The Building Knowledge Graph

RealEstateCore (REC) is an ontology, which can most easily be thought of as a standard and interoperable schema for building knowledge graphs; it defines the types (classes) of nodes you can have in your graph, the types of relations (object properties) these nodes have with one another, and the types of data values (data properties) associated with them. By using REC, you get two distinct advantages:

  1. You do not need to spend a lot of time and resources on designing your own data model; we’ve already done it for you.
  2. Your data is automatically compatible with that of other REC users (e.g., your suppliers or your customers), greatly simplifying data and process integration (doubly so if you also utilize the REC APIs to exchange data).

RealEstateCore is used with RDF-based knowledge graphs. There is a wealth of information about RDF available on the net, which we will not duplicate here; suffice to say that in RDF, a knowledge graph is made up of a (potentially very large) set of statements, each of which consist of three components: the subject of the statement, the predicate that captures what the statement intends to say about the subject , and the object that carries the value of that assertion. E.g., a graph might include the statement WhiteHouse ownedBy USGovernment, or IBM hasNumberOfEmployees 345900. Due to their structure, a statement is often known as a triple.

In the below we introduce some key REC classes and exemplify their use with RDF triples.

Modularity

RealEstateCore consists of a set of individual ontology modules, tied together by a shared core. The intent is that users be able to utilize only those modules that their particular use case requires.

The REC module nanespaces are organized using the following pattern https://w3id.org/rec/{MODULE}/, with entities being minted using slashes as module-entity delimiter, e.g., https://w3id.org/rec/building/Balcony. To differentiate beteween versions we utilize version IRIs; a specific version of a specific module can be resolved as https://w3id.org/rec/{MODULE}/{VERSION}/.

We also provide a composed ontology consisting of all modules (RealEstateCore Full), as we’ve found many users prefer to include everything by default. REC Full is available at https://w3id.org/rec/full/.

Note that we use HTTP content negotation to automatically return an appropriate representation of the above URIs in accordance with what the client requests. E.g., if you are using a web browser to access those links, you will get the HTML-based ontology documentation; but if you load them in a tool that requests RDF (e.g., Protégé or TopBraid Composer, you will get that ontology representation instead.

Key Classes

RealEstateCore ontology overview

The figure above (borrowed from the DTDL port of REC) illustrates a subset of the key classes in RealEstateCore, which are described in greater detail below.

RealEstateCore employs a set of design principles shared across these classes:

  • Parthood is described using the core:hasPart and core:isPartOf object properties. Restrictions are defined on top-level classes such that instances of these classes can only have other instances of the same class as parts; e.g., assets can only have assets as parts, spaces can only have spaces, and so forth. This provides a simple a consistent way for developers to navigate the building topology. To jump across these topologies (e.g., to indicate that an asset has a spatial location, other specific properties are used, see below.)
  • Spatial location is described using core:hasLocation and core:isLocationOf object properties together with some core:Space instance.
  • Administrative parthood, i.e., membership in a core:Collection, is described using the object properties core:includedIn and core:includes.
  • To indicate that a capability or asset has a coverage or impact area (e.g., a camera covering a lobby, an HVAC unit covering an apartment), or other functional relation to some REC entity, we utilize the core:serves and core:servedBy object properties. These properties are at present comparatively ambiguous.

For exhaustive documentation, see the PyLODE docs.

Spaces

A core:Space is contiguous part of the physical world that has a 3D spatial extent and that contains or can contain sub-spaces. For example a Region can contain many pieces of Land, which in turn can contain many Buildings, which in turn can contain Levels and Rooms. This concept is comparable to a Zone in the BOT ontology.

:JU-E-Building      rdf:type        core:Building
:JU-E-Building-L1   rdf:type        core:Level
:E2412              rdf:type        core:Room
:E2412              core:isPartOf   :JU-E-Building-L1
:JU-E-Building-L1   core:isPartOf   :JU-E-Building

Space example as a graph

Building components

A core:BuildingComponentis a part that constitutes a piece of a building’s structural makeup, for example Facade, Wall, Slab, RoofInner, etc.

Assets

A core:Asset is an object which is placed inside of a building, but is not an integral part of that building’s structure. We provide a substantial hierarchy of assests, for example architectural, furniture, equipment, systems, etc.

:AHU1               rdf:type        asset:AirHandlingUnit
:MaintenanceRoom1   rdf:type        core:Room
:MainLobby          rdf:type        core:Room
:AHU1               core:locatedIn  :MaintenanceRoom1
:AHU1               core:serves     :MainLobby

Asset example as a graph

Logical device

core:LogicalDevice: A physical or logical object defined as an electronic equipment or software that communicates and interacts with a digital twin platform. A logical device could be an integrated circuit inside of a smart HVAC unit, or a virtual server running on a Kubernetes cluster. Logical devices can have Capability instances (through hasCapability) that describe their input/output capabilities. If Logical Devices are embedded within Asset entities (through the hostedBy property) such capabilities typically denote the capabilities of the asset.

Capabilities, sensing and actuating

A core:Capability indicates the capacity of a entity, be it a Space, an Asset, or a LogicalDevice, to produce or ingest data. This is equivalent to the term “point” in Brick Schema and generic Building Management System. Specific subclasses specialize this behavior: core:Sensor entities harvest data from the real world, core:Actuator entities accept commands from a digital twin platform, and core:Parameter entities configure some capability or system. Capabilities are connected to spaces or assets using the core:isCapabilityOfand core:hasCapability object properties.

The core:Event class includes subclasses that can be utilized to represent individual observations or actuations in a REC-based system, e.g., core:Observation or actuation:ActuationRequest. For a further discussion on RealEstateCore-based actuation, see our tutorial on the subject.

:TemperatureSensor1     rdf:type                        device:TemperatureSensor
:E2414                  rdf:type                        core:Room
:TemperatureSensor1     core:isCapabilityOf             :E2414
:TemperatureSensor1     qudt:hasQuantityKind            quantitykind:Temperature
:TemperatureSensor1     qudt:unit                       unit:DEG_C
:Obs9442234442          rdf:type                        core:Observation
:Obs9442234442          core:observationGeneratedBy     :TemperatureSensor1
:Obs9442234442          core:value                      "19.8"

Capability example as a graph

Collections

core:Collection covers administrative groupings of entities that are adressed and treated as a unit for some purpose. These entities may have some spatial arrangement (e.g., an apartment is typically contiguous) but that is not a requirement (see, e.g., a distributed campus consisting of spatially disjoint plots or buildings).

Agents, persons, and organizations

core:Agent describes any basic types of stakeholder that can have roles or perform activities, and is subclassed into, e.g., people, companies, departments. This bears some resemblance to FOAF, which it was originally inspired by, but has since diverged due to requirements shift.

Measurement units and data values

We heavily reuse QUDT.org for observable property quantity kinds ( Temperature, Pressure, Angle, etc.) and for measurement units. In order to reduce complexity and dependency on remote IRIs we have opted to clone their design into REC as-is, maintaining the original namespaces.

Ontology Documentation

For every REC release, we auto-generate documentation directly from the ontology and its annotations; see the documentation site, where each version is listed, at https://doc.realestatecore.io.

For an overview of RealEstateCore (or other ontologies) we recommend the use of WebVOWL. A version of WebVOWL is embedded in the autogenerated documentation.