SIST ISO 19155-2:2020

INTERNATIONAL ISO
STANDARD 19155-2
First edition
2017-08
Geographic information — Place
Identifier (PI) architecture —
Part 2:
Place Identifier (PI) linking
Information géographique — Architecture d’identifiants de lieu (IL) —
Partie 2: Liaison d’identifiants de lieu (IL)
Reference number
©
ISO 2017
© ISO 2017, Published in Switzerland
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ii © ISO 2017 – All rights reserved

Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 2
4 Abbreviated terms . 2
4.1 Abbreviated terms . 2
4.2 UML Notation . 3
4.3 Backward compatibility . 3
5 Conformance . 3
5.1 General . 3
5.2 Linking mechanism: gml:id . 3
5.3 Linking mechanism: UUID . 3
5.4 Linking mechanism: URL . 3
6 Place Identifier (PI) concept . 4
6.1 General . 4
6.2 PI structure. 4
7 PI linking . 4
7.1 Overview . 4
7.2 PI linking directionality . 5
7.3 PI linking model . 7
8 PI linking mechanisms . 7
8.1 Overview . 7
8.2 Linking mechanism: gml:id . 8
8.2.1 Overview . 8
8.2.2 Linking from a PI . 8
8.2.3 Linking to a PI . 8
8.2.4 Instance examples using gml:id . 8
8.3 Linking mechanism: UUID . 9
8.4 Linking mechanism: URL . 9
Annex A (normative) Abstract test suite .10
Annex B (normative) Encoding using gml:id to link with GML .11
Annex C (normative) Encoding using gml:id to link with GML application schemas .16
Annex D (normative) Encoding using UUID for linking .19
Annex E (normative) Encoding using URL for linking .21
Annex F (informative) Use case examples .22
Annex G (informative) RDF examples of linking PIs .27
Bibliography .34
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
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ISO collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular the different approval criteria needed for the
different types of ISO documents should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www .iso .org/ directives).
Attention is drawn to the possibility that some of the elements of this document may be the subject of
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on the ISO list of patent declarations received (see www .iso .org/ patents).
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For an explanation on the voluntary nature of standards, the meaning of ISO specific terms and
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World Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT) see the following
URL: w w w . i s o .org/ iso/ foreword .html.
This document was prepared by Technical Committee ISO/TC 211, Geographic information/Geomatics.
A list of all parts in the ISO 19155 series can be found on the ISO website.
iv © ISO 2017 – All rights reserved

