TagDoc:Tag Categorization

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TagDoc categorizes tags and keys in OpenStreetMap in the dimensions levels, types, geometry conventions, closed way default and geographic domains and if the way direction is significant.

Tag level

  • primary tag: A feature with this tag applied as the only tag has a well defined meaning.
  • secondary tag: A feature with this tag only has a well defined meaning when it is combined with other tags.

Tag type

  • discrete classification tag: The tag represents a discrete classification of the feature it is applied to. Such tags are documented on TagDoc with a separate page for each class.
  • quantitative tag: The tag value assigns a quantitative value to the feature. Such tags are documented with a key based page here.
  • individually characterizing tag: The tag value contains an individual identifier (usually unique to the feature on some level - though not necessarily on any larger geographic scale) - Like a name or a consecutive number. Such tags are documented with a key based page here.
  • for human reading: The tag value contains a text string intended for human reading to provide unstructured information about the feature. Like a note, description or comment. Such tags are documented with a key based page here.
  • technical tag: The tag meaning relates purely to the OpenStreetMap data model, not to the geography.
  • tag: The tag stores information related to the data and its production or provenance, not about the geography.
  • catch-all classification, key carries meaning: The tag carries a meaningful classification in the key independent of the value used. There are very few keys for which this applies, like building=* and shop=*.

Geometry conventions

There are different conventions in OpenStreetMap how to represent different real world geography elements with features in the OpenStreetMap database in consideration of the basic paradigms of local mapping and local verifiability and one feature, one OSM element. Which of these conventions applies usually depends on the primary tag used, sometimes also on secondary tags. With regards to the most common non-Point geometry types (lines/linear ways and polygons) these conventions are:

  • Tags with divisible geometry: Here the established convention is that a geometry with this tag can be split in any way necessary to characterize different parts of it in a differentiated fashion. Good examples are most roads (highway=*) which may be split to indicate certain properties differ between different sections of the road (like speed limits, surface material) or landcover mapping like with natural=wood, which can be split to map local differences in leaf_type=* or similar. Any other secondary tags would be identical (and therefore duplicate) between the components.
  • Tags with singular geometry: Here the convention is that a geometry with this tag may not be split and the mapped geographic object is to be represented with a single feature in OpenStreetMap. Iconic example are buildings (building=*) where a specific tagging exists (building:part=*) which can be used for locally characterizing specific parts of the building while the building itself remains a singular entity.
  • Tags with mixed geometry conventions: Primary tags where the geometry conventions depend on secondary tags as well as secondary tags where the geometry conventions depend on the primary tag. Most secondary tags fall under this. Example for a mixed convention primary tag is natural=water - which when used to map standing water (like in combination with water=lake) is by convention a singular tag but when used to map flowing water, i.e. riverbank polygons (with water=river) is a divisible tag.

Closed way default

In the OpenStreetMap data model the distinction if a closed way (first and last node identical) represents a linestring or a polygon is based on the presence of an area=* tag, or - in the absence of such - implicit defaults based on the tags of the feature. These defaults are subject to changing convention. TagDoc shows these conventions as they are implemented in popular tools and applications:

The possible defaults displayed are:

  • Polygon way.png - closed ways are by default interpreted as polygons.
  • Linear way.png - closed ways are by default interpreted as lines/linestrings.
  • Special way.png - closed ways are by default interpreted in a special way.

Way direction significance

For the interpretation of some tags the way direction has significance in OpenStreetMap while for others it does not. This is documented here. For example most basic road tags (highway=*) do not give significance to the way direction while the secondary tag oneway=yes does. This is a simple yes/no classification.

Geographic domains

TagDoc also classifies tags according to the geographic domain they provide information on. These domains can be overlapping, a tag can belong to several. Currently the following domains are being categorized:

  • Physical Geography - Tags that physically characterize the the geography independent of human activities.
  • Hydrography - Tags that characterize water related elements of the physical geography.
  • Topography - Tags that characterize the geometry of the earth land surface.
  • Glaciology - Tags that characterize permanent ice and glaciers.
  • Oceanography - Tags that characterize the ocean and maritime physical geography.