CEN TC 287 Date: 1998-07 CR 287002:1998 CEN TC 287 Secretariat: AFNOR Geographic Information Fundamentals Overview Geoinformation Übersicht Information géographique Vue d'ensemble ICS: Descriptors: Document type: CEN Report Document subtype: Document stage: Publication / (?) Document language: E \\MAGNOLIA\UAN\dms\dtic\xdom370\en\CR 287002 (E).doc STD Version 1.0
Contents 1 Scope... 5 2 References... 5 3 Definitions... 6 4 Abbreviations... 7 5 Standardisation in the field of geographic information... 7 5.1 Objectives... 7 5.2 Principles... 7 5.3 Work programme of CEN/TC 287... 8 6 Fundamentals... 8 6.1 Reference model... 8 6.2 Terminology... 9 7 Data description... 9 7.1 Techniques... 9 7.2 Geometry... 10 7.3 Quality... 10 7.4 Metadata... 11 7.5 Transfer... 11 7.6 Rules for application schemas... 11 8 Referencing... 12 8.1 Position... 12 8.2 Spatial referencing... 12 8.3 Time... 13 9 Processing... 13 2
Foreword This document has been prepared by CEN /TC 287, "Geographic Information". 3
Introduction Geographic information concerns objects and phenomena directly or indirectly related to a location on or near the surface of the Earth. Geographic Information Systems (GIS) are computer systems for the handling of geographic information. There is widespread recognition among users of information technology that indexing data by location is a fundamental way to organise and to use digital data. Digital data from a wide variety of sources is being referenced to locations for use in a diversity of applications including administration, the provision of utility and other services, the environment, transport, demographics and defence. Consequently, there is an increasing need for geographic information standards. These are needed to enable universal usage of digital geographic information, to enhance the ability to integrate geographic information with other digital information and applications, and to incorporate geoprocessing functionality into a broad spectrum of existing and emerging information technologies. In general, the underlying principle of such standards is to use formal description techniques for describing the information, data transformations and service interfaces. The standards foresee a new generation of GIS providing various data encodings and transformations, based on end-user demand described by formal models. 4
1 Scope This CEN Report provides an overview of the family of European Prestandards (ENVs) for geographic information. It describes the areas of application to which those ENVs apply, and explains the overall context within which the family will operate. This family of ENVs provides a means to represent geographic information, along with the necessary mechanisms and definitions to enable geographic data to be used by different computer systems, in different environments and for different applications. The following are within scope of this CEN Report : - an overview of this family of ENVs ; - the structure of this family of ENVs ; - an overview of data description methods including the EXPRESS data specification language ; - an overview of how geographic data is referenced with respect to position and time ; - an overview of definition of processing carried out on geographic data. The scopes of the other ENVs in this family are defined within those individual ENVs. 2 References This CEN Report incorporates by dated or undated reference, provisions from other publications. These normative references are cited at the appropriate places in the text and the publications are listed hereafter. For dated references, subsequent amendments to or revisions of any of these publications apply to this CEN Report only when incorporated in it by amendment or revision. For undated references the latest edition of the publication referred to applies. ENV 12009:1997, Geographic Information Reference Model. ENV 12160:1997, Geographic Information Data description Spatial schema. ENV 12656:1998, Geographic Information Data description Quality. ENV 12657:1998, Geographic Information Data description Metadata. ENV 12658:1998, Geographic Information Data description Transfer. ENV 12661:1998, Geographic Information Referencing Geographic identifiers. ENV 12762:1998, Geographic Information Referencing Direct position. ENV 287006:1998, Geographic Information Data description - Rules for application schema. CR 12660:1998, Geographic Information Processing Query and Update : spatial aspects. CR 287003:1998, Geographic Information Vocabulary. CR 287005:1996, Geographic Information - Data description Conceptual schema language. ENV ISO 10303-11, Information automation systems Product data representation and exchange Part 11 : Description methods : The EXPRESS language reference manual. 5
3 Definitions For the purposes of this CEN Report, the following definitions apply : 3.1 application schema conceptual schema for a specific field of interest within the field of geographic information [ENV 12009] 3.2 conceptual schema result from a conceptual modelling of geographic data [ENV 12009] 3.3 gazetteer directory of instances of locations [ENV 12661] 3.4 geographic identifier unique identifier for a location [ENV 12261] 3.5 geographic information information concerning phenomena directly or indirectly associated with a location relative to the surface of the Earth [ENV 12009] 3.6 graphical notation formal language using graphical symbols [CR 287005] 3.7 lexical language formal language using words and mathematical symbols [CR 287005] 3.8 location identifiable part of the real world [ENV 12661] 3.9 metadata data about a geographic dataset or geographic datasets [ENV 12657] 6
