METADATA AND GIS. A Classification of Metadata for GIS. Stéphane COULONDRE, Thérèse LIBOUREL and Laurent SPÉRY

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1 METADATA AND GIS A Classification of Metadata for GIS Stéphane COULONDRE, Thérèse LIBOUREL and Laurent SPÉRY The notion of metadata is more and more present in information systems. Metadata is usually defined as data over data. This notion is also used within geographic information, but in a more classical way. Thus, in the case of interoperability between GIS, metadata should be used as in other information systems. As a result, and since it seems to us that semantics of the medata concept are diverse and not always well-defined, we propose in this paper, within the framework of GIS, a classification of metadata based on abstraction levels of perception. This should lead to a classification of the use of metadata. KEYWORDS: Metadata, Interoperability, Classification, Data transfer, Ontology. INTRODUCTION Generally speaking, information systems represent and store data and processes. However, these last years the notion of metadata has emerged. Metadata is defined in straightforward terms as data over data. Actually, it can give complex informations concerning structure description, semantics and contents of data items, their associated processes and more widely the studied domain of these various informations. Geographical information systems also obey these rules and use more and more this notion of metadata. Various works on standardization for GIS include a metadata section. In particular, they devote themselves to describing structure and contents of metadata. GIS tools have taken on a huge development, and metadata production and exploitation tools have been progressively proposed as a complement. In the section "Use of metadata in GIS", we carry out a quick survey on metadata within GIS, as well as its common use. However, considering the development of open systems, we think that metadata can play a more important part, which we explain in the section "Metadata and interoperability". As the semantics of the word "metadata" is various and not well-defined, we propose in the last section a classification of metadata based on different abstraction levels, that should lead to a classification of the use of metadata. USE OF METADATA IN GIS Nowadays, metadata usage is very widespread. In the first subsection, we see how metadata is defined, and in which context it is used. In the second subsection, we focus on several metadata standards. Definition and usage of metadata Metadata is often defined as data about data. The International Organization for Standardization (ISO) proposes several closely related definitions [14]: data describing and documenting data, data about datasets and usage aspects of it, data about the content, quality, condition, and other characteristics of data. This definition is similar to that proposed by [4]. In the context of these definitions, the documenting role of metadata is fundamental. This information must give decision elements to the user in order to choose the most appropriate dataset. Indeed, as stated in [17], in the case of large amounts of data, it is difficult to analyse data content in a straight way. Metadata then gives appreciation or description elements of the information in the dataset. It is held in catalogues, as presented in [19] or [11] which reference the datasets. Some metadata browsing tools have been developed [22]. However, metadata role is not restricted to documenting information. For the Federal Geographic Data Committee (FGDC), metadata must also allow: data acquisition and transformation which are complex steps for a producer. Metadata can, on one hand, represent