ISO/TR 14825:1996

TECHNICAL ISOTTR
REPORT 14825
First edition
1996-11-15
Geographic Data Files (GDF)
Fichiers de don&es ggographiques
Reference number
ISOTTR 14825: 1996(E)

---------------------- Page: 1 ----------------------
ISOmR 14825:1996(E)
Foreword
IS0 (the International Organization for Standardization) is a worldwide
federation of national standards bodies (IS0 member bodies). The work of
preparing International Standards is normally carried out through IS0
technical committees. Each member body interested in a subject for which
a technical committee has been established has the right to be represented
on that committee. International organizations, governmental and non-
in liaison with ISO, also take part in the work. IS0
governmental,
collaborates closely with the International Electrotechnical Commission
(IEC) on all matters of electrotechnical standardization.
The main task of technical committees is to prepare International
Standards. In exceptional circumstances a technical committee may
propose the publication of a Technical Report of one of the following types:
the required S upport cannot
- type 1, when be obtai ned for the
publication of an Internationa IS tandard, despite eated efforts;
rep
- type 2, when the subject is still under technical development or where
for any other reason there is the future but not immediate possibility of
an agreement on an International Standard;
- type 3, when a technical committee has collected data of a different
kind from that which is normally published as an International
Standard (“state of the art”, for example).
Technical Reports of types 1 and 2 are subject to review within three years
of publication, to decide whether they can be transformed into International
Standards. Technical Reports of type 3 do not necessarily have to be
reviewed until the data they provide are considered to be no longer valid or
useful.
ISOTTR 14825, which is a Technical Report of type 2, was prepared by the
European Committee for Standardization (CEN) in collaboration with IS0
Technical Committee ISOmC 204, Transport information and control
systems, in accordance with the Agreement on technical cooperation
between IS0 and CEN (Vienna Agreement).
This document is being issued in the Technical Report (type 2) series of
publications (according to subclause G.3.2.2 of part 1 of the ISO/IEC
0 IS0 1996
All rights reserved. Unless otherwise specified, no part of this publication may be reproduced
or utilized in any form or by any means, electronic or mechanical, including photocopying and
microfilm, without permission in writing from the publisher.
International Organization for Standardization
Case postale 56 l CH-1211 Geneve 20 l Switzerland
Internet central @ isocs.iso.ch
x.400 c=ch; a=400net; p=iso; o=isocs; s=central
Printed in Switzerland
II

---------------------- Page: 2 ----------------------
lSO/TR 14825: 1996(E)
0 IS0
Directives, 1995) as a “prospective standard for provisional application” in
the field of transport information and control systems because there is an
urgent need for guidance on how standards in this field should be used to
meet an identified need.
This document is not to be regarded as an “International Standard”. It is
proposed for provisional application so that information and experience of
its use in practice may be gathered. Comments on the content of this
document should be sent to the lSO/TC 204 Secretariat.
A review of this Technical Report (type 2) will be carried out not later than
three years after its publication with the options of: extension for another
three years; conversion into an International Standard; or withdrawal.

---------------------- Page: 3 ----------------------
ISO/TR 14825: 1996(E)
INTRODUCTION
The Geographic Data Files (GDF) standard has been developed to meet the needs of professionals and
organizations involved in the creation, update, supply and application of referenced and structured road network
data.
It has been created in order to improve the efficiency of the capture, the production and handling of road related
geographic information. This increase in efficiency is obtained by supplying a common reference model on which
users can base their requirements and producers can base their product definition. In addition to this, the standard
facilitates the exchange of information, defined according to this reference model. For this it contains the definition
of an exchange format which avoid compatibility problems at both the users and the producersS side of the
information flow. In this respect, producers and users should not be viewed as two totally distinct groups- It is
envisaged that an important application of the standard will be the combination of information present in already
existing geographic information data bases into one, more comprehensive source of road-related information”
To ensure maximum compatibility with these already existing sources and also to make use of the strengths of the
structure in which these sources have been defined, the basic foundation of the standard is based on a general,
non-application specific data model. On top of this data model, a road network specific application model has been
built. Together they make up the GDF standard. Due to its general character, the data model also is able to support
other types of applications, thus facilitating the future creation of a wide variety of geographic information sources
which can be combined with maximum flexibility.
The standard consists of twelve chapters, of which six (chapter 5 to 10) together form the reference model. They
each contain the elaboration and definition of one clearly identifiable aspect of road related information. Chapter 4
contains a more general description of the standard. It contains the definition of the general data model as well as
the definition of the basic components of the standard, thus explaining the structure of the rest of the standard.
Chapter 11 describes the logical data structure by which the conceptual models as defined in the previous chapters
can be represented.
Chapter 12 defines the exchange format by which the information can be exchanged.

