Open Geospatial Consortium
Submission Date: <2014-11-12
Approval Date:n/a
Publication Date:2014-11-12
External identifier of this OGC® document:http://www.opengis.net/def/doc-type/standard/1.0
Internal reference number of this OGC® document:14-107
Version: 1.0
Category: OGC® Discussion Paper
Editors:Peter Broßeit, Matthias Müller
A Conceptual Model and Text-based Notation for Temporal Geometry
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This document is not an OGC Standard. This document is distributed for review and comment. This document is subject to change without notice and may not be referred to as an OGC Standard.
Document type: OGC®Discussion Paper
Document subtype: n/a
Document stage: Draft
Document language: English
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Contents
1. Introduction 5
2. Scope 5
3. References 6
4. Terms and Definitions 7
5. Conventions 7
5.1 Abbreviations 7
6. Simple Temporal Geometry 7
6.1 Geometric elements 8
6.1.1 Points in time and temporal intervals 8
6.1.2 Instant 9
6.1.3 Period 9
6.1.4 Multi-instant 9
6.1.5 Multi-period 9
6.1.6 Duration 9
6.1.7 Regular multi-instant 10
6.1.8 Regular multi-period 10
6.2 Temporal references systems 10
6.3 Topological relations 11
7. Text Representation for Temporal Geometry 12
7.1 BNF Introduction 12
7.2 General Structure of the Text Representation 12
7.3 Time Representations in Calendar Reference Systems 13
7.4 Time Representations in Temporal Coordinate Systems 14
7.5 Time Representations in Ordinal Reference System 15
7.6 Representation of Temporal Reference Systems 16
7.7 Qualified Time Representations 17
8. Examples and use cases 18
8.1 Extended temporal references for WMS layers 18
8.1.1 Annual rainfall statistics 18
8.1.2 Historical maps: Valid times 18
8.1.3 Maps at the geological time scale 18
8.1.4 Map overlay with different temporal reference systems 19
8.1.5 Complex temporal references: Monthly means in a climatology dataset 19
8.2 Metadata records: temporal extent and sparse data sets 19
8.3 Spatio-temporal databases 20
8.3.1 Example 1: Fruit markets – opening times 20
8.3.2 Example 2: Occurrences of forest fires per administration unit 20
8.4 Time series data – extent, sampling interval and resolution 20
Annex: Discussion of issues with current time encodings 22
A.1 Issues with GML time 22
A.2 Issues with ISO 8601 22
A.3 Issues with ISO 19108 23
A.4 Issues with WMS time 23
i. Abstract
This document addresses the representation of temporal information in the geospatial data encoding and standards and applications. It defines a conceptual model for temporal geometry primitives and complexes as well as a text-based encoding for the defined temporal types. As a supplement, shortcomings of existing standards and practices are discussed.
ii. Keywords
The following are keywords to be used by search engines and document catalogues.-
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):
Technische Universität Dresden
v. Submitters
All questions regarding this submission should be directed to the editor or the submitters:
Name / AffiliationPeter Broßeit / Technische Universität Dresden
Matthias Müller / Technische Universität Dresden
vi. Revision history
Date / Release / Author / Description2014-06-10 / Peter Broßeit, Matthias Mueller / Initial draft
2014-06-19 / Peter Broßeit, Matthias Mueller / Revised draft reflecting discussions after Geneva TC
2014-11-10 / Matthias Mueller, Peter Broßeit / Rewritten BNF encoding to comply with BNF productions used by OGC 12-063r4, added examples, provided extended discussion of issues with existing standards.
2014-03-12 / Chris Little / Corrections and issues according to the discussion at the Tokyo TC meeting. Editorial corrections.
2015-01-30 / Matthias Mueller / Corrections to BNF productions for qualified time. Corrected Examples, added missing references.
1. Introduction
This document defines a conceptual model for temporal geometry and a corresponding a text-based encoding. Its purpose is to stimulate a debate about the temporal schema defined in ISO 19108, the GML encoding of time and the widely used encoding standard ISO 8601. As described in Annex A, the application of the cited standards may lead to ambiguous representations of temporal object and provide insufficient support for periodic and composite temporal geometries.