3.10 quality totality of characteristics of a product that bear on its ability to satisfy stated or implied needs [ENV 12656] 3.11 query operation to select and retrieve data from a database without changing their contents [CR 12660] 3.12 spatial reference label or alphanumeric code that identifies a location 3.13 transfer move from one location to another 3.14 update operation to amend or replace data in a dataset [CR 12660] 4 Abbreviations EDI SDAI SQL STEP Electronic Data Interchange Standard Data Access Interface Structured Query Language Standard for Technical Exchange of Product data 5 Standardisation in the field of geographic information 5.1 Objectives Geographic information concerns objects and phenomena directly or indirectly associated with a location relative to the surface of the Earth. The basic objective of standardisation in this field is to enable geographic information to be accessed by different users, applications and systems, and from different locations. This requires a standard way of defining and describing this information, a standard method for structuring and encoding it, and a standard way of accessing, transferring and updating it via geographic information processing and communication functions, independent of any particular computer system. Standardisation enables consistent implementations across multiple applications and systems, but permits different implementation technologies to be used for storing data in computer systems. 5.2 Principles The Geographic Information European Prestandards are intended for use by suppliers of data and developers of systems and applications as well as users. They are based on existing information systems standards and methodologies, especially those relating to open systems interconnection. To enable databases and applications that are different in structure, form and content to interconnect and inter-operate, a distinction is made between internal and external applicability of this standard, with the major emphasis placed on the external aspects. Additional benefits may be gained from application of this standard internally to databases and applications. 7
5.3 Work programme of CEN/TC 287 The work programme of TC 287 is divided into four main parts: fundamentals, data description, referencing and processing. Under the main heading of fundamentals, there are three work items : Work Item 287001 : Fundamentals - Reference Model Work Item 287002 : Fundamentals - Overview Work Item 287003 : Fundamentals - Definitions Under the main heading of data description, there are five work items describing and defining geographic data : Work Item 287006 : Data description - Rules for application schemas Work Item 287007 : Data description - Geometry Work Item 287008 : Data description - Quality Work Item 287009 : Data description - Metadata Work Item 287010 : Data description - Transfer Under the main heading of referencing, there are three work items : Work Item 287011 : Referencing - Position Work Item 287012 : Referencing - Time Work Item 287014 : Referencing - Indirect positioning systems Under the main heading of processing there is one work item : Work Item 287013 : Processing - Query and Update 6 Fundamentals 6.1 Reference model ENV 12009:1997 describes the basis of the whole area of standardisation for geographic information, stating the main items of interest and how they interrelate. It identifies and defines a structured set of concepts and components enabling the definition, description, querying and updating, and the transfer of geographic information and information about geographic information. A reference model is a framework defining and explaining some of the important concepts and components, and giving a unified generic vision, the key ideas and scope of something. In the standards arena, a reference model is often used as a broad map on which more specific standards (and lack of standards) can be positioned and compared. The reference model for geographic information provides a framework for development of standards in this field, and in particular it: - describes the basic concepts of the field of geographic information ; - defines the specific items subject to standardisation ; 8
- shows how these fit together ; - allows new components to be added to extend the field of geographic information. The standard is based on established information technology basics such as the conceptual schema, the threeschema architecture, open edi, open systems interconnection, the data management reference models, the information resource dictionary system (IRDS) framework, and the levelling and layering concepts commonly used in the information technology world. However, this standard does not prescribe any specific standard, serve as an implementation specification for systems nor does it serve as a basis for appraising the conformance of implementations. 6.2 Terminology CR 287003 contains all terms and definitions used in the Geographic Information European Prestandards. It establishes a harmonised concept system that will aid the development of standards for the representation of geographic information, along with the necessary mechanisms and definitions to enable geographic data to be transferred. It is applicable to any standard relating to geographic information. Each individual standard contains its own relevant definitions. 7 Data description 7.1 Techniques Data description has a central role in the communication of data whether this happens within an information system or between information systems. The data description specifies the structure of data to be transferred and thus overcomes different ideas or expectations regarding the semantics of the data. Whether communication takes place between humans or computers, the aim of data description is to ensure both common understanding of the content and automatic translation between different representations. Common methods for describing any data are required. The approach of employing a data description language provides flexibility compared with the traditional way of prescribing a fixed data format. Data described in a standardised way can be handled by generic tools which operate on the descriptions. The power of standardized description techniques becomes evident when more than one dataset is accessed in a communication environment. The data description process is termed modelling. This includes describing and defining data contents as well as structures and rules applicable to data. The result of the data description process is called a schema. Modelling at any specific level of abstraction takes place on the basis of a level specific formalism. The description techniques used for presentation shall be compatible with the formalism used in modelling. A lexical language which ensures consistency, avoids ambiguity and allows computer processing of the descriptions and processing of data based on the descriptions is required. A generic language ensures that the variety of applications involving geographic information is supported. EXPRESS as defined in ENV ISO 10303-11:1994 is used to describe geographic information in conceptual schema. The conceptual language EXPRESS has been developed for the purpose of information modelling and description as part of the STEP standard which supports the computer-interpretable representation and transfer of product data. The objective of EXPRESS is to provide a neutral mechanism capable of describing data throughout the life cycle of a product, independent of any particular system implementation. 9