---------------------- Page: 4 ----------------------
@ IS0
ISO/TR 14825: 1996(E)
I. SCOPE
This standard specifies a system for the interchange of digital road related geographic information. It
takes into account all the requirements of applications in the road transport and traffic telematics
(RTTT) field. Within this field, the standard is application independent. The standard contains the
following detailed specifications :
a) A reference model according to which the information covered by the standard shall be defined.
The core of the reference model is formed by a data model and a data dictionary in which the
individual information components and their interrelations are defined.
A specification of ways of representation of the information components contained.
4
) The specification how to define meta information. An important aspect is the quality of the
information defined according to the standard. This aspect is dealt with separately in the sense that
the methodology is defined with which the quality of the information components contained can be
measured.
A specification for an exchange format reflecting the reference model.

---------------------- Page: 5 ----------------------
@ IS0
ISO/TR 148253 996(E)
2 R REFERENCES
21 . Normative References
Working Party to Produce National Standards for the Transfer of Digital Map Data; National
II
Transfer Format, Release 1 .I ; Ordnance Survey; Southampton, U.K.; January 1989
National Committee for Digital Cartographic Data Standards et al.; Spatial Data Transfer Standard;
21
U.S. Geological Survey, National Mapping Division; Columbus, Ohio, USA; June 1990
International Federation of Library Associations and Institutions, Joint Working Group on the
31
International Standard Bibliographic Description for Cartographic Materials; ISBD(CM):
International Standard Bibliographic Description for Cartographic Materials; IFLA International
Office for UBC; London, U.K.; 1977
Bibliographic Systems Office, Working Group on Bibliographic Codes; Revised List of Languages
41
and Language Codes; Library of Congress, Bibliographic SystemsOffice; Washington D.C., U.S.A;
1977
ISO; International Standard IS0 690, Documentation -Bibliographic references - Content, form and
51
structure; International Organisation for Standardisation; 1987
ISO; International Standard IS0 6709, Standard representation of latitude, longitude and altitude
61
for geographic point locations; International Organisation for Standardisation; 1983
ISO; International Standard IS0 3166; Codes for the representation of names of countries.;
71
International Organisation for Standardisation; 1993
ISO; International Standard IS0 2108, Information and documentation - International Standard
81
Book Numbering (ISBN); International Organisation for Standardisation; 1992
ISO; International Standard IS0 3297; Documentation - International Standard Serial Numbering
91
(ISSN); International Organisation for Standardisation; 1986
lSO/IEC; International Standard 8859-I) Information processing - 8-bit single-byte coded graphic
101
character sets - Part 1: Latin alphabet No. 1; International Organisation for Standardisation; 1987
ISO; International Standard 2859, Sampling procedures for inspection by attributes; International
111
Organisation for Standardization; 1985-l 995 (all parts)
AFNOR; Norme expkrimentale: Echanges de donneirs informatisees dans le domaine de
121
I’lnformation Geographique - EDlG&O; Association Franc;aise de Normalisation;Paris, 1991;
2

---------------------- Page: 6 ----------------------
0 IS0
ISO/TR 14825: 1996(E)
2.2 Digital Geographic Information Working Group; Digital Geographic
Information Exchange Standard (DIGEST); December 1989; Other references
131 FIG; Technical Dictionary, terms and definitions as used in surveying and mappingin Germany;
lnstitute for Applied Geodesy; Frankfurt am Main; 1971
141 ICA; Multilingual Dictionary of Technical Terms in Cartography; InternationalCartographic
Association; Wiesbaden; 1973
151 DTV; DTV-Lexikon; Deutscher Taschenbuchen-Verlag; 1975
161 The Shorter Oxford English Dictionary
171 Claude Berge; Graphs; North-Holland Mathematical Library, Volume 6 - Part 1; North Holland;
Amsterdam, New York, Oxford; 1985
181 Mehdi Behzad, Gary Chartrand; Introduction to the Theory of Graphs; Allyn and Bacon Inc.;
Boston, Massachusetts; 1971
191 Nijssen G.M, Halpin T.A.; Conceptual Schema and Relational Database Design - A fact oriented
approach; Prentice Hall, Sydney; 1989

