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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 ==

* [[Image(source:/trunk/images/data/node.png)]] - a  '''node'''  is an individual point with a defined position.

* [[Image(source:/trunk/images/data/way.png)]] - 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'''.

* [[Image(source:/trunk/images/data/relation.png)]] - 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 `stop` for a bus stop in the bus route, or `forward` for 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

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