The main components and features of EXPRESS are : - entity data types, including subtypes and (multiple) supertypes ; - attributes, including derived attributes - uniqueness and domain rules can be defined for attributes ; - relationships, appearing as 1:n relationships except in aggregations ; - global rules to specify constraints that have a wider scope than one entity ; - data types, which are grouped in simple data types, aggregation data types, constructed data types and generalised data types ; - functions and procedures, including built-in functions and built-in procedures. NOTE An evaluation of languages suitable to describe geographic information at the conceptual level is given in CR 287005. 7.2 Geometry ENV 12160 establishes the principles for defining geometry sub-schemas. The geometry sub-schema specifies the primitives and their constructs for representing geometry and topology of geographic objects. The geometry of an object is represented by geometric primitives. An object may be modelled as a geometric primitive such as a point, a line, a face or a volume in a two or three dimensional space. The topology or relative position of objects is represented by structure primitives. The space may be divided into different tessellations with similar geometric primitives such as grid-cells, triangles, hexagons or cubes which are then handled as objects. Alternatively, space can be divided into geometric primitives homogeneous to their properties, so that objects can be the result of analysis. Geometric primitives have topology when they are related to each other. Lines may have a shared node, faces may have a shared edge, grid-cells may have neighbours. The geometric primitives and topological associations are an important part of the formalism of modelling. The geometry concepts defined in ENV 12160 are as follows : - points, lines, arcs (in particular circular arcs, bsplines, clotoides) and surfaces ; - shortest ways between two given points in a surface ; - pixel, raster band, grid. The topological concepts defined in ENV 12160 are : - nodes, edges and faces. Outside the scope of ENV 12160 are higher dimensional manifolds, e.g. three-dimensional bodies and their volumes, and higher dimensional complexes. 7.3 Quality ENV 12656 establishes the general principles for describing the quality of geographic information. It is concerned with presenting information appropriate for judging the quality of geographic information. This includes details of the derivation and usage of particular datasets and quantitative measures of quality, either against a pre-established scale or relative to similar information. The purpose of describing the quality of geographic information is to allow producers to define how well their data product meets its specification, and for users to define their requirements in the same way. 10
Geographic data is normally created through a process of defining a 'nominal ground' that characterises the required abstraction from real world phenomena. The quality of geographic information is concerned with the performance of a particular set of data against the nominal ground. To judge the usefulness of data for particular requirements, first the specification needs to be considered and second, the actual performance of data against the specification needs to be evaluated. ENV 12656 is limited to providing a model by which the information necessary to judge the quality of geographic information against its specification can be defined, in order to ensure that the data is suitable for its intended purpose. The quality model is designed primarily for use with digital geographic datasets but the principles can also be used to describe geographic information in other forms, such as paper maps and lists. This does not specify how measures of quality shall be made or the quality expected of an individual product. Neither does it include techniques for quality control or assessment, nor concern itself with quality systems or management. However, it provides a mechanism for maintaining quality information for quality management systems, although it does not provide any mechanism for managing quality itself. 7.4 Metadata ENV 12657 defines a conceptual schema for metadata for geographic datasets. Metadata is data about datasets. It includes information about the content, representation, extent (both geometric and temporal), spatial reference system, quality and administration of the dataset. ENV 12657 identifies those items that are mandatory for describing geographic datasets - the minimum set of metadata. It gives examples of how the standard may be applied but does not concern itself with the construction of databases for holding metadata. It is designed primarily for use with digital geographic datasets, but the principles can also be used to describe geographic information in other forms such as paper maps or lists. Metadata describes the structuring of geographic data to support the searching and ordering of the data. It allows producers of datasets to describe them in a consistent way and enables users to assess availability and suitability for their own particular purposes. Lack of awareness about existing datasets can lead to geographic information being under used or mis-used. 7.5 Transfer ENV 12658 defines the transfer schemas, implementation mechanisms and encoding rules for the transfer of geographic data. It allows users to transfer spatial and non-spatial components of geographic information, together with their data dictionary and metadata. The data dictionary contains a formal description of the application schema for the data being transferred. This data dictionary complements the description of the data that exists in the metadata as defined in the metadata standard. A large number of transfer formats exist, offering varying levels of applicability and ease of use. All have limitations in the data structures supported, and they all require the recipient of the data to have additional knowledge about the data being transferred. The first of these limitations