---------------------- Page: 7 ----------------------
ISO/TR 14825: 1996(E)
3. DEFINITIONS
The numbers between square brackets refer to the reference documents mentioned in 2.
3.1 General terms
3.3 .I Accuracy
The closeness of results of observations, computations or estimates to the true values or the values as
accepted as being true [2]
3.1.2 Cartography
The art, science and technology of making maps, together with their study as scientific documents and
works of art[l5]
3.1.3 Cartographic Primitive
Atomic construction element in a cartographic representation, i.e. Node, Edge and Face.
3.1.4 Data Fiie
A collection of related data records.[2] The records shall have a homogeneous structure.
3.1.5 Data Retard
A record containing feature related data
3.1 x6 Data set
A large set of data covering a particular geographic area
3.1.7 Entity
A real world phenomenon that is not subdivided into phenomena of the same kind (e.g. a bridge) [2]
3.1.8 Error Rate
The percentage of falsehoods
3.1.9 Field
A specified part of a record containing a unit of data. The unit of data may be a data element or a data
item [I]
3.1 .I0 Geudesy
Science of determination of the shape and gravity field of the earth and of survey and mapping of the
physical surface of the earth [14]
3.1 .I 1 Geography
Science of phenomena of the earth’s surface, its being and growing and its manifold relations [16]
3.1 .I 2 Geumetry
Science of the characteristics of spatial figures [16]
4

---------------------- Page: 8 ----------------------
@ IS0
ISO/TR 14825:1996(E)
3.1 .I3 Global Record
A record that logically precedes the data records and contains control parameters, data definition and
documentation necessary to interpret companion data records [ISO 821 I]
3.1 .+I4 information Unit
A collection of information that may be regarded as an undivided whole, e.g. 1 data set, I section, 1
layer
3.1 .I5 Logical Dumain
The range of attribute values to which a meaning has been assigned.
3.1 .I6 Logical Unit
A collection of data that may be regarded as a logically undivided whole, e.g. 1 logical record.
3.1.17 Medium Unit
An object for data storage that can be considered as a physically undivided whole, e.g. 1 floppy disk, 1
magnetic tape etc.
3.1 .I8 Physical Unit
A unit of data storage that may be regarded as physically undividable.
3.1 .I 9 Precision
The closeness of measurements of the same phenomenon repeated under exactly the same
conditions and using the same techniques.
3.1.20 Primitive
Fundamental form from which all other forms can be derived [17]
3.121 Repeating Attribute Type
An attribute type that may have multiple values associated to one and the same instance of a particular
feature type.
3.1.22 Resolution
The smallest unit which can be detected. It fixes a limit to precision and accuracy.
3.1.23 Spatial Domain
The description of the limits of a geographical area to which a particular set of data spatially belongs to.
3.124 Source Material
The origin of data in analogue or digital representation, stored on any kind of data medium.
3.1.25 Topography
The technical and conceptual registration of the terrain, its features and properties of the landscape
.
WI
3.1.26 Topology
The field of mathematics that deals with characteristics of geometric structures that keep preserved
after continual variation [ 161

---------------------- Page: 9 ----------------------
0 IS0
ilSO/rR 14825:1996(E)
3.1.27 Transcription
Render ,ing of geographic na ,mes from a non-alphabetic script into an alphabetic one or vice versa. The
term is also applied to initial reco rding script of hitherto unwritten names [15]
3.1.28 Up-to-dateness
The closeness in time of the (geographic) data to the present reality.

