README.adoc

Openstreetmap Semantics

Scope and goal of this document

This document proposes axiomatic semantics which describe the meaning, purpose, and representation of data in Openstreetmap (OSM). The proposal follows a hierarchy from the most general principles about what OSM does, to how in particular this is achieved at more distinct levels of detail. We are therein trying to strike a balance between the following goals:

• The semantics should be simple, intuitive and clean in order for users to be able to internalize them easily, such that mappers and software authors can effortlessly act and design in accordance.

• The semantics should respect the current consensus in the OSM community as much as possible in order to be adoptable by a larger group and reach acceptance quickly.

• The semantics should form a precise, consistent foundation, meaning they unambiguously define all aspects up to a certain level by a reasonable metric of generality, whence the scope of this document ends.

• The semantics should be growable, meaning that, as the fixation within scope of this document facilitates consensus outside of its scope, that consensus can eventually be manifested and incorporated into a new, larger scope.

This proposal does not attempt to change the conventional, community-driven nature of OSM nor to forcefully impose specific mapping techniques upon the community. It’s adoption is meant entirely through voluntary, constructive intent by everyone individually, as it is the accepted way in the OSM community.

This proposal does, however, differ from the also community-maintained OSM-Wiki in that it is deliberately designed and revised by a small group of authors to guarantee consistency and definiteness. It aims to provide a definite, stable, and reliable basis for a limited scope at the cost of potentially equally definite disagreement by some. We, the authors, hope that the reader, even where they may disagree, can see the appeal of such a normative basis and agree, that the latter’s benefits will outweigh potential shortcomings and oversights (which certainly exist).

Data Content

The scope of Openstreetmap (OSM) is primarily physical or geospatial, consensual information and identification.

• Primarily means that exceptions from the rule are permissable for reasons of practicality, but these exceptions should unanimously be considered undesireable and rectified as it is possible.

• Information are geospatial if they entirely are or relate to information which are formed entirely from coordinates.

• Information are physical if they entirely are or relate to information which describe a physical reality or to geospatial information.

• Data is consensual if it can be agreed upon within the audience of OSM.

• Identification refers to the information which is contained within a name or designation of an entity.

A natural feature such as a lake is a physical information. A building, its form, material and color is a physical information. A country border is a geospatial information. A name of a country is not a physical or geospatial information per-se, but a valid identification of the country’s border. A telephone number, website URL, opening times, the cuisine of a restaurant and similar data are not of the kind described above; they are stored in OSM for practical reasons alone. They should optimally be stored in a different database to which the entity’s identification provides a relational link.

Data representation

The geospatial extent of volumes, surfaces, curves or points (extents) and their nature is represented by primitives and a set of their associated properties. Primitives are vertices and their locations, and relations.

Relations can group sequences of vertices and other relations and, as such, allow to express complex hirarchies to represent more complex geospatial information and relationships.

Contexts and properties

The meaning and semantics of properties are only defined within contexts, meaning they can only be used on a primitive if the primitive provides that context. Initially, vertices provide a vertex context and relations provide a relation context. When properties, which pertain to any of these contexts, are added to the primitives, they can spawn new contexts or dismantle existing contexts.

A context is generally a global concept. It can transcend the primitive by whose property it was spawned and extend to any other primitive to which it has a defined relationship.

The formals of a property are written as

…​ in …​ [ spawns …​ [ for …​ ] ] [ dismantles …​ [ for …​ ] ]

where we omit the “for …​” part if its equal to the “in …​” part and thus applies to everywhere the original context was spawned.

A property is called physical if its existence within the given context carries any implications about the physical reality.

Any hypothetical property such cold=yes can be physical or not, depending on the context where it is used. By any reasonable (hypothetical) definition, is would be physical, if used on a primitive which describes a physical volume or surface (thus providing the according context). It would be unphysical, if used on a primitive which describes a two- or one-dimensional entity or even on an abstract relation.

Commutativity of contexts

Properties can dismantle any context within their parent chain of contexts and no ambiguity from the lack of ordering must arise. Similarly, no ambiguity must be created by defining the same property differently in different contexts, multiple of which can exist at the same time. Unambiguity is guaranteed if every property is valid in exactly one context within its parent chain.

Non-primitive, standardized geometric types

With the usual geometric interpretations, the following definitions are made for convenience:

• A relation of a sequence of only vertices is called a line. A line is degenerate iff all vertices have the same location. A line is proper iff every intersection between any pair of its segments occurs only on one of the segements' endpoints. A line is self-intersecting in the usual sense, but not without crossing over. A line is closed if the first vertex is added to the end of the sequence.

  a sequence of only vertices in relation spawns line

• A line with property area=yes is called an area. The geometric interpretation of an area follows from its closed line. An area is degenerate iff its closed line can be split into two congruent lines. An area is proper or self-intersecting in the sense of its closed line.

  area=yes in line spawns area

Compatibilty of properties and accumulation of contexts

A pair of properties is orthogonal to eachother, if adding both properties to a primitive describes the same data as duplicating the primitive and adding one of each property to each duplicate. That means that orthogonal properties can be added to spawn contexts without semantic interference.

An area can carry the properties parking=* and building=*, spawning contexts parking and building for the surface which is designated by the area. Unless we maintain that parking=* and building=* can’t be at the same place, a good definition for that situation would be a parking inside of a building. This is the same as using two congruent areas for parking and building.