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Objects - The basic building blocks of OSM data and maps

JOSM ti aiuta ad inserire dati di oggetti reali nel geo-database di OSM. 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.

Nodes, ways, and relations

Nodes

A node is an individual point with a defined position.

Ways

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 represent a river, the boundaries of a country, or even a building.

Closed Ways (Areas)

If a way is closed, i.e. if the last node is the same as the first node, it is also called an area.

Relations

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 assigned a role, for instance stop for a bus stop in the bus route, or forward for a road segment which is part of the bus route and which is only used in forward direction.

Tags

A tag is a name/value pair assigned 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 some 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 Tags/Membership 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.

• Under Preferences -> Display Settings -> Look and Feel it is possible to en- or disable the display of objects ids in the toggle dialogs.
• With File -> Download object ... it is possible to download objects by Ids.
• View -> Advanced info opens separated windows with informations about selected objects in text format.
• View -> Advanced info (web) opens the objects' pages of selected objects on the ​OSM website in your web browser.

Alternatively, you can browse the objects directly by entering following address in your web browser:

http://www.openstreetmap.org/browse/[node/way/relation]/[ID]

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.

• View -> History opens the History Browser. The History Browser will show you the versions of a particular object.
• Alternatively, View -> History (web) opens a page on the ​OSM website in your web browser which displays the object history, too.

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