Semantic Models

Status

Normative

Purpose

SCRA Semantic Models define the canonical meanings of concepts used within the Smart City Reference Architecture (SCRA).

Each semantic model represents a distinct concept independently of any implementation technology.

Semantic models define what a concept means, not how it is represented, exchanged, stored, or exposed through an API.

Relationship to Other Specifications

The principal SCRA Schema specification layers are:

Architecture Decision Records
            |
            v
Semantic Modeling Principles
            |
            v
      Semantic Models
            |
            v
Semantic Mapping Specifications
            |
            v
NGSI-LD / JSON-LD / RDF / MCP

Architecture Decision Records explain why particular semantic decisions were adopted.

Semantic Modeling Principles define the rules governing the development of semantic models.

Semantic Models define what the canonical concepts mean and what constitutes their semantic identity.

Semantic Mapping Specifications define how source standards and implementation technologies correspond to those concepts.

Core Public Transport Models

The initial SCRA public transport semantic model includes:

The following additional concepts are established by Architecture Decision Records and will receive normative semantic model pages:

  • PublicTransportRoute
  • PublicTransportStop
  • TransportStation
  • JourneyPattern
  • PointInJourneyPattern

Additional models may be introduced as the specification expands into other smart-city domains.

Model Categories

Planning Concepts

Planning concepts describe recurring or long-term transport plans.

Current planning concepts include:

  • ServiceJourney
  • PublicTransportRoute
  • JourneyPattern
  • PublicTransportStop

Dated Operational Concepts

Dated operational concepts describe plans associated with a particular operating day.

Current dated operational concepts include:

  • DatedVehicleJourney

Monitored Operational Concepts

Monitored operational concepts describe the observed, predicted, or reported execution of transport operations.

Current monitored operational concepts include:

  • MonitoredVehicleJourney
  • PassingEvent

Physical Concepts

Physical concepts describe persistent real-world transport assets and locations.

Current physical concepts include:

  • PhysicalVehicle
  • TransportStation

Principal Relationships

The principal journey relationships are:

ServiceJourney
        |
        | has dated occurrence
        v
DatedVehicleJourney
        |
        | is monitored as
        v
MonitoredVehicleJourney
        |
        | is operated by
        v
PhysicalVehicle

A MonitoredVehicleJourney may have multiple operational events:

MonitoredVehicleJourney
        |
        | has passing event
        v
   PassingEvent
        |
        | occurs at
        v
PublicTransportStop

A ServiceJourney may be associated with a route and normally follows a journey pattern:

PublicTransportRoute
        ^
        |
   ServiceJourney
        |
        v
   JourneyPattern
        |
        v
PublicTransportStop

These relationships are conceptual and independent of any implementation technology.

Semantic Identity

Every normative semantic model shall contain an Identity section.

Semantic identity defines:

  • the conditions under which two instances represent the same concept;
  • the changes that do not alter that identity; and
  • the changes that result in a distinct semantic instance.

Semantic identity is independent of source identifiers, database keys, NGSI-LD Entity identifiers, URIs, and other implementation-specific identifiers.

Adoption of Existing Standards

SCRA Schema adopts existing semantic concepts whenever they accurately represent the intended meaning.

For public transport, Transmodel is the principal domain reference model.

Other relevant standards and vocabularies may include:

  • NeTEx
  • SIRI
  • NGSI-LD
  • Smart Data Models
  • Schema.org
  • SOSA / SSN
  • GTFS
  • GTFS-Realtime

SCRA follows this order:

  1. Adopt an existing concept.
  2. Harmonize compatible concepts.
  3. Extend an existing concept where necessary.
  4. Create a new concept only when existing standards are insufficient.

© SCRA Project