Open Geospatial Consortium

Submission Date: 2021-07-15

Approval Date:   2021-07-15

Publication Date:   2021-09-13

External identifier of this OGC® document: http://www.opengis.net/doc/UG/CityGML-user-guide/3.0

Internal reference number of this OGC® document:    20-066

Version: 1.0

Category: OGC® User Guide

Editor:   Charles Heazel

OGC City Geography Markup Language (CityGML) 3.0 Conceptual Model Users Guide

Copyright notice

Copyright © 2021 Open Geospatial Consortium

To obtain additional rights of use, visit http://www.opengeospatial.org/legal/

Warning

This document is not an OGC Standard. This document provides guidance on the use of the OGC CityGML: 3.0 Conceptual Model Standard. This document is a non-normative resource and not an official position of the OGC membership. It is subject to change without notice and may not be referred to as an OGC Standard. Further, User Guides should not be referenced as required or mandatory technology in procurements.

Document type:    OGC® User Guide

Document subtype:   

Document stage:    Approved

Document language:  English

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Table of Contents

i. Abstract

CityGML is an open conceptual data model for the storage and exchange of virtual 3D city models. It is defined through a Unified Modeling Language (UML) object model. This UML model extends the ISO Technical Committee 211 (TC211) conceptual model standards for spatial and temporal data. Building on the ISO foundation assures that the man-made features described in the City Models share the same spatial-temporal universe as the surrounding countryside within which they reside. The aim of the development of CityGML is to reach a common definition of the basic entities, attributes, and relations of a 3D city model. This is especially important with respect to the cost-effective sustainable maintenance of 3D city models, allowing the reuse of the same data in different application fields.

This Users Guide provides extended explanations and examples for the individual concepts that are defined in the CityGML 3.0 Conceptual Model Standard. Both documents, the Conceptual Model Standard and the Users Guide, are mutually linked to facilitate navigation between corresponding sections in these documents.

ii. Keywords

The following are keywords to be used by search engines and document catalogues.

ogcdoc, OGC document, CityGML, 3D city models

iii. Preface

Attention is drawn to the possibility that some of the elements of this document may be the subject of patent rights. The Open Geospatial Consortium shall not be held responsible for identifying any or all such patent rights.

Recipients of this document are requested to submit, with their comments, notification of any relevant patent claims or other intellectual property rights of which they may be aware that might be infringed by any implementation of the standard set forth in this document, and to provide supporting documentation.

iv. Submitting organizations

The following organizations submitted this Document to the Open Geospatial Consortium (OGC):

  • Heazeltech LLC

v. Submitters

All questions regarding this submission should be directed to the editor or the submitters:

Table 1. Submission Contact Points
Name Institution

Charles (Chuck) Heazel

HeazelTech LLC

vi. Contributors

The following individuals contributed content to the CityGML 3.0 Users Guide:

Table 2. Participants in the Users Guide development
Name Institution

Emmanuel Devys

Institut national de l’information géographique et forestière (IGN), France

Charles (Chuck) Heazel

HeazelTech LLC

Tatjana Kutzner

Chair of Geoinformatics, Technical University of Munich, Germany

1. Introduction

An increasing number of cities and companies are building virtual 3D city models for different application areas like urban planning, mobile telecommunication, disaster management, 3D cadastre, tourism, vehicle and pedestrian navigation, facility management and environmental simulations. Furthermore, in the implementation of the European Environmental Noise Directive (END, 2002/49/EC) 3D geoinformation and 3D city models play an important role.

In recent years, most virtual 3D city models have been defined as purely graphical or geometrical models, neglecting the semantic and topological aspects. Thus, these models could almost only be used for visualization purposes but not for thematic queries, analysis tasks, or spatial data mining. Since the limited reusability of models inhibits the broader use of 3D city models, a more general modeling approach had to be taken in order to satisfy the information needs of the various application fields.

