pair. Any contributor is free to propose and discuss new tags to describe real-world features [60], resulting in a bottom-up tagging approach, indicating that there is no traditionally, enforced tagging limitation with which mappers have to comply. However, a large number of suggested key-value combinations that are widely used in the community are provided in a wiki [61] that helps to standardize certain objects in OSM.