Introduction
The Place Identifier (PI) architecture (ISO 19155) defined the conceptual model of a place and
specified normative encodings, for Place Identifiers, not specific to any type of geographic feature. In
this document, three mechanisms are presented that define how Place Identifiers can be linked with
features or objects in other encodings. Even though the identifiers of those features or objects may not
specifically be a place, they may be referred to conceptually as “other identifiers.”
Figure 1 depicts the abstractions of linking mechanisms among feature/object encoding rules.
a) Without the use of a Place Identifier b) With the use of a Place Identifier
Key
encoding rules
linkages
Figure 1 — Linkages with other encoding rules
The linking mechanisms presented in this document are based on accepted information technology
for object identification and reference using gml:id, UUID, or URL. By using these linking mechanisms
with the rules defined here, and according to the type of encoding rule being linked to, Place Identifiers
can more uniformly be related to features and objects — other identifiers — in other encodings.
This extends the functionality of those other identifiers, in different encodings, by linking with Place
Identifiers encoded in GML.
Existing PI data can complement a range of other encodings through the use of the linking mechanisms
defined in this document.
For example, a group of Place Identifiers representing stores in a shopping mall can be associated with
the specific locations inside the shopping mall represented by a GML data set.
INTERNATIONAL STANDARD ISO 19155-2:2017(E)
Geographic information — Place Identifier (PI)
architecture —
Part 2:
Place Identifier (PI) linking
1 Scope
This document defines the following three mechanisms for linking Place Identifiers (PIs) (see ISO 19155)
to features or objects existing in other encodings:
— Id attribute of a GML object (gml:id) as defined in ISO 19136;
— Universally Unique Identifier (UUID) as defined in IETF RFC 4122;
— Uniform Resource Locator (URL) as defined in IETF RFC 1738.
These PI linking mechanisms are enabled using xlink: href as defined in W3C XML Linking Language
(XLink).
While the identifiers of these features or objects can sometimes identify a place, within the scope of
this document, the identifiers of features or objects existing in other encoding domains are referred to
conceptually as other identifiers.
This document further defines that when PIs are encoded, as specified in ISO 19155, using the
Geography Markup Language (GML) (ISO 19136), they are linked using gml:id to other GML encoded
features. The details of encoding GML instances using gml:id are specified in a normative annex.
Additional normative annexes define encodings for linking Place Identifiers to other identifiers using
UUID and URL and present examples for their use.
This document is applicable to location-based services, linked open data, robotic assisted services
and other application domains that require a relationship between PIs and objects in either the real or
virtual world.
This document is not about creating a registry of Place Identifiers linked to specific features or
objects, and support of linking mechanisms other than gml:id, UUID, and URL is out of the scope of this
document.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
ISO 19103, Geographic information — Conceptual schema language
ISO 19136:2007, Geographic information — Geography Markup Language
ISO 19155:2012, Geographic information — Place Identifier (PI) architecture
IETF, Universally Unique IDentifier (UUID) URN Namespace, RFC 4122, July 2005
IETF, Uniform Resource Locators (URL), RFC 1738, December 1994
W3C XML Linking Language (XLink) Version 1.1 — Recommendation, 06 May 2010
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 19155 and the following apply.
ISO and IEC maintain terminological databases for use in standardization at the following addresses:
— IEC Electropedia: available at http:// www .electropedia .org/
— ISO Online browsing platform: available at http:// www .iso .org/ obp
3.1
Place Identifier (PI) link
relationship established between PIs and other identifiers in different encoding domains
Note 1 to entry: While the identifiers of these features or objects can sometimes identify a place, within the
scope of this document, the identifiers of features or objects existing in other encoding domains are referred to
conceptually as “other identifiers”.
Note 2 to entry: These “other identifiers” can exist outside of the PI architecture.
3.2
Place Identifier (PI) linking mechanism
means used to define a place identifier (PI) link (3.1)
4 Abbreviated terms
4.1 Abbreviated terms
BIM Building Information Model
CSV comma-separated values
IFC Industry Foundation Class
GUID Globally Unique IDentifier
OGC Open Geospatial Consortium
PI Place Identifier
RDF Resource Description Framework
SVG Scalable Vector Graphics
UML Unified Modeling Language
URI Uniform Resource Identifier
URL Uniform Resource Locator
URN Uniform Resource Name
UUID Universally Unique IDentifier
XML eXtensible Markup Language
2 © ISO 2017 – All rights reserved