---------------------- Page: 10 ----------------------
@ IS0
lSO/TR 14825: 1996(E)
3.2 Mathematical terms
3.2.1 Area Feature
A two dimensional feature. An area feature is defined by one or more faces.’
3.2.2 Edge
A directed sequence of non-intersecting line segments with nodes at each end [2]
3.2.3 Encfave
Small part of an area enclosed by another area seen from the area to which that part belongs [ 141
3.2A Excfave
Small part of an area enclosed by another area seen from the enclosing area [14]
3.2.5 Face
or more non-
A two-dimensional element bounded by a closed set of edges and zerc
dimensional
intersection inner closed set of edges. The face is the atomic two element.
3.2.6 Graph
A set of points and a set of arrows, with each arrow joining one point to another. The points are called
nodes of the graph, and the arrows are called the edges of the graph [18].
3.2.7 Intermediate
A Point, not being a Node, that bounds the line segments belonging to an Edge
3.2,8 Line Featwe
A one-dimensional feature. A line feature is defined by one or more edges.
3.2.9 Luup
An edge which is bounded at both ends by one and the same node.
3.2.10 Nude
A zero-dimensional element that is a topological junction of two or more edges, or an end point of an
edge PI
3.2.1 I Non planar graph
A graph which is not planar
3.2.12 Path
A finite, alternating sequence of nodes and edges, such that every arc is immediately preceded and
succeeded by the two vertices with which it is incident and in which no vertex is repeated, except
(possibly) the first and the last one. [I 81
3.2.13 Planar Graph
A graph G is planar if it can be embedded in a plane. That means that it can be drawn on the plane so
that edges intersect only at a node mutually incident with them.
7

---------------------- Page: 11 ----------------------
ISO/TR 14825:1996(E)
3.2A4 Plane Graph
A planar graph embedded in the plane
3.2.15 Point
A zero-dimensional element that specifies geometric location. One coordinate pair or triplet specifies
the location [2]
3.2.16 Segment
The direct connection between exactly 2 Intermediate Points
3.2.17 Valency (or Degree)
The number of edges which are incident with a particular Node.

---------------------- Page: 12 ----------------------
@ IS0
ISO/TR 14825: 1996(E)
3.3 Geodetical terms
3.3.1 Cuntrul Points
Points in the real world that are identical with points in a map or aerial photograph.
3.3.2 Ellipsoidal Height
The distance between a point and the reference ellipsoid (measured along the ellipsoidal normal).
3.3.3 Geudetic datum
The position and orientation of a particular reference ellipsoid.
3.3=4 Geuid
A model of the figure of the earth, that coincides with the mean sea level over the oceans and
continues in continental areas as an imaginary sea level surface, defined by spirit level.
At every place it is perpendicular to the pull of gravity. The shape is irregular, but can for most
purposes be approximated by an oblate ellipsoid.
3.3.5 Geuid Ondulation
The difference between the orthometric height and the ellipsoidal height, measured along the ellipsoid
normal.
3.3.6 Height
The (vertical) distance between a point and the reference height level or the reference ellipsoid. On
land maps the reference level is commonly the mean sea level.
3.3.7 Horizontal Reference System
A reference system for positions
3.3.8 Magnetic Declination
Angle between Magnetic North and True North [14]
3.3.9 Map Projectian
The transformation method used to represent the curved earth surface on a plane
3.3.10 offset
A pair of values, subtracted from all coordinate values in order to shorten these coordinate values.
3.3.1 I Orthumetric Height
The distance between a point and the geoid (measured along the perpendicular line.
3.3.12 Reference Ellipsoid
An oblate ellipsoid of revolution that is used to approximate the figure of the geoid. It is specified by two
parameters: a semi-major axis “a” (equatorial radius of the earth) and a semi-minor axis “b” (polar
radius).
The flattening ‘7 is defined as: f = (a-b).a
3.3.13 Reference Height Level
The level to which all terrestrial heights are referred. It changes from country to country and it forms
part of the national coordinate system for surveying and mapping.

---------------------- Page: 13 ----------------------
@ IS0
lSO/TR 14825: 1996(E)
3.3.14 Reference System
A coordinate system on which a national survey is based 1141
3.3.A 5 Vertical Reference System
A reference system for elevations
3.3.16 World Geodetic System (WGS)
A three-dimensional Cartesian coordinate system, its origin being the geocentre. WGS is related to a
specified gravity model.
10

---------------------- Page: 14 ----------------------
0 IS0 ISO/TR 14825:1996(E)
3.4 GDF Terms
Note: All feature classes mentioned below are defined in the Feature Catalogue (Chapter 5)
3.4.1 Album
A collection of related Volumes.
3.4.2 Attribute
A characteristic of a feature which is independent of other features[2]
3.4,3 Attribute Code
An alphanumeric identifier for an attribute type [I]
3.4.4 Attribute Name
A name associated to an attribute type [I]
3.4.5 Attribute Type
A defined characteristic of a Feature, which is independent of the other features.
3.4.6 Attribute Value
A specific quality or quantity assigned to an attribute [2]
3.4.7 Completeness
Extent to which all specified features are present.
3.4.8 correctness
Indication of whether a data item is correctly recorded according to a specified data catalogue.
3.4.9 Data Set
A collection of related data files
3x4.1 0 Feature
A database representation of a real world object. ( see feature catalogue)
3.4.11 Feature Category
Type of representation of a feature. I.e. Point, Line, Area or Complex Feature.
3.4.12 Feature Class
An alphanumeric identifier for a feature class.
3.4.13 Feature Code
An alphanumeric identifier for a feature class [I]
3.4.14 Feature Name
A name associated with a feature class [2]
3.4.15 Feature Theme
A specified group of related features.
11