CityGML is a common semantic information model for the representation of 3D urban objects that can be shared over different applications. The latter capability is especially important with respect to the cost-effective sustain-able maintenance of 3D city models, allowing the possibility of selling the same data to customers from different application fields. The targeted application areas explicitly include city planning, architectural design, tourist and leisure activities, environmental simulation, mobile telecommunication, disaster management, homeland security, real estate management, vehicle and pedestrian navigation, and training simulators.

CityGML is an open conceptual data model for the storage and exchange of virtual 3D city models. It is defined through a Unified Modeling Language (UML) object model. This UML model extends the ISO Technical Committee 211 (TC211) conceptual model standards for spatial and temporal data. Building on the ISO foundation assures that the man-made features described in the City Models share the same spatial-temporal universe as the surrounding countryside within which they reside.

CityGML defines the classes and relations for the most relevant topographic objects in cities and regional models with respect to their geometrical, topological, semantical, and appearance properties. “City” is broadly defined to comprise not just built structures, but also elevation, vegetation, water bodies, “city furniture”, and more. Included are generalization hierarchies between thematic classes, aggregations, relations between objects, and spatial properties. CityGML is applicable for large areas and small regions and can represent the terrain and 3D objects in different levels of detail simultaneously. Since either simple, single scale models without topology and few semantics or very complex multi-scale models with full topology and fine-grained semantical differentiations can be represented, CityGML enables lossless information exchange between different GI systems and users.

The CityGML 3.0 standard consists of several parts: 1) The CityGML 3.0 Conceptual Model standard that defines the conceptual model in UML and that is described in more detail within this Users Guide. 2) A separate Encoding standard for each Encoding to be defined. This will be the GML Encoding in the beginning, further encoding specifications (e.g., relational database schema, JSON-based representation) will follow in the future.

2. How To Use This Resource

The Users Guide to the CityGML 3.0 Conceptual Model Standard is not intended to be read from start to finish. Rather, it is a resource structured to provide quick answers to questions which an implementer may have about the CityGML 3.0 Standard.

The CityGML 3.0 Standard includes hyperlinks which can be used to navigate directly to relevant sections of the Users Guide.

Some content in the Users Guide has been copied from the CityGML 3.0 Conceptual Model Standard to make the content more accessible to the user. In order to make clear which content in the Users Guide has been copied, the copied text is provided within grey boxes.

This text has been copied from the CityGML 3.0 Conceptual Model Standard.

All other texts are provided exclusively in this Users Guide.

3. Scope

This document provides Engineering Guidance on the use of the CityGML 3.0 Conceptual Model Standard.

The OGC Conceptual Model Standard specifies the representation of virtual 3D city and landscape models. The CityGML 3.0 Conceptual Model is expected to be the basis for a number of future Encoding Standards in which subsets of the Conceptual Model can be implemented. These Encoding Standards will enable both storage and exchange of data.

The CityGML 3.0 Conceptual Model Standard was designed to be concise and easy to use. As a result, most non-normative content has been removed. The purpose of this Users Guide is to capture that non-normative content and make it easy to access if and when needed.

4. References

The following documents contain provisions that, through reference in this text, constitute provisions of this Users Guide. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. For undated references, the latest edition of the document referred to applies.

  • IETF: RFC 2045 & 2046, Multipurpose Internet Mail Extensions (MIME). (November 1996),

  • IETF: RFC 3986, Uniform Resource Identifier (URI): Generic Syntax. (January 2005)

  • INSPIRE: D2.8.III.2 Data Specification on Buildings – Technical Guidelines. European Commission Joint Research Centre.

  • ISO: ISO 19101-1:2014, Geographic information - Reference model - Part 1: Fundamentals

  • ISO: ISO 19103:2015, Geographic Information – Conceptual Schema Language

  • ISO: ISO 19105:2000, Geographic information – Conformance and testing

  • ISO: ISO 19107:2003, Geographic Information – Spatial Schema

  • ISO: ISO 19108:2002/Cor 1:2006, Geographic information – Temporal schema — Technical Corrigendum 1

  • ISO: ISO 19109:2015, Geographic Information – Rules for Application Schemas

  • ISO: ISO 19111:2019, Geographic information – Referencing by coordinates

  • ISO: ISO 19123:2005, Geographic information — Schema for coverage geometry and functions

