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We use term Elements when we talk about data model behind OpenStreetMap, low-level abstractions of the physical world. They consist of:
- nodes - defining points in space (Where it is?)
- ways - defining linear features and how nodes connected with each other (How does it connected with other objects?)
- relations - which are sometimes used to explain how other elements work together (How does this public transport circulate? How does turn restriction works here? and several others)
All of the above can have one of more associated tags (which describe the meaning of a particular element).
Main article: Node/Beginners
- represents a specific point on the earth's surface defined by its latitude and longitude. Each node comprises at least an id number and a pair of coordinates.
- can be used to define standalone point features
- are also used to define the shape of a way
- can be included as member of relation. The relation also may indicate the member's role: that is, the node's function in this particular set of related data elements.
- with amenity=bench represent bench in park from physical world
- with natural=tree may represent single tree
But remember: this is only our abstraction of real world, bench (and also tree) has non-zero height, width and depth. This simplification proven to be useful for humans.
Main article: Way/Beginners
A way is an ordered list of between 2 and 2,000 nodes. Ways are used to represent linear features such as rivers and roads.
Ways can also represent the boundaries of areas such as buildings or forests.
Areas with holes, or with boundaries of more than 2,000 nodes, cannot be represented by a single way. Instead, the feature will require a more complex multipolygon relation data structure.
Main article: Relation/Beginners
A relation is a multi-purpose data structure that documents a relationship between two or more data elements (nodes, ways, and/or other relations). Examples include:
- A route relation, which lists the ways that form a major (numbered) highway, a cycle route, or a bus route.
- A turn restriction that says you can't turn from one way into another way.
- A multipolygon that describes an area (whose boundary is the 'outer way') with holes (the 'inner ways').
Thus, relations can have different meanings. The relation's meaning is defined by its tags. Typically, the relation will have a 'type' tag. The relation's other tags need to be interpreted in light of the type tag.
The relation is primarily an ordered list of nodes, ways, or other relations. These objects are known as the relation's members.
Each element can optionally have a role within the relation. For example, a turn restriction would have members with "from" and "to" roles, describing the particular turn that is forbidden.
A single element such as a particular way may appear multiple times in a relation.
Main article: Tag/Beginners
All types of data element (nodes, ways and relations) can have tags. Tags describe the meaning of the particular element to which they are attached.
A tag consists of two free format text fields; a 'key' and a 'value'. Each of these are Unicode strings of up to 255 characters. For example, highway=residential defines the way as a road whose main function is to give access to people's homes.
There is no fixed dictionary of tags, but there are many conventions documented on this wiki (starting with the Map Features page). Tag usage can be measured with the Taginfo application. If there is more than one way to tag a given feature, it's probably best to use the most common approach.