Volunteered Geographic Information (VGI), is one of the characteristics of the Web 2.0. Every person having local spatial expertise can act as a "living sensor". If this local knowledge is collected on a regional or global level in a structured way a valuable and comprehensive information repository is established for a multitude of use cases.

The OpenStreetMap (OSM) project started  currently is one of the most successful VGI projects. Up to now approximately 1.8 billion GPS points have been uploaded by more than 250.000 registered users, converted to 700 Million nodes (points) and 56 Million ways (linear features) .

In contrary to the approach used in Geographic Information Systems (GIS) where geo-objects (features) are categorized into feature classes or layers, OSM is based on a surprisingly simple data model consisting of just four data types:  Nodes, representing points of interest (POIs) and acting as vertices, ways, and relations which aggregate nodes, ways and other relations to larger units. Relations are used for geo-features comprising several sub-features, similar to a feature collection in the Geographic Markup Language (GML), and are among others used to model turn restrictions for further routing. These structural elements are used to build polygons as well as topological relationships.

In order to create map features users can tag nodes, ways, and relations in a very flexible way by key value pairs, e.g. key=amenity, value=Parking or key=Highway, Value=Residentia. In addition they are uniquely identified by an identification number.

The tags are based on a consensus principle, i. e. there is no fixed catalogue of features classes which is defined by a custodian instance. In contrary, it is based on user agreement and is subject to some userdebates, modifications, and developments encompassing the evolvement of the whole project. For orientation a list of commonly used features is listed in the wiki (http://wiki.openstreetmap.org/wiki/Tag). This flexibility of course has some disadvantages but enables easily to capture new kind of features.

Typically data collection is based on using GPS devices. In addition it is possible to use licence free maps or records for data collections. In addition Yahoo!’s geo-imagery or freely available web map services can be used for digitization. According to different level of user experience and requirements different editors for data capture and update are available. For many purposes, the Flash-enabled browser-based editor Potlach can be used. The Merkaartor editor offers some more functionality which is still exceeded by the Potlach editor, at the expense of complexity.

All data are stored in a common geo-database. Through an application program interface it is possible to obtain the whole dataset (the so-called planet file), incremental updates, features for certain regions on the earth, and features with specific tags or other characteristics. Also a web-based graphical user interface for data export in different formats.

OpenStreetMap has its own data format, mirroring the structure of nodes, ways, relations, and tags in its own XML based markup language. Conversion programs support the transformation in other formats and the bulk loading into geo-databases. Some Geographical Information Systems already support import of OSM data.

Closely related to OpenStreetMap are some software packages supporting the rendering of the data. Thi visualization is based on style rules which can be adapted for many cases of cartographic needs. The generation and provision of tiles allows the integration in maping application, e. g. the browser based OpenLayers mapping solution.

Its data usage is  not restricted by any restrictive licence. This flexibility and freedom (see Creative-Commons-Attribution-ShareAlike-Licence (CC-BY-SA, http://creativecommons.org/­licenses/­by-sa/2.0/) encourage the usage of OpenStreetMap data and tools in many fields of application. One core theme is the application for humanitarian needs, e. g. natural hazards or man-made crisis situations.