4.2 UML Notation
In this document, conceptual schemas are presented in the Unified Modeling Language (UML). The user
shall refer to ISO 19103 for the specific profile of UML used in this document.
4.3 Backward compatibility
This document uses the concepts defined in the Place Identifier (PI) Architecture (ISO 19155) without
modification. Therefore, no backward compatibility issues exist in this document.
5 Conformance
5.1 General
This document defines three conformance classes, specified in 5.2 to 5.4, matching the requirements
classes of the three linking mechanisms defined in Clause 8. Any PI linking mechanism for which
conformance with this document is claimed shall pass all of the requirements of the abstract test suite
specified in Annex A.
5.2 Linking mechanism: gml:id
PI linking for which gml:id conformance is claimed shall pass all of the requirements specified in the
abstract test suite in A.2.
Table 1 — Conformance class: Linking mechanism: gml:id
Conformance class /conf/19155-2/5/5.2
Dependency A.1
Requirements /req/linking_mechanism/GML_ID
Tests A.2
5.3 Linking mechanism: UUID
PI linking for which UUID conformance is claimed shall pass all of the requirements specified in the
abstract test suite in A.3.
Table 2 — Conformance class: Linking mechanism: UUID
Conformance class /conf/19155-2/5/5.3
Dependency A.1
Requirements /req/linking_mechanism/UUID
Tests A.3
5.4 Linking mechanism: URL
PI linking for which URL conformance is claimed shall pass all of the requirements specified in the
abstract test suite in A.4.
Table 3 — Conformance class: Linking mechanism: URL
Conformance class /conf/19155-2/5/5.4
Dependency A.1
Requirements /req/linking_mechanism/URL
Tests A.4
6 Place Identifier (PI) concept
6.1 General
A “place” is defined as an identifiable part of any space (ISO 19155), either in the real world or
virtual world.
ISO 19155:2012, 6.1 states that the same place can be identified with multiple Place Identifiers (PIs).
If the place is identified with coordinates, it is called “position” and if the place is identified with
geographic identifiers, it is called “location.” Additionally, the place may be identified with online
resource identifiers such as URI.
In ISO 19109:2015, 7.4, “position” is a spatial attribute of a feature, “location” is a location attribute of a
feature, and a virtual identifier, such as a URI, is a thematic attribute of a feature. Therefore, a PI can be
considered as an attribute of a feature.
6.2 PI structure
Figure 2 shows the structure of the PI, as defined in ISO 19155:2012, 7.2.2. The PI_PlaceIdentifier type
is the basic data type for the PI.
Figure 2 — PI_PlaceIdentifier
The PI_PlaceIdentifier class defines the following attributes.
a) The attribute rs specifies the reference system of the PI.
b) The attribute value is the identifier for a place and is unique within the context of the defined
reference system.
c) The optional attribute validPeriod specifies the period in which the identifier is valid for the place.
The validPeriod can be used to set the expiration of the identifier which can be validated through
implementation. The property of uniqueness of the identifier is determined by the reference system (rs).
7 PI linking
7.1 Overview
There are two main concepts that are standardized in this document.
The first concept is a mechanism for connecting PIs which identify the same place, where the connection
is directly embedded into the PI.
PI matching (ISO 19155:2012, 4.11) is the matching of multiple PIs that represent the same place. This
matching is accomplished through the use of the PI matching service (ISO 19155:2012, 7.3.2) and when
implemented, is performed within the PI architecture. The logical structure of “PI matching” is shown
in ISO 19155:2012, Figure 8, where a “PI_MatchedPISet” connects more than two “PI_PlaceIdentifier”s
which identify the same place. The one way association from “PI_MatchedPISet” to “PI_PlaceIdentifier”
enables users to make links independently with the instances of PI_PlaceIdentifier, and the data
4 © ISO 2017 – All rights reserved

independence of the connection ensures the use of existing place identifiers because existing data need
not change their schemas.
However, in many situations, relationships exist between PIs encoded using ISO 19155 and other
identifiers in different encodings, existing independently or outside of the PI architecture. As these
other identifiers are usually features or objects in different encodings, it is not possible for them to be
included in or managed by the PI architecture. In these situations, users may want to simply embed the
connection into their PI. Depending on the encodings of these other identifiers, a set of clearly defined
mechanisms is required to expose and use those other identifiers for PI linking.
This is referred to as a Place Identifier (PI) link (3.1), similarly, a Place Identifier (PI) linking mechanism
(3.2) is a means of defining a Place Identifier (PI) link.
While one specific linking mechanism is not suitable for all other identifiers in other encodings,
similarities in linking mechanisms exist, such as the use of XLink for XML-based encodings.
The second concept standardized by this document is the structure of data instances for PI Linking.
This document defines how to use existing constructs to enable linking to PIs without modification to
those encoding rules for object identification and reference.
7.2 PI linking directionality
In this document, all links are directional. In cases where a bi-directional link is required, two distinct
directional links shall be made, one in each direction.
/req/linking_directionality/direction- where a bi-directional link is required, two distinct
al_links directional links shall be made, one in each direction
A combination of directionality and encoding format limits the applicability of this document, as shown
in the following cases:
a) if both identifiers are encoded in GML (ISO 19136:2007), the linking is normative;
b) if one identifier is in another encoding, the linking from a PI to that identifier is normative;
c) if one identifier is in another encoding, the linking from that identifier to a PI is informative.
In Figure 3, the direction of the link between a PI encoded using ISO 19155 and an identifier in another
encoding is shown using an arrow representing the directionality of the link. In addition, normative
linking is shown using a solid line and informative linking is shown using a dashed line.
The normative linking mechanisms are:
a) gml:id: for linking between a PI and another identifier encoded in GML, see 8.2.2 and 8.2.3;
b) UUID or URL: for linking from a PI to an identifier in another encoding, see 8.3 and 8.4, respectively.
Figure 3 depicts linking to PIs from IFC(BIM) and SVG using IfcDocumentReference and rdf: resource
as linking mechanisms. These mechanisms are shown as informative because those encodings are not
being modified by this document.
Key
encoding rules (examples in this document)
other encoding rules
linking methods (normative)
linking methods (informative)
ISO 19155-2 scope (normative content)
ISO 19155 (normative PI encoding)
Figure 3 — Overview of content in ISO 19155-2
6 © ISO 2017 – All rights reserved