  • ISO: ISO 19156:2011, Geographic information – Observations and measurements

  • ISO: ISO/IEC 19505-2:2012, Information technology — Object Management Group Unified Modeling Language (OMG UML) — Part 2: Superstructure

  • ISO/IEC 19507:2012, Information technology — Object Management Group Object Constraint Language (OCL)

  • ISO: ISO/IEC 19775-1:2013 Information technology — Computer graphics, image processing and environmental data representation — Extensible 3D (X3D) — Part 1: Architecture and base components

  • Khronos Group Inc.: COLLADA – Digital Asset Schema Release 1.5.0

  • OASIS: Customer Information Quality Specifications - extensible Address Language (xAL), Version v3.0

  • OGC: The OpenGIS® Abstract Specification Topic 5: Features, OGC document 08-126

  • OGC: The OpenGIS™ Abstract Specification Topic 8: Relationships Between Features, OGC document 99-108r2

  • OGC: The OpenGIS™ Abstract Specification Topic 10: Feature Collections, OGC document 99-110

5. Terms and Definitions

For the purposes of this document, the following additional terms and definitions apply.

2D data
geometry of features is represented in a two-dimensional space
NOTE In other words, the geometry of 2D data is given using (X,Y) coordinates.
[INSPIRE D2.8.III.2, definition 1]

2.5D data
geometry of features is represented in a three-dimensional space with the constraint that, for each (X,Y) position, there is only one Z
[INSPIRE D2.8.III.2, definition 2]

3D data
Geometry of features is represented in a three-dimensional space.
NOTE In other words, the geometry of 2D data is given using (X,Y,Z) coordinates without any constraints.
[INSPIRE D2.8.III.2, definition 3]

application schema
A set of conceptual schema for data required by one or more applications. An application schema contains selected parts of the base schemas presented in the ORM Information Viewpoint. Designers of application schemas may extend or restrict the types defined in the base schemas to define appropriate types for an application domain. Application schemas are information models for a specific information community.
OGC Definitions Register at http://www.opengis.net/def/glossary/term/ApplicationSchema

codelist
A value domain including a code for each permissible value.

conceptual model
model that defines concepts of a universe of discourse
[ISO 19101-1:2014, 4.1.5]

conceptual schema

  1. formal description of a conceptual model
    [ISO 19101-1:2014, 4.1.6]

  2. base schema. Formal description of the model of any geospatial information. Application schemas are built from conceptual schemas.
    OGC Definitions Register at http://www.opengis.net/def/glossary/term/ConceptualSchema

Implementation Specification
Specified on the OGC Document Types Register at http://www.opengis.net/def/doc-type/is

levels of detail
quantity of information that portrays the real world
NOTE The concept comprises data capturing rules of spatial object types, the accuracy and the types of geometries, and other aspects of a data specification. In particular, it is related to the notions of scale and resolution.
[INSPIRE Glossary]

life-cycle information
set of properties of a spatial object that describe the temporal characteristics of a version of a spatial object or the changes between versions
[INSPIRE Glossary]

Platform (Model Driven Architecture)
the set of resources on which a system is realized.
[Object Management Group, Model Driven Architecture Guide rev. 2.0]

Platform Independent Model
a model that is independent of a spacific platform
[Object Management Group, Model Driven Architecture Guide rev. 2.0]

Platform Specific Model
a model of a system that is defined in terms of a specific platform
[Object Management Group, Model Driven Architecture Guide rev. 2.0]

Universally Unique Identifier
A 128-bit value generated in accordance with this Recommendation | International Standard, or in accordance with some historical specifications, and providing unique values between systems and over time. [ISO/IEC 9834-8:2014, Rec. ITU-T X.667 (10/2012)]

universe of discourse
view of the real or hypothetical world that includes everything of interest
[ISO 19101-1:2014, definition 4.1.38]

6. Conventions

6.1. Identifiers

The normative provisions in this document are denoted by the URI
All requirements and conformance tests that appear in this document are denoted by partial URIs relative to this base.