is reduced if there is a mechanism to allow users to define the data structure of their application. The second is removed if a description of the data structure is included with the data being transferred. To allow geographic data to be re-used in different application fields and to maximise its use, standard methods of transferring, encoding and describing the data are needed. ENV 12658 contains conceptual schema for encoding an application schema into the data dictionary, encoding data into an exchange structure and incorporating external files. It also contains examples of its use. 7.6 Rules for application schemas ENV 287006 defines the rules about the use of the data description techniques for developing application schemas for geographic information. To aid the description of specific applications and to ensure that such development does not create unnecessary conflicts or restrictions, it is necessary to create rules to ensure that the best use is made of standards, in any application. The rules are not able to cover all aspects of the applications schemas which could be used, but there are core elements of most applications which can be standardised. 11
This standard describes how the user can define entities and their properties in an application schema. It also explains how to integrate the standardized schemas (spatial, quality, position, geographic identifiers) with the application-dependent schemas (semantic, quality constraint) into a full application schema. 8 Referencing 8.1 Position ENV 12762 defines the basic concepts related to position information based on coordinates and defines how that position information may be described and identified. The choice of any particular system is outside the scope of ENV 12762. There are many different ways by which position can be defined. Some are based on coordinates, and a standardised method is required for the description of locational reference systems. A planar Cartesian reference system, a three dimensional Cartesian reference system, geographical coordinate systems or an astronomical reference system may be used as reference system. There is a need to create a list of existing geodetic reference system (ellipsoid, datum, projection, etc) with a unique identifier, in order to make it possible to pass that identifier in a dataset to be transferred without the parameters themselves. ENV 12762 will enable geodetic referencing systems to be described in such a manner that data can be transformed from one reference system to another, enabling integration of datasets. 8.2 Spatial referencing ENV 12661 describes methods of documenting and disseminating systems for spatial referencing using geographic identifiers. In many real-world applications, position is referenced not by coordinates, but by a geographic identifier, which is a label which identifies a real-world object. Examples are the names of monument points (e.g. The Eiffel Tower), street names and town names. Such spatial references are often termed "indirect", but are more correctly termed "non-coordinate", in that they are not based explicitly on coordinates. A coordinate reference can be created by assigning coordinates to a representation of the object (as point, line or area) or by defining the boundary of an area by a sequence of coordinates. Whilst a spatial reference can usually be mapped onto a coordinate system to define a precisely defined position, it may refer to a fuzzy object which can be only approximately defined (e.g. a hill). Some spatial references identify physical objects, by means of a label which may require additional structured information to make it unique. Others identify arbitrary locations, against which data may be referenced by means of a coding scheme (e.g. postcodes and census enumeration districts). Such location classes are usually hierarchically structured so that data may be aggregated by larger location classes. Spatial references depend on an agreed method of enumeration, and it is not possible for an independent observer to establish the spatial reference of an object by the application of a scientific method of measurement. The identifier must be retrieved from some authoritative source such as a signpost, nameplate or an official list (gazetteer). ENV 12661 specifies how systems of spatial references using geographic identifiers are described, and defines the contents of a gazetteer of locations. It enables producers of data to define locations in a consistent manner, and assists users to understand the spatial references used in datasets. It also enables gazetteers of instances of locations to be constructed in a consistent manner. 12
8.3 Time Time identifies ways in which the temporal dimension of geographic data may be handled. An object may be dynamic, changing its position, shape or other properties. Much geographic information changes with time, and there is currently no commonly identified way by which the temporal dimension can be handled. It manifests itself in different ways in different datasets and applications. Some data has a time stamp. Some contains a continuous time series of values. Some has a life history based on discrete events. In the beginning, it is created or observed, during its lifetime an object my have some changes resulting in versions, and in the end it can cease to exist. Some data is subject to continuous update and requires a capability to roll backward and forward through time. NOTE This work item has now been suspended in CEN/TC 287. The result of the work of ISO/TC 211 on this topic will be considered for implementation as an EN. 9 Processing CR 12660 defines the requirements for query and update of geographic information, and identifies the spatial operators that a standard data manipulation language may use to process geographic information. It also identifies the minimum requirements that the server and client shall agree to ensure that the integrity of the client database is maintained. Every geographic information system and every database has some way of querying and updating the data. However, existing standards do not fully cater for the requirements of geographic data. CR 12660 defines the necessary additional facilities to access and maintain geographic data, consistent with established standards for non-geographic data. This may aid the development of the additional functionality, and enable users to anticipate its arrival. 13