7.3 PI linking model
The conceptual model in Figure 4 defines the structure of PI linking.
The PI_PlaceIdentifier class defined in this document is an extended data type for the Place Identifier
which enables users to connect a PI directly to other PIs.
The PI_PlaceIdentifier class also has a mandatory association “Linking” in addition to the derived three
attributes. The association “Linking” is a mechanism for connecting PIs which identify the same place,
where the connection is directly embedded into the PI.
NOTE While “PI_MatchedPISet” defined in ISO 19155 has an association to connect more than two “PI_
PlaceIdentifier”s which identify the same place, “PI_PlaceIdentifier” defined in ISO 19155 itself does not have a
property to connect other “PI_PlaceIdentifier”s.
Figure 4 — PI linking model
8 PI linking mechanisms
8.1 Overview
This document defines three mechanisms for linking Place Identifiers to features or objects existing in
other encodings. These PI linking mechanisms are enabled using the following information technologies
for object identification and reference:
— using a gml:id linking both from and to a PI, see 8.2;
— using a Universally Unique Identifier (UUID) linking from a PI, see 8.3;
— using a Uniform Resource Locator (URL) linking from a PI, see 8.4.
When using the linking mechanisms defined in this document, the PIs shall be encoded using ISO 19136,
which is the recommended normative encoding option, as specified in ISO 19155:2012, Annex B.
/req/linking_mechanisms/encoding existing Place Identifiers shall be encoded using ISO 19136
as specified in ISO 19155:2012, Annex B
Implementation requirements are specified in 8.2, 8.3, and 8.4.
The encodings presented in this document are suitable for linking with a PI but do not represent a
conclusive list and other encodings may also be suitable for linking with a PI using one of the defined
mechanisms. Linkages may also exist directly between multiple encodings and can make use of a PI.
For the purpose of this document, linkages not using a PI are out of scope.
Use case examples of PI linking are presented in Annex F.
The PI architecture and PI linking concepts have implications for use in the semantic web and linked
data. To provide additional descriptions, Annex G includes RDF examples for PIs and PI linking.
8.2 Linking mechanism: gml:id
8.2.1 Overview
If gml:id is used, linking is defined as bi-directional, both from and to a PI, and linking Place Identifiers
with other features encoded in GML and related application schema is specified based on the following
content from ISO 19136:2007, 8.1:
Xlink components are used in GML to implement associations between objects by reference. GML
property elements (ISO 19136:2007, 7.2.3) may carry XLink attributes, which support the encoding of
an association relationship by reference, the name of the property element denoting the target role in
the association.
/req/linking_mechanisms/GML_ID gml:id shall be used when linking to Place Identifiers
encoded in GML
8.2.2 Linking from a PI
An xlink: href property in a PI instance defines a link that shall be used to point to a discrete GML
instance and the value in that discrete GML instance is referenced by traversing the link.
/req/linking_mechanisms/GML_ID/from- the xlink: href property in a PI instance shall use a
a-PI gml:id to define a link that points to a discrete GML
instance
8.2.3 Linking to a PI
The converse is also defined by this document, where an xlink: href property in a GML instance defines
a link that shall be used to point to a discrete PI instance and the value in that discrete PI instance is
referenced by traversing the link.
/req/linking_mechanisms/ GML_ID/to-a-PI the xlink: href property in a GML instance shall use
a gml:id to define a link that points to a discrete PI
instance
8.2.4 Instance examples using gml:id
Examples showing the use of gml:id for linking between PIs and other GML encoded instances are
presented in Annex B.
GML feature instances conforming to application schemas, such as OGC CityGML (OGC 12-019) and OGC
IndoorGML 1.0 (OGC 14-005r3), may also be linked with a PI, as shown in Annex C.
8 © ISO 2017 – All rights reserved

