Homonymous keys

Some keys serve a different purpose or have a different data type depending on the primary tag of the element. In other cases, a homonymous key may have different semantics when tagged on a node versus a way.

Causes

These conflicts have multiple causes, depending on the nature of the incompatibility. See the list below for details.

Disadvantages

An editor's developers may find it difficult to implement a field for a key that has a homonym associated with a different preset, especially if the homonymous keys have different data types or acceptable values. iD's fields suggest likely values based on taginfo lists of most common values by key. Since taginfo does not segment these lists by primary feature tag, irrelevant suggestions may appear.

If an element is tagged with multiple feature tags and each feature tag can potentially be used with a different homonym, a renderer or other data consumer may produce inconsistent, nondeterministic, or nonsensical results. For example, if a feature is simultaneously tagged building=yes and attraction=carousel, min_height=* may indicate either the height of the space beneath the carousel (if it sits on an elevated structure) or the minimum height required to ride the carousel. Indeed, every renderer that conforms to the Simple 3D buildings specification inaccurately depicts 161214224 161214224 floating above the ground. A renderer may need to ignore the tag when set to a typical child height, which may be neither desirable nor feasible.

As an extreme example, various uses of type=* had to be deprecated in favor of other keys because it was impossible to indicate the type of a reservoir etc. that is mapped as a multipolygon, which requires type=multipolygon.

Mitigation

Some tagging schemes have made use of namespaces, subkeys, or more descriptive keys to avoid key conflicts. For example, construction=* is now used as a namespace to quality most feature tags; its use as a key only continues with certain feature keys that are unlikely to be tagged on the same feature. aerodrome=* replaced type=* on aeroway=aerodrome features, making it possible to map an airport as a multipolygon relation. man_made=flagpole features are tagged flag:type=* and potentially tower:type=*.

Confusion may still arise despite namespacing. For example, building:condition=* indicates the building's state of (dis)repair, while parking:condition=* indicates the condition that must be satisfied to park.

Some mappers avoid tagging multiple primary feature tags on a single feature, instead mapping multiple features to represent the same physical object. However, this approach can run counter to the "One feature, one OSM element" principle.

List of homonymous keys

Due to the ever-changing nature of OSM (see any tags you like), this list will always be incomplete; however, please help document these namespace collisions whenever you encounter them.

Homonyms in English

These conflicts arise when "iterative refinement" tagging schemes use words that have homonyms in the English language:

• branch=*
  • when used with amenity=bank;restaurant;fuel, shop=* – the name of the store branch
  • when used with community=hutterite – the branch of Hutterites that occupies the place
  • when used with railway=* – the name of the branch line

• department=*
  • when used as a primary feature tag – to identify the feature as a section of a shop=department_store; see OpenLevelUp/Use cases/Shopping mall
  • when used with building=university;college – the name of the university department that occupies the building; see amenity=university

• line=*
  • when used with power=line – the type of assembly that connects power lines
  • when used with railway=* – the name of the railway line

• service=*
  • when used with highway=service – the type of traffic that uses the road
  • when used with railway=* – the track layout
  • when used with shop=car;car_repair – the service or services offered at the shop
  • when used with waterway=canal – the canal's function

Refining the feature versus something tangential to it

• admin_level=*
  • when used with boundary=* – the administrative level of the boundary itself
  • when used with government=* – the administrative level of the government unit that operates the feature

• brewery=* – whether the establishment is or has a brewery, or the name of the brewery that supplies beer to the establishment

• distance=*
  • when used with highway=milestone – the distance along the road denoted on the milestone
  • when used with route relations – the length of the route

• highway=*
  • when used as a primary feature tag – the road/path classification (e.g., highway=motorway), or a piece of road furniture closely associated with a road (e.g., highway=bus stop)

• maxspeed=* (foot=*, bicycle=*, oneway=*, zone:maxspeed=*, etc.)
  • when used with highway=* – speed limit when traveling along the roadway
  • when used with traffic_sign=* – speed limit denoted on the sign

• min_height=*
  • when used with attraction=* – minimum height required to ride the attraction (some mappers are using minimum_height_requirement=* to specify this information)
  • when used with building=* – height above ground level at which the building begins

• model=*
  • when used with shop=model – the variety of hobby models or kits sold (discussed but not documented)
  • when used with various primary feature tags (such as man made=street cabinet) – the model name or number of the feature itself

• school=*
  • when used with amenity=school – the type or level of instruction given at the school
  • when used with route=* – whether a route is intended to transport students (e.g., a school bus route)

Containment versus refinement

These conflicts often arise when a crude tagging scheme is established before a more granular micromapping technique is introduced later on:

• bench=*
  • when used with amenity=shelter, highway=bus_stop – whether a bench is available on-site (yes/no)
  • when used with amenity=bench – the type of bench (e.g. bench=wave_lounger)

• cemetery=*
  • when used as a primary feature tag – a grave or sector within the cemetery
  • when used with landuse=cemetery – the type of cemetery (e.g. cemetery=war_cemetery)