6.2. UML Notation

The CityGML Conceptual Model (CM) Standard is presented in this document through diagrams using the Unified Modeling Language (UML) static structure diagram (see Booch et al. 1997). The UML notations used in this standard are described in the diagram in <<figure-1>>.
UML Notation
Figure 1. UML notation (see ISO TS 19103, Geographic information - Conceptual schema language).
All associations between model elements in the CityGML Conceptual Model are uni-directional. Thus, associations in the model are navigable in only one direction. The direction of navigation is depicted by an arrowhead. In general, the context an element takes within the association is indicated by its role. The role is displayed near the target of the association. If the graphical representation is ambiguous though, the position of the role has to be drawn to the element the association points to.
The following stereotypes are used in this model:
  • «ApplicationSchema» denotes a conceptual schema for data required by one or more applications. In the CityGML Conceptual Model, every module is defined as a separate application schema to allow for modularization.

  • «FeatureType» represents features that are similar and exhibit common characteristics. Features are abstractions of real-world phenomena and have an identity.

  • «TopLevelFeatureType» denotes features that represent the main components of the conceptual model. Top-level features may be further semantically and spatially decomposed and substructured into parts.

  • «Type» denotes classes that are not directly instantiable, but are used as an abstract collection of operation, attribute and relation signatures. The stereotype is used in the CityGML Conceptual Model only for classes that are imported from the ISO standards 19107, 19109, 19111, and 19123.

  • «ObjectType» represents objects that have an identity, but are not features.

  • «DataType» defines a set of properties that lack identity. A data type is a classifier with no operations, whose primary purpose is to hold information.

  • «Enumeration» enumerates the valid attribute values in a fixed list of named literal values. Enumerations are specified in the CityGML Conceptual Model.

  • «BasicType» defines a basic data type.

  • «CodeList» enumerates the valid attribute values. In contrast to Enumeration, the list of values is open and, thus, not given inline in the CityGML UML Model. The allowed values can be provided within an external code list.

  • «Union» is a list of attributes. The semantics are that only one of the attributes can be present at any time.

  • «Property» denotes attributes and association roles. This stereotype does not add further semantics to the conceptual model, but is required to be able to add tagged values to the attributes and association roles that are relevant for the encoding.

  • «Version» denotes that the value of an association role that ends at a feature type is a specific version of the feature, not the feature in general.

    In order to enhance the readability of the CityGML UML diagrams, classes are depicted in different colors. The following coloring scheme is applied:

ColorScheme yellow

Classes painted in yellow belong to the Requirements Class which is subject of discussion in that clause of the standard in which the UML diagram is given. For example, in the context of <<ug-model-core-section>>, which introduces the _CityGML Core_ module, the yellow color is used to denote classes that are defined in the _CityGML Core_ Requirements Class. Likewise, the yellow classes shown in the UML diagram in <<ug-model-building-section>> are associated with the _Building_ Requirements Class that is subject of discussion in that chapter.

ColorScheme blue

Classes painted in blue belong to a Requirements Class different to that associated with the yellow color. In order to explicitly denote to which Requirements Class these classes belong, their class names are preceded by the UML package name of that Requirements Class. For example, in the context of the _Building_ Requirements Class, classes from the _CityGML Core_ and the _Construction_ Requirements Classes are painted in blue and their class names are preceded by _Core_ and _Construction_, respectively.

ColorScheme green

Classes painted in green are defined in the ISO standards 19107, 19111, or 19123. Their class names are preceded by the UML package name, in which the classes are defined.

ColorScheme grey

Classes painted in grey are defined in the ISO standard 19109. In the context of this standard, this only applies to the class _AnyFeature_. _AnyFeature_ is an instance of the metaclass _FeatureType_ and acts as super class of all classes in the CityGML UML model with the stereotype &#171;FeatureType&#187;. A metaclass is a class whose instances are classes.