8.3 Linking mechanism: UUID
If UUID is used, linking is defined in a single direction, from a PI, where a Namespace URN shall conform
to IETF RFC 4122, and a Universally Unique Identifier (UUID) is stored in the attribute of the linkedPI,
as shown in the following example fragment.
…

TokyoInternationalAirport_POI_list.net
Terminal1_Gate15


…
/req/linking_mechanisms/UUID when a UUID is used, the xlink: href property in a PI
instance shall use a UUID to define a link from a PI
that points to a discrete instance of another identifier
Examples showing the use of UUID for linking PIs are presented in Annex D.
8.4 Linking mechanism: URL
If a URL is used, linking is defined in a single direction, from a PI, where a URL conforming to IETF
RFC 1738, is stored in the attribute of the linkedPI, as shown in the following example fragment.
…

gazetteer_tokyo.net
Roppongi


…
/req/linking_mechanisms/URL when a URL is used, the xlink: href property in a PI
instance shall use a URL to define a link from a PI that
points to a discrete instance of another identifier
Examples showing the use of a URL for linking PIs are presented in Annex E.
Annex A
(normative)
Abstract test suite
A.1 Conformance test for PI linking mechanisms
a) Test purpose: Verify a defined PI linking mechanism.
b) Test method: Check that the PI linking mechanism follows requirements specified in this document.
c) Reference: Clause 7.
d) Test type: Basic.
A.2 Conformance test for a PI linking mechanism using gml:id
a) Test purpose: Verify the defined PI linking mechanism using gml:id. Each PI encoded as GML shall
be implemented based on the requirements of this document when it is necessary to have a linkage
between other identifiers in different encoding domains.
b) Test method: Check the document to confirm that the xlink: href attribute is used for the linking
between the other identifiers when the document is encoded as GML and that the xlink: href points
to a gml:id.
c) Reference: 8.2; ISO 19155:2012; ISO 19136:2007, 8.1.
d) Test type: Capability.
A.3 Conformance test for a PI linking mechanism using UUID
a) Test purpose: Verify the defined PI linking mechanism using UUID. Each PI encoded as GML shall be
implemented based on the requirements of this document when it has a linkage between the other
identifiers using a UUID.
b) Test method: Check the document to confirm that the xlink: href attribute is used for linking to
another identifier pointed to by a UUID.
c) Reference: 8.3; ISO 19155:2012; ISO 19136:2007, 8.1.
d) Test type: Capability
A.4 Conformance test for a PI linking mechanism using URL
a) Test purpose: Verify the defined PI linking mechanism using URL. Each PI encoding as GML shall be
implemented based on the requirements of this document when it has a linkage between the other
identifiers using a URL.
b) Test method: Check the document to confirm that the xlink: href attribute is used for linking to
another identifier pointed to by a URL.
c) Reference: 8.4; ISO 19155:2012; ISO 19136:2007, 8.1
d) Test type: Capability
10 © ISO 2017 – All rights reserved

Annex B
(normative)
Encoding using gml:id to link with GML
B.1 Encoding introduction
This annex defines the encoding procedures for using gml:id for linking, as specified in 8.2.
In the following encoded instance fragments, the PIs are encoded using ISO 19136 (GML), the normative
option, specified in ISO 19155:2012, Annex B.
B.3 contains XSD and XML documents of the encoding samples shown in B.2.
All components of the GML schema for PI are defined in the name space with the identifier
" ht t p:// st and ar d s .iso .org/ iso/ 19155/ gpi/ 1 .0", for which the prefix gpi or the default namespace is used
within this document.
B.2 Instance encodings
B.2.1 Link from a PI
As specified in 8.2.2, a PI can be linked to a discrete GML instance using gml:id with the following
constructs.
A PI encoded in GML shall link to a GML feature instance using xlink: href as specified in
ISO 19136:2007, 7.2.3.