• cycleway=*
  • when used with highway=cycleway – the type of cycling infrastructure
  • when used with highway=primary, secondary, etc. – the type of cycling infrastructure that accompanies the main carriageway

• emergency=*
  • when used as a primary feature tag – the type of public safety feature
  • when used with barrier=* or highway=* – whether emergency vehicles are allowed
  • when used with amenity=hospital — whether the hospital offers emergency care

• footway=*
  • when used with highway=footway – the type of pedestrian infrastructure
  • when used with highway=primary, secondary, etc. – the type of pedestrian infrastructure that accompanies the main carriageway

• junction=*
  • when used as a primary feature tag – whether two or more ways form a junction at the node
  • when used with highway=* – the type of junction

• kerb=*
  • when used as a primary feature tag – the type of kerb
  • when used with crossing=* – whether there is a kerb at either end of the crossing (and optionally also what type of kerb it is)

• shop=*
  • when used as a primary feature tag – the type of shop
  • when used with amenity=fuel – whether a nearby shop=convenience is associated with the gas station (see shop=yes)

• swimming pool=*
  • when used with leisure=fitness_centre, tourism=hotel – whether a swimming pool is available on-site (yes/no)
  • when used with leisure=swimming pool – the type of swimming pool

Incompatible formats

These conflicts arise due to "iterative refinement" tagging schemes:

• construction=*
  • when used with building=construction, highway=construction, landuse=construction, railway=construction – the value is what will be moved to the primary key once construction is complete
  • when used with other primary tags as a lifecycle prefix – the entire primary tag is namespaced, e.g. construction:amenity=*

• direction=*
  • when used with highway=mini_roundabout – the direction of rotation
  • when used with e.g. highway=traffic_signals, highway=stop, highway=give_way, traffic_sign=* (as a node on a way) or cycleway=asl – the direction of traffic along the way to which the feature applies (forward/backward or both for highway=traffic_signals)
  • when used with route=road – the signposted cardinal direction of travel along the subrelation
  • when used with manoeuvre relations – a relative turning direction
  • when used with tourism=viewpoint – one or more arcs visible from the viewpoint (0-360 is the most common value)
  • when used with other primary tags – an absolute direction as a numeric value in degrees (or a cardinal value like N/E/S/W), usually the direction in which the object or its front is pointing, e.g. amenity=bench or traffic_sign=* (as a separate node not connected with a way)

• name=*
  • when used with route=train;subway;monorail route=tram;bus;trolleybus route=ferry – a strict format including the network, route number, origin, destination, and some ASCII art
    • This is causing major problems as some of this routes have actual names
  • when used with other primary tags – what the feature is called, not how it is described

• network=*
  • when used with amenity=atm, amenity=bicycle_rental – the name of an organization that coordinates operation in various locations
  • when used with route=road;bus;train etc. – an identifier for a coherent system of routes, typically based on the containing region's region code
  • when used with route=bicycle;hiking;mtb etc. – the route's scope and mode of transportation
  • when used with type=network – the route's scope and/or mode of transportation

• symbol=*
  • when used with route=hiking;horse etc. – a human-readable description of osmc:symbol=*
  • when used with route=road – often a URL to a typical route shield image

Other nuances

• bus=*
  • when used with amenity=bus_station, highway=bus_stop, public_transport=* – whether the facility is designed for buses
  • when used with highway=motorway;tertiary;service etc. – whether buses are allowed
  • when used with route=bus relations – the type of bus that serves the route

• capacity=*
  • when used with amenity=fuel;parking;taxi – the number of vehicles the facility can accommodate at a time
  • when used with amenity=kindergarten;school – the number of students the facility can enroll at a time
  • when used with leisure=stadium, amenity=shelter, room=* – the maximum (standing or seating) occupancy
  • when used with tourism=camp_site, but not tourism=hotel – the number of visitors the facility can accommodate at a time
  • when used with e.g. man made=storage_tank or man made=grit_bin (and some others) – volume measurement

• destination=*
  • when used with highway=motorway_link;trunk_link;primary_link etc. – the signposted names of the places to which the road leads
  • when used with waterway=* – the name of the body of water into which the waterway flows, not necessarily signposted

• maxweight=*
  • when used with highway=motorway;residential;cycleway etc. – legal access restriction by vehicle weight
  • when used with highway=elevator, aerialway=zip_line, attraction=roller_coaster – access restriction by passenger weight

• type=*
  • when used on a relation – the technical type of relation
  • when used with various primary feature tags – the type of aerodrome, reservoir, tree, etc.

Homonymous roles

Some relation types also document roles inconsistently, with similar consequences for editors and data consumers:

• via
  • when used with type=restriction;connectivity;turnlanes:turns, type=destinationsign – the intersection node or internal intersection way at the decision point
  • when used with type=destination_sign – the internal intersection way at the decision point, but not the intersection node

See also

• Synonymous tags
• Counterintuitive keys and values
• Skunked tags