## 2 Volunteered Geographic Information: The OpenStreetMap Project  
The OSM project is one of the most popular and well-known VGI platforms on the Internet. The main goal of the project since its initiation in 2004 is to create a freely available database of geographic features [4]. Contributions and edits to OSM can be made by any internet user that is registered to the project. This open approach to data contribution allowed the project to gradually attract new members and grow rapidly in the past few years to more than one million registered members at the time of writing [5]. Aside from the aforementioned registration to the project to be able to make edits to the map, the volunteers are commonly equipped with GPS enabled devices that allow the collection of new information in the field. Others prefer to trace new data from aerial imagery from their home computer. The imagery information is provided by a variety of sources and allows the active users to trace data even for areas that are not close to the editor's location. In addition to digitized features, attribute information about the created objects can be simultaneously added to the database. For a certain number of countries, contributions to the project were achieved through large data imports from commercial or governmental data sources whose licenses are compatible to the OSM license. Some examples can be found in the United States, the Netherlands and Austria, where partial or complete data representations of the road networks were enabled through this approach. For France, building information and the CORINE (Coordination of Information on the Environment), land cover information were imported to OSM in 2009. There are no strict limitations, rules or standards to the type of information which should be contributed to the OSM project. Merely a de facto standard, represented by the \"Map-Features\", helps to guide the contributors with their work [6]. This guide describes the most common elements and objects and their corresponding attributes that can be found in the OSM project. The map features are mostly attributed with a key and value combination also referred to as \"tags\". The collected real world information in the database is represented by three data types. Nodes, which represent any point feature, Ways which represent lines such as roads and areas such as buildings, and Relations which contain information about how Nodes and Ways are related to each other. In recent years, the OSM project, data, and contributors have been the center of attention for many research disciplines. In 2009, early outcomes showed a density of OSM data in Germany with potential applicability for 3D location based services [7]. The results also showed some first indications of a correlation between improved data quality in areas with a higher population density. Similar results were conducted for London and entire England in 2008-2009, highlighting the decrease in data quality when moving away from bigger cities and that: \"more affluent areas and urban locations are better covered than deprived or rural locations\" [8,9]. In 2010, findings for Germany resembled the pattern of urban locations found in England [10]: \"If coverage is needed only in the densely populated urban areas of Germany, OpenStreetMap may already be an interesting and very cost-efficient-alternative\". A similar statement was made for England [11]: \"Most accurate tiles are located in major urban areas such as London, Liverpool, Manchester or Birmingham\". A different analysis for Germany [12] stated that: \"At the national level, the quality of OSM is highest regarding relative object completeness\" and \"quality differs locally, and even in a single town the different aspects of quality may vary\". A study conducted for France showed a heterogeneous OSM data pattern, which \"is particularly explained by the coexistence of different data sources, processes of capture, and contributors' profiles, highlightingthe importance of following accepted and well-defined specifications\" [13].