park-registry.net
Hibiya Park



In the instance example above, a link is established to the gml:id, as shown in the following fragment,
which is part of a larger GML file


Hibiya Park

If there is another GML point instance which identifies Hibiya Park, the PI_PlaceIdentifier can have a
link to the GML point instance.

park-registry.net
Hibiya Park



In the previous example, the second linked GML instance is also part of a larger GML file, which identifies
the position of Hibiya Park.


srsName="http:// www .opengis .net/ def/ crs/ EPSG/ 0/ 4326">
35.674393, 139.756522

B.2.2 Link to a PI
As specified in 8.2.3, a GML instance can be linked to a PI instance using gml:id with the following
constructs. The default encoding of a link from a GML instance to a PI uses a property element defined
in a GML application schema. The property element to link to a PI or PIs should be declared in the GML
application schema.
In the examples below, landmarkName and landmarkPosition both have a linkedPI property
that is used to link to a PI.

Hibiya Park





35.674393, 139.756522




The referred PI instance should be declared in the specified location:


park-registry.net
Hibiya Park

In this case, an attribute for PI linking should be declared in a GML application schema:



gpi: PI _PlaceIdentifier



An entire GML application schema that includes the property for PI linking is shown in B.3.1.
NOTE It can be difficult to add a new property for PI linking to GML application schemas that already exist.
In these cases, the "PI_MatchedPISet" class (ISO 19155:2012, 7.2.5) can be used to connect more than one PI
without needing to modify the already existing GML application schema.
12 © ISO 2017 – All rights reserved

B.3 Sample GML application schema and data
B.3.1 Sample GML application schema
The element which refers to a PI should be declared in a GML application schema to realize PI linking.
A sample GML application schema which contains two features that refer to a PI as one of their
properties is shown in the listing below.
In this example, the feature Park has a geographic name property landmarkName, which has a link to
a PI, and the feature Shop has a geometric property landmarkPosition, which also has a link to a PI.

xmlns: xsd = "http:// www .w3 .org/ 2001/ XMLSchema" targetNamespace="http:// sample .net/ sample"
elementFormDefault="qualified" version="1.0">








































maxOccurs="unbounded">


gpi: PI _PlaceIdentifier







substitutionGroup="gml: AbstractGML"/ >





maxOccurs="unbounded">


gpi: PI _PlaceIdentifier








B.3.2 Sample GML application data
Sample application data based on the GML application schema in B.3.1.

xmlns: xsi = "http:// www .w3 .org/ 2001/ XMLSchema -instance" xmlns: xlink = "http:// www .w3 .org/ 1999/
xlink" xsi: schemaLocation = "http:// sample .net/ sample sample_appschema">


Hibiya Park





Hibiya Restaurant
+81-0-1234-5678



35.674393, 139.756522

















35.673824 139.756163




35.6733163 139.7577562
35.6746498 139.7587111
35.6766972 139.7551579
35.6723144 139.7530255
14 © ISO 2017 – All rights reserved

35.6710583 139.7559275
35.6733163 139.7577562




Annex C
(normative)
Encoding using gml:id to link with GML application schemas
C.1 Linking with CityGML
C.1.1 General
CityGML is a model for the representation of 3D objects commonly found in an urban environment.
CityGML is implemented as an application schema of GML 3. The current release of CityGML is revision
2.0 (12-019, April 2012). Work on CityGML is managed through the Open Geospatial Consortium.
CityGML supports different Levels of Detail (LOD). As specified in CityGML (12-019:6.2 Multi-scale
modelling):
LODs are required to reflect independent data collection processes with differing application
requirements. Further, LODs facilitate efficient visualisation and data analysis. In a CityGML dataset,
the same object may be represented in different LOD simultaneously, enabling the analysis and
visualisation of the same object with regard to different degrees of resolution.
Furthermore, two CityGML data sets containing the same object in different LOD may be combined and
integrated. However, it will be within the responsibility of the user or application to make sure objects
in different LODs refer to the same real-world object.
The coarsest level LOD0 is essentially a two and a half d
...