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 representing a bus route different from a relation representing a cycling route.

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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 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

A tag is a name/value pair assigend to an object. Tags are used to describe an object in detail. If we write foo=bar we mean The tag with key foo is assigned the value bar. Here are two examples:

• name=Main road assigned to a way. This tag indicates that the name of the way is Main road.
• amenity=restaurant assigned to a node. This tag indicates that the node represents a restaurant in the real-world.
• route=bus assigned to a relation. This tag indicates that the relation represents a bus route.

Tags can be combined. An object can carry as many tags as necessary. Here's a more complex example:

highway=living_street
name=Main road
name:de=Hauptstrasse
maxspeed=30
hgv=no
oneway=yes

Assigned to a way, this set of tags indicate, that the way represents a living street with name Main road (the german name be Hauptstrasse). Speed is limited to 30km/h and heavy goods vehicles (hgv) are not allowed. Furthermore, it's a one way street.

You are free to assign an object whatever tag you feel necessary. However, there is a list of so called ​Map Features, i.e. a canonical list of tags with a well-defined meaning which are often used by mappers. Whenever possible you are adviced to stick to these tags because map renderers, routing engines, and other pieces of software rely on them.

In the Properties/Memberships Dialog you can assign tags to an object, remove tags from an object, or edit assigned tags.

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.

In the History Dialog you can load the history of an object and launch the History Browser. The History Browser will show you the versions of a particular object.

Alternatively, Tools->Object history redirect you to a page on the ​OSM website which displays the object history too.

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