ColorScheme white

The color white is used for notes and <<iso19507,Object Constraint Language>> (OCL) constraints that are provided in the UML diagrams.
The example UML diagram in <<figure-2>> demonstrates the UML notation and coloring scheme used throughout this standard. In this example, the yellow classes are associated with the _CityGML Building_ module, the blue classes are from the _CityGML Core_ and _Construction_ modules, and the green class depicts a geometry element defined by ISO 19107.
UML Notation Example
Figure 2. Example UML diagram demonstrating the UML notation and coloring scheme used throughout the CityGML Standard.

7. CityGML Foundations

This standard defines an open CityGML Conceptual Model (CM) for the storage and exchange of virtual 3D city and landscape models. These models include the most relevant entities of the urban space like buildings, roads, railways, tunnels, bridges, city furniture, water bodies, vegetation, and the terrain. The conceptual schema specifies how and into which parts and pieces physical objects of the real world should be decomposed and classified. All objects can be represented with respect to their semantics, 3D geometry, 3D topology, appearances, and their changes over time. Different spatial representations can be provided for each object (outdoor and indoor) in four predefined Levels of Detail (LOD 0-3). The CityGML 3.0 Conceptual Model (<<ug-citygml-model-section>>) is formally specified using UML class diagrams, complemented by a data dictionary (<<data-dictionary-section>>) providing the definitions and explanations of the object classes and attributes. This Conceptual Model is the basis for multiple encoding standards, which map the concepts (or subsets thereof) onto exchange formats or database structures for data exchange and storage.
While the CityGML Conceptual Model can be used for 3D visualization purposes, its special merits lie in applications that go beyond visualization such as decision support, urban and landscape planning, urban facility management, Smart Cities, navigation (both indoor and outdoor), Building Information Modeling (especially for as-built documentation), integration of city and BIM models, assisted and autonomous driving, and simulations in general (cf. <<Kolbe2009>>). A comprehensive overview on the many different applications of virtual 3D city models is given in [<<Biljecki2015>>]. Many of the applications already use and some even require using CityGML.
In the CityGML CM, all 3D city objects can easily be enriched with thematic data. For example, street objects can be enriched with information about traffic density, speed limit, number of lanes etc., or buildings can be enriched by information on the heating and electrical energy demand, numbers of households and inhabitants, the appraised building value etc. Even building parts such as individual roof or wall surfaces can be enriched with information e.g., about solar irradiation and thermal insulation parameters. For many application domains specific extensions of the CityGML CM have already been created (cf. <<Biljecki2018>>).

7.1. Modularization

The CityGML Conceptual Model provides models for the most important types of objects within virtual 3D city and landscape models. These feature types have been identified to be either required or important in many different application areas. However, implementations are not required to support the complete CityGML model in order to be conformant to the standard. Implementations may employ a subset of constructs according to their specific information needs. For this purpose, modularization is applied to the CityGML CM.
ModuleOverview
Figure 3. CityGML 3.0 module overview. The vertical boxes show the different thematic modules. Horizontal modules specify concepts that are applicable to all thematic modules.
The CityGML conceptual model is thematically decomposed into a _Core module_ and different kinds of _extension modules_ as shown in <<figure-moduleoverview>>. The Core module (shown in green) comprises the basic concepts and components of the CityGML CM and, thus, must be implemented by any conformant system. Each red colored module covers a specific thematic field of virtual 3D city models.
The CityGML CM introduces the following eleven thematic extension modules: _Building_, _Bridge_, _Tunnel_, _Construction_, _CityFurniture_, _CityObjectGroup_, _LandUse_, _Relief_, _Transportation_, _Vegetation_, and _WaterBody_. All three modules _Building_, _Bridge_, and _Tunnel_ model civil structures and share common concepts that are grouped within the _Construction_ module. The five blue colored extension modules add specific modeling aspects that can be used in conjunction with all thematic modules:
  • The Appearance module contains the concepts to represent appearances (like textures and colors) of city objects.

  • The PointCloud module provides concepts to represent the geometry of city objects by 3D point clouds.

  • The Generics module defines the concepts for generic objects, attributes, and relationships.

