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Objects - The basic building blocks of OSM data and maps
JOSM helps you to enter data about real-world objects in the OSM geo-database. Despite the overhelming complexity of real-world objects you can map, JOSM provides only three kinds of objects as basic building blocks for maps: nodes, ways and relations.
You can assign tags to objects. It's the assigned set of tags which makes a node representing a restaurant different from a node representing a church, or a way representing a residential road different from a ways representing a river, or a relation represeting a bus route different from a relation representing a cycling route.
Nodes, ways, and relations
- a node is an individual point with a defined position.
- a way is a sequence of nodes. Despite its name a way doesn't represent ways in the real-world only. An OSM way is also used to respresent a river, the boundaries of a country, or even a building. If a way is closed, i.e. if the last node is the same as the first node, it is also called an area.
- a relation is an sequence of other objects, for instance a sequence of ways (representing road segments) and nodes (representing bus stops) which represent a bus route. Each object participating in a relation can be assigend a role, for instance stopfor a bus stop in the bus route, orforwardfor a road segmente which is part of the bus route from the start to the terminal station.
Tags
tbd
Object ids
The OSM server assigns each object a unique number, the object ID or OSM ID. A new object doesn't have an ID (it's ID is 0) until it is uploaded the first time to the OSM server.
- TODO: how to enable displaying of object ids
- TODO: download object you now the id of
- TODO: browse an object with a specific id on the server
Object versions
Each object has a version. The version is incremented whenever the object is uploaded to the server and the OSM server not only stores the most recent version of an object, but it's complete history.
- TODO: how to view the history in JOSM
- TODO: how to view the history on the OSM server
