Th e OSM contributors provide two diff erent data types: (1) two-dimensional geometries and (2) additional (semantic or metric) information. For the creation of real geometries, so-called ‘nodes’can be utilised. Each node describes a distinct geotagged point (with latitude and longitude). For the creation of linear or polygonal geometries, several nodes can be combined to ‘ways’, whereby a closed way (i.e. fi rst node equals last node) represents a polygon and
a non-closed way represents a line. Complex polygons with holes or relationships between OSM features can be mapped with ‘relations’. Enabling the community to provide an arbitrary amount of additional information, OSM applies a concept of freely defi nable key-value pairs.
Th ereby, a key (describing an information domain) with a distinct value (refi ning this information) is attached to the corresponding OSM feature. For example, a closed way with the key ‘building’ and the value ‘yes’ represents a building footprint in OSM. Th e amount of key-value pairs is unlimited.