Homonymous keys

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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 often arise when "iterative refinement" tagging schemes use words that have homonyms in the English language.

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 way 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


Homonyms in English

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
  • distance=*
  • 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)
  • min_height=*
    • when used with attraction=* – minimum height required to ride the attraction
    • when used with building=* – height above ground level at which the building begins
  • school=*
    • when used with amenity=school – the kind 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

Differing formats

Other nuances

Homonymous roles

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

See also