  • Versioning adds concepts for the representation of concurrent versions, real world object histories and feature histories.

  • The Dynamizer module contains the concepts to represent city object properties by time series data and to link them with sensors, sensor data services or external files.

    Each CityGML encoding can specify support for a subset of the CityGML modules only. If a module is supported by an encoding, then all concepts should be mapped. However, the encoding specification can define so-called _null mappings_ to restrict the use of specific elements of the conceptual model in an encoding. Null mappings can be expressed in an encoding specification for individual feature types, properties, and associations defined within a CityGML module. This means that the corresponding element will not be included in the respective encoding.
    Note that also CityGML applications do not have to support all modules. Applications can also decide to only support a specific subset of CityGML modules. For example, when an application only has to work with building data, only the modules _Core_, _Construction_, and _Building_ would have to be supported.

7.2. General modeling Principles

7.2.1. Semantic modeling of Real-World Objects

Real-world objects are represented by geographic features according to the definition in ISO 19109. Geographic features of the same type (e.g., buildings, roads) are modeled by corresponding feature types that are represented as classes in the Conceptual Model (CM). The objects within a 3D city model are instances of the different feature types.
In order to distinguish and reference individual objects, each object has unique identifiers. In the CityGML 3.0 CM, each geographic feature has the mandatory _featureID_ and an optional _identifier_ property. The _featureID_ is used to distinguish all objects and possible multiple versions of the same real-world object. The _identifier_ is identical for all versions of the same real-world object and can be used to reference specific objects independent from their actual object version. The _featureID_ is unique within the same CityGML dataset, but it is generally recommended to use globally unique identifiers like UUID values or identifiers maintained by an organization such as a mapping agency. Providing globally unique and stable identifiers for the _identifier_ attribute is recommended. This means these identifiers should remain stable over the lifetime of the real-world object.
CityGML feature types typically have a number of spatial and non-spatial properties (also called attributes) as well as relationships with other feature or object types. Note that a single CityGML object can have different spatial representations at the same time. For example, different geometry objects representing the feature's geometry in different levels of detail or as different spatial abstractions.
Many attributes have simple, scalar values like a number or a character string. However, some attributes are complex. They do not just have a single property value. In CityGML the following types of complex attributes occur.
  • Qualified attribute values: For example, a measure consists of the value and a reference to the unit of measure, or e.g., for relative and absolute height levels the reference level has to also be named.

  • Code list values: A code list is a form of enumeration where the valid values are defined in a separate register. The code list values consist of a link or identifier for the register as well as the value from that register which is being used.

  • Attributes consisting of a tuple of different fields and values: For example, addresses, space occupancy, and others.

  • Attribute value consisting of a list of numbers: For example, representing coordinate lists or matrices.

    In order to support feature history, CityGML 3.0 introduces bitemporal timestamps for all objects. In CityGML 2.0, the attributes _creationDate_ and _terminationDate_ are supported. These refer to the time period in which a specific version of an object is an integral part of the 3D city model. In 3.0, all features can now additionally have the attributes _validFrom_ and _validTo_. These represent the lifespan a specific version of an object has in the real-world. Using these two time intervals a CityGML dataset could be queried both for how did the _city_ look alike at a specific point in time as well as how did the _city model_ look at that time.
    The combination of the two types of feature identifiers and bitemporal timestamps enables encoding not only the current version of a 3D city model, but also the model's entire history can be represented in CityGML and possibly exchanged within a single file.

7.2.2. Class Hierarchy and Inheritance of Properties and Relations

In CityGML, the specific feature types like __Building__, __Tunnel__, or _WaterBody_ are defined as subclasses of more general higher-level classes. Hence, feature types build a hierarchy along specialization / generalization relationships where more specialized feature types inherit the properties and relationships of all their superclasses along the entire generalization path to the topmost feature type __AnyFeature__.
Note: A superclass is the class from which subclasses can be created.

