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 overwhelming 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 Areasthe 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

Please don't hurry reading this, you can consult this page again at any second.

Tags

Tags provide meaning to the objects. Without them, any other object would be useless and meaningless. Each of three other data primitives can have multiple tags.

Often tags used in combinations; usually these combinations can be found in presets. But it may be true that tags you need to use are not in presets and you need to know them first.

Simplest example is ​drinking_water=yes - it is present globally and can be applied to every object that provides water. Other example is ​leaf_cycle=* of trees, it will have less coverage globally (as trees do not grow just everywhere).

From time to time you will encounter tags that found somewhere else but not in your locality; it is good idea to check wiki what is tag about, use: ​Essential links tagging as starting point.

We will cover tags in more detail below, but first will cover all other data primitives.

Nodes

A node is an individual point with a defined position. With respective tags it can be used to indicate shops, benches, individual trees or points of drinking water.

Ways

A way is a sequence of nodes. OSM way used to represent linear objects, with respective tags it can used to describe: a river bank, tree row, kerbs in the streets (among other uses).

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. Old term for this is Closed Ways (it is still present at wiki, used in software and by some mappers; other part uses "geometry" when talking just about shape of the objects, without tags).

Sometimes single area is not enough to represent an object (for example, a lake with an island in it), then you have to use a multi-polygon

Relations

In most cases you don't need them

If some object can be represented with single node or way and multiple tags supported by software, then there is no need in relations. But there are some rare cases where it is theoretically and practically impossible only with nodes, ways and tags (ex. turn restrictions, will be covered below).

Simple explanation

A relation:

• is a sequence of other objects (nodes, ways and relations)
• as other objects (node, way) - every relation can have their own "tags"
• can be used as members in other relations (nested or hierarchical structures)
• tag type=* used to define one of ​many types of relations

Additionally, each object participating in a relation can be assigned a role, for instance "via" node and "from", "to" ways in type=restriction relation.

Most popular types

type=multipolygon

A water object (natural=water), a building (building=*) or administrative boundary (admin_level tag).

(stub)

type=route

Used to describe bus routes and train routes along other transportation services. (stub)

type=restriction

Can used to describe turn restriction according to signs and road marking. JOSM has a plug-in to support this type of relations: ​http://wiki.openstreetmap.org/wiki/JOSM/Plugins/Turnrestrictions

​http://wiki.openstreetmap.org/wiki/Relation:restriction#Examples

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=residential
name=Main road
name:en=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 road in a residential area with name Main road (the German name be Hauptstraße, the English name, which in this example equals to name, is Main road). 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 advised 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:

https://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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