2. Scope
The idea is inspired by the concept of the Well-known Text Representation for Geometry as defined in OGC 06-103r4[1] which is built on a sound conceptual model and provides a lean text-based encoding of spatial geometry. The objectives of this document are
· to provide a conceptual UML model for temporal geometries (similar to temporal objects defined by ISO 19108) that allows for temporal geometry complexes,
· to provide a text-based notation for the conceptual model that is largely compatible with the ISO 8601 extended date/time format, including the precision aspect,
· to specify or collate extensions to ISO 8601 for complex temporal objects,
· to specify or collate extensions to ISO 8601 for temporal coordinate systems and ordinal systems, and
· the discussion of issues that arise with current time encodings used in conjunction with OGC standards.
This discussion paper intends to contribute to future revisions of ISO 19108 and GML to provide (1) a consistent representation of time and (2) the ability to express temporal geometry complexes. For a brief review of current shortcomings in these standards, see Annex 1.
Future work should clarify the appropriate handling of time zone references for calendar dates. The current version of ISO 8601 does not provide time zones for dates but only for combined date-time statements. This causes unnecessary uncertainty in date representations since the exact start and end time of a particular calendar day depend on the time zone.
This issue is not resolved in this paper. A discussion, including possible solutions can be found in section 7.3. A final resolution should be provided in a future revision of ISO8601.
Not in scope is the definition of temporal references systems. This document assumes the existence of well-defined calendar / time systems, temporal coordinate systems, and ordinal time systems. The definition and maintenance of such reference systems would be subject to other resources. The provided temporal reference system names serve for illustration purposes only.
3. References
The following normative documents contain provisions that, through reference in this text, constitute provisions of this document. For dated references, subsequent amendments to, or revisions of, any of these publications do not apply. For undated references, the latest edition of the normative document referred to applies.
ISO 8601:2004, Data elements and interchange formats - Information interchange - Representation of dates and times.
ISO 19108:2005: Geographic information - Temporal schema.
OGC 12-063r4: Geographic information - Well known text representation of coordinate reference systems, version 0.11.4 (2014-04-16). (Alternative name: ISO/DIS 19162)
OGC 07-036: OpenGIS® Geography Markup Language (GML) Encoding Standard, version 3.2.1.
OGC 06-121r8: OGC Web Services Common Standard, version 1.2.0.
OGC 06-042: OpenGIS Web Map Server Implementation Specification, version 1.3.0.
Allen, J. F. (1983): Maintaining knowledge about temporal intervals. In: Communications of the ACM 26(11): 832–843.
Denis, P., Muller, P. (2010): Comparison of different Algebras for inducing the temporal structure of texts, COLING '10 Proceedings of the 23rd International Conference on Computational Linguistics, pp 250-258.
4. Terms and Definitions
This document uses the terms defined in Sub-clause 5.3 of [OGC 06-121r8], which is based on the ISO/IEC Directives, Part 2, Rules for the structure and drafting of International Standards. In particular, the word “shall” (not “must”) is the verb form used to indicate a requirement to be strictly followed to conform to this standard.
5. Conventions
This sections provides details and examples for any conventions used in the document. Examples of conventions are symbols, abbreviations, use of XML schema, or special notes regarding how to read the document.
5.1 Abbreviations
BNF Backus-Naur Form
GML Geography Markup Language
WMS Web Map Service
XML Extensible Markup Language
6. Simple Temporal Geometry
Basic entities for modelling time of geographic features are specified in ISO 19108, clause 5.2.3 as temporal geometry objects which serve the modelling of absolute positions in time. These geometric object types are TM_Instant and TM_Period, which conduce to the modelling of points and intervals in time and can be used with different types of temporal reference systems.
This section re-defines the ISO 19108 definitions of instants and periods in a way that is compatible with ISO 8601, by allowing instants with reduced precision. Based on these primitives, four additional geometry types are defined which are frequently encountered in practice. MultiInstant and MultiPeriod may be used to describe congruent states of an object at multiple times. RegularMultiInstant and RegularMultiPeriod may be used to express events and state changes (such as shop opening times or sampling intervals) that recur on a regular basis.
6.1 Geometric elements
6.1.1 Points in time and temporal intervals
It is assumed that time is defined on continuous axis. Similarly to point and line geometries in space, point in time and temporal intervals can be distinguished on the temporal axis. The set theoretic distinction between both types is very clear.
A point in time has a topological dimension of 0; it has an interior of one point, an empty boundary set, and an exterior of all other points.
A temporal interval has a topological dimension of 1; it has a boundary of two points (begin and end), an interior of all points that lie after start and before end, and an exterior of all other points.