7.2.3. Relationships between CityGML objects

In CityGML, objects can be related to each other and different types of relations are distinguished. First of all, complex objects like buildings or transportation objects typically consist of parts. These parts are individual features of their own, and can even be further decomposed. Therefore, CityGML objects can form aggregation hierarchies. Some feature types are marked in the conceptual model with the stereotype _&#171;TopLevelFeatureType&#187;_. These constitute the main objects of a city model and are typically the root of an aggregation hierarchy. Only top-level features are allowed as direct members of a city model. The information about which feature types belong to the top level is required for software packages that want to filter imports, exports, and visualizations according to the general type of a city object (e.g., only show buildings, solitary vegetation objects, and roads). CityGML Application Domain Extensions should also make use of this concept, such that software tools can learn from inspecting their conceptual schema what are the main, i.e., the top-level, feature types of the extension.
Some relations in CityGML are qualified by additional parameters, typically to further specify the type of relationship. For example, a relationship can be qualified with a URI pointing to a definition of the respective relation type in an Ontology. Qualified relationships are used in CityGML, among others, for:
  • General relationships between features – association relatedTo between city objects,

  • User-defined aggregations using CityObjectGroup. This relation allows also for recursive aggregations,

  • External references – linking of city objects with corresponding entities from external resources like objects in a cadastre or within a BIM dataset.

    The CityGML CM contains many relationships that are specifically defined between certain feature types. For example, there is the _boundary_ relationship from 3D volumetric objects to its thematically differentiated 3D boundary surfaces. Another example is the _generalizesTo_ relation between feature instances that represent objects on different generalization levels.
    In the CityGML 3.0 CM there are new associations to express topologic, geometric, and semantic relations between all kinds of city objects. For example, information that two rooms are adjacent or that one interior building installation (like a curtain rail) is overlapping with the spaces of two connected rooms can be expressed. The CM also enables documenting that two wall surfaces are parallel and two others are orthogonal. Also distances between objects can be represented explicitly using geometric relations. In addition to spatial relations logical relations can be expressed.

7.2.4. Definition of the Semantics for all Classes, Properties, and Relations

The meanings of all elements defined in the CityGML conceptual model are normatively specified in the data dictionary in <<data-dictionary-section>>.

7.3. Representation of Spatial Properties

7.3.1. Geometry and Topology

Spatial properties of all CityGML feature types are represented using the geometry classes defined in ISO 19107. Spatial representations can have 0-, 1-, 2-, or 3-dimensional extents depending on the respective feature type and Levels of Detail (LOD). The LOD concept is discussed in <<ug-levels-of-detail-section>> and <<ug-geometry-lod-section>>. With only a few exceptions, all geometries must use 3D coordinate values. Besides primitive geometries like single points, curves, surfaces, and solids, CityGML makes use of different kinds of aggregations of geometries like spatial aggregates (_MultiPoint_, _MultiCurve_, _MultiSurface_, _MultiSolid_) and composites (_CompositeCurve_, _CompositeSurface_, _CompositeSolid_). Volumetric shapes are represented in ISO 19107 according to the so-called _Boundary Representation_ (B-Rep). For further explanation see <<Foley2002>>.
The CityGML Conceptual Model does not put any restriction on the usage of specific geometry types as defined in ISO 19107. For example, 3D surfaces could be represented in a dataset using 3D polygons or 3D meshes such as triangulated irregular networks (TINS) or by non-uniform rational B-spline surfaces (NURBS). However, an encoding may restrict the usage of geometry types. For example, curved lines like B-splines or clothoids, or curved surfaces like NURBS could be disallowed by explicitly defining _null encodings_ for these concepts in the encoding specification (c.f. <<ug-modularization-section>> above).
Note that the conceptual schema of ISO 19107 allows composite geometries to be defined by a recursive aggregation for every primitive type of the corresponding dimension. This aggregation schema allows the definition of nested aggregations (hierarchy of components). For example, a building geometry (_CompositeSolid_) can be composed of the house geometry (_CompositeSolid_) and the garage geometry (_Solid_), while the house’s geometry is further decomposed into the roof geometry (_Solid_) and the geometry of the house body (_Solid_). This is illustrated in <<figure-recursiveaggregation>>.