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= Objects - The basic building blocks of OpenStreetMap data and maps =

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

[[PageOutline(2-10,Table of Contents)]]

== Nodes, ways, and relations == #NodesWaysRelations

=== Tags (also an OSM data primitive) === #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'''.

[=#presets Often tags used in combinations; '''usually these combinations can be found] in [wiki:Presets 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 [osmwiki:Key:drinking_water drinking_water=yes] - it is present globally and can be applied to every object that provides water. Other example is [osmwiki:Key:leaf_cycle leaf_cycle]=* of trees, it will have less coverage globally (as trees do not grow just everywhere).

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

[#Tags1 We will cover tags in more detail below], but first will cover all other data primitives.

=== Nodes (also an OSM data primitive) === #Nodes
[[Image(source:trunk/images/data/node.png,20,bottom,margin-right=15,nolink)]] 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.

Only nodes have [[Coordinates]] in the OSM data model. Ways and relations simply refer to nodes.

==== Common nodes ====

Nodes can be re-used between multiple ways. Multiple ways can refer to a single node (common node).

A practical meaning of this is demonstrated at the [[Help/Action/Draw#Commonnodes Draw Nodes]] page. The [Help/Action/MergeNodes Merge Nodes] tool can be used to create common nodes.

=== Ways (also an OSM data primitive) === #Ways
[[Image(source:trunk/images/data/way.png,20,middle,margin-right=15,margin-bottom=5,nolink)]] A **way** is a sequence of nodes ([https://en.wikipedia.org/wiki/Polygonal_chain a "polygonal chain" from ordinary geometry]). 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).

==== Direction ====

Since nodes are ordered in OSM; every way in OSM have a direction. This gives us possibility to add tags (details about object) such as [osmwiki:Key:conveying conveying=*] - to specify direction of escalators and moving walkways or [osmwiki:Key:incline incline=*] which can be used toghter with roads and footways. There [osmwiki:Category:Way_Direction_Dependent more examples] and we can't cover them all.

==== Segment ====

Is '''a fragment between two consequent points of the way''' ([https://en.wikipedia.org/wiki/Line_segment "line segment" from ordinary geometry]). This is not a part of OSM data model, but users often distinguish because of tools than operate on segment level (rather than on complete way); please, distinguish a segment(s) from complete "way"s.

==== Areas ==== #ClosedWays
[[Image(source:trunk/images/data/closedway.png,20,middle,margin-right=15,margin-bottom=5,nolink)]] 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 [wiki:Help/Concepts/Object#typemultipolygon multi-polygon]

=== Relations (also an OSM data primitive) === #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).

This means '''you can [#Tags1 skip this big "Relations" section], until you decide to add or edit a relation'''.

==== Simple explanation ====
[[Image(source:trunk/images/data/relation.png,20,middle,margin-right=15,margin-bottom=5,nolink)]]  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 [http://wiki.openstreetmap.org/wiki/Types_of_relation 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.

[wiki:Help/Relations Help/Relations] covers the most basic tools to create relations in JOSM ([wiki:Help/Dialog/RelationList Relations window], [wiki:Help/Dialog/SelectionList Selection], [wiki:Help/Dialog/RelationEditor Relation editor])

==== Most popular types ====

===== type=multipolygon =====

A water object ({{{natural=water}}}), a building ({{{building=*}}}) or administrative boundary ({{{admin_level=*}}} tag) usually consist of the  multiple spatially separated parts belonging to the one object. More importantly, each of these parts can have exclusions ([https://en.wikipedia.org/wiki/Enclave_and_exclave enclaves/exclave] or simply islands or swamps in the water basins).

* Parts are created using "{{{outer}}}" role. ''Each multi-polygon should have at least one outer part''.
* Holes inside parts - using "{{{inner}}}" role. ''Each part can have from 0 to N holes''.
* Ways that form a ring where all ways are connected between each other using common nodes (JOSM will warn you about mistakes during validation)
* Rarely, holes can be nested. If nesting is more complex than ''single outer'' and ''single inner ring'', role swapping can continue: outer-inner-outer-inner-...
* Outer and inner rings shouldn't have common nodes (but validator will warn you about this and other mistakes)

Multi-polygon specific tools:
* [wiki:Help/Action/CreateMultipolygon create a multipolygon]
* [wiki:Help/Action/UpdateMultipolygon update existing multipolygon]

===== 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 == 
More precisely, a **tag** is a name/value pair assigned to an object. 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`. 
* `natural`=`tree` assigned to a node. This tag indicates that the node represents a tree 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.

In the [[Image(source:trunk/images/dialogs/propertiesdialog.svg,18,middle)]] [Dialog/TagsMembership Tags/Membership Dialog] you can assign tags to an object, remove tags from an object, or edit assigned tags.

== Technical properties ==

=== Object ids ===
The OSM server assigns each object a unique number, the **object ID** or **OSM ID**.

In JOSM, 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 **''[[Image(source:trunk/images/preference.png,20,middle)]] [Action/Preferences Preferences] (F12) -> [[Image(source:trunk/images/preferences/display.png,20,middle)]] [Preferences/Display Display Settings] -> Look and Feel''** it is possible to en- or disable the display of objects ids in the toggle dialogs.
* With **''File -> [[Image(source:trunk/images/downloadprimitive.png,20,bottom)]] [Action/DownloadObject Download object ...]''** it is possible to download **objects** by Ids.

=== Object versions and history ===

Version numbers (versions) were introduced to simplify wiki-style editing. Unfortunately, to track changes to ways/relations you need to use more complex tools than simple comparison by version number.

* Contents of the relation can change without change in version or id of the relation (a way can be changed, a node can be changed, a child relation can be changed).
* Contents of the way can change without change in version or id of the way (a node can be moved several times).

But you can always track complete history of one node based on history. 
Also, you can use time sliced data (region extracts by date) to inspect how objects were changing (and refer to object id and point in time from the database).

==== Information about latest version ====
* **''View -> [[Image(source:trunk/images/info.png,18,bottom)]] [Action/InfoAboutElements Advanced info]''** opens separated windows with informations about selected objects in text format.
* **''View -> [[Image(source:trunk/images/info.png,18,bottom)]] [Action/InfoAboutElementsWeb Advanced info (web)]''** opens the **objects' pages** of selected objects on the [osmwww: 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]
}}}

==== History of the object ====

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 -> [[Image(source:trunk/images/dialogs/history.png,18,bottom)]] [Action/ObjectHistory History]''** opens the History Browser. The History Browser will show you the **versions** of a particular object.
* Alternatively, **''View -> [[Image(source:trunk/images/dialogs/history.png,18,bottom)]] [Action/ObjectHistoryWeb History (web)]''** opens a page on the [osmwww: OSM website] in your web browser which displays the object history, too.

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