---------------------- Page: 15 ----------------------
lSO/T=R 14825:1996(E)
3.4A 6 Field
A set of characters representing one unit of data
3.4.17 Layer
A certain subset of a section based upon information contents. The collection of level-0 elements
present in a layer together should form one planar graph.
3x4.l 8 Manoeuvre
An ordered sequence of a Road Element, a Juncfion and one or more Road Elements.
3.4.19 Record
An implementation dependent construct that consists of an identifiable collection of one or more
related fields [2]
3.4.20 Semantic Relationship
A characteristic of a feature involving other features.
3.4.21 Section
A certain subset of a dataset based on geographical co-ordinates
3.4.22 Relationship
Semantic relationship
3.4.23 Relationship Code
An alphanumerical identifier for a (Semantic) Relationship.
3.4.24 Refationship Name
A name associated to a relationship type [2]
3.4.25 Relationship Type
A defined characteristic of a feature which is depenedent of ofther features.
3.4.26 Trarrsportation Element
A Road Element, Railway Element, Water Boundary Element, Junction, Rail Junction and Water
Boundary Junction
3.4.27 Turn
An ordered sequence of a Road Elemenf, a Juncfion and a Road Element
3.4.28 Volume
The smallest physical unit of medium. For example a magnetic tape, a floppy disk etc. A single
Volume may contain one or more GDF Datasets depending on Dataset size.
12

---------------------- Page: 16 ----------------------
@ IS0
ISO/TR 14825:1996(E)
4. GENERAL DATA MODEL
4.1 GDF History
The first version of the draft GDF standard was released in October 1988 (Geographic Data Files
Release 1 .O; 1988-10-01). A product of the EUREKA project DEMETER, it was designed to specify the
data content, means of data representation and structure of data supply for vehicle navigation systems.
Based on a comprehensive data model (derived from the UK National Transfer Format [NTF] Model)
the first draft standard was used extensively in the EUREKA project CARMINAT and in the DRIVE
projects PANDORA and Task Force European Digital Road Map (EDRM). Both the DRIVE-1 projects
put forward requests for changes to GDF 1 .O.
GDF 2.0 has been developed under work package 3532 of the Task Force EDRM. The GDF 2.0
Working Group consisted of representatives of EDRM partners Daimler Benz (Prime Contractor),
Bosch, Philips, Renault, Tele Atlas and Intergraph and MVA Systematica, a participant of the
PANDORA project. The other PANDORA project partners were Philips, Bosch, Ordnance Survey and
the Automobile Association.
In parallel with DRIVE projects, an informal working group of mapping organizations (both state and
private), electronics and vehicle manufacturers, academics and other experts has been developing a
conceptual data model upon which to standardize the exchange of geographical data in Europe.
Termed the “European Transfer Format” (ETF) model, it diverges from the GDF 1.0 data model in
certain key areas. The second release of GDF endorses the ETF conceptual data model, and is
therefore the first exchange format to be compliant with the ETF model.
GDF 2.0 differs from GDF I .O both in its treatment of the architecture of certain data structures and in
its use of terminology. It is also far more extensive in specification. It defines standards for the
description, classification and encoding of features of the road environment, suitable to support a
family of application areas. These include requirements for Vehicle Navigation Systems, Highway
Maintenance Systems, Road Transport Informatics, and Advanced Road Transport Telematics (ATT).
RTI and ATT are known in the US as Intelligent Vehicle Highway Systems (IVHS). Accommodation of
other applications in the future is envisaged.
GDF 2.1 was released in October 1992 and consists of a revision of Volume 3, the Attribute Catalogue
and Volume 9, the Media Record Specifications. The major differences with respect to 2.0 are the
introduction of the Segmented Attribute concept and the Time Domain concept.
GDF 2.2 was released in November 1994. The Feature, Attribute and Relationship Catalogue have
been considerably extended. The changes mainly consisted of the introduction of new information
items. GDF 2.2 was at the time of release , officially submitted to CEN TC278 for approval as a prENV.
For this, the CEN member countries reviewed the document during a three months period. The
comments received were subsequentely incorporated. The result of this was published in July 1995 as
GDF 2.3.
GDF 2.3 was subject to a final editorial review in September 1995.
GDF 3.0 was the result of the incorporation of comments from this review and submitted to CEN
TC278 for official approval as ENV in October 1995.
The work was undertaken by EDRM2 project partners and the members of CEN TC278 WG 7 with
considerations given to the needs of highway authorities in the later stages.
13

---------------------- Page: 17 ----------------------
ISO/TR 14825: 1996(E)
4.2 Structure of GDF
GDF is divided into 12 chapters :
Scope, Normative References and Definitions
123
4 . General Data Model
5. The GDF Feature Catalogue
6 . The GDF Attribute Catalogue
7. The GDF Relationship Catalogue
8 . The GDF Feature Representation Scheme
9 . Quality Description Specifications
Global Data Catalogue
10 .
11 . Logical Data Structures
Media Record Specifications
12 .
Throughout each chapter, /talks are used to indicate a reserved meaning.
14

---------------------- Page: 18 ----------------------
@ IS0
lSO/TR 14825: 1996(E)
4.3 Contents of GDF
The Feature Catalogue provides a definition of the “real world objects” such as Roads, Buildings’
Administrative Areas and Settlements that have significance in the broad area of applications for this
standard. They all relate to the road environment. Suppliers of data destined for Vehicle Navigation
Systems, Highway Maintenance Systems and other applications will find a concise description and
classification of features in this catalogue.
The Attribute Catalogue defines a number of characteristics of features and possibility of
relationships. Some attributes are dedicated to one particular feature class. Other attributes may be
more generally applied. Some attributes themselves describe a certain characteristic completely.
Others have to be combined to reach this goal.
The Relationship Catalogue describes relations between features that may be used to convey
information in a realistic manner. For example, a relationship may exist between Road Elements and
Buildings such that a Building “is along” a Road Element. This serves to enrich the model to represent
the real world.
In the Feature Representation Scheme, instruction is given on how to represent an object: as a Point,
Line, Area or Complex Feature.
The Quality Measuring Specifications describe rules and methods on how to measure the quality of
and validate a GDF dataset.
In the Global Data Catalogue, a description is given on how meta information such as geodetic
references and data sources must be modelled. Data dictionary specifications are also provided in this
catalogue so that GDF will be able to meet diverse needs in the future.
The Volume Logical Data Structures describe how the information modelled according to the rules
described in the previous volumes, can be represented by a nested set of data types. This specification
is independent of any particular media record specification.
Finally, information needed to supply GDF data as physical data records is provided in the Media
Record Specification.
15

---------------------- Page: 19 ----------------------
@ IS0
ISO/TR 14825: 1996(E)
4.4 Data Models in GDF
Many of the concepts relevant to GDF are best expressed diagrammatically. Those concepts
concerned with the data models of GDF are represented using a variant of Entity-Relation Modelling
referred to as NIAM (see section 4.5).
Where required, detailed sub-schema’s are given to reduce the complexity of the figures. The
complete data model and views pertinent to each of the volumes are presented and explained in detail
in 4.5.
16

---------------------- Page: 20 ----------------------
@ IS0 ISO/TR 14825: 1996(E)
4.5 Understanding the NIAM diagrams
Figure 4.2 gives a diagram of the Conceptual Data Model. The diagram has been constructed
according to the conventions of NIAM modelling (Nijssens Information Analysis Method) [20].
Figure 4.1 provides the key to figure 4.2 and all other data models contained in this standard.
In the centre of the model resides “the feature” which is a geographic object that has a location’ such
as a road or a building. Note that in this diagram with “feature” is meant feature instance’ i.e. an
individual occurrence of a geographic object, such as the Eiffel Tower in Paris.
Each feature belongs to a certain feature class, i.e. a set of feature instances of the same class. Each
feature belongs to not more than one feature class: hybrid features are not allowed. This constraint is
indicated by the arrow above “belongs to”. Each feature also must belong to a feature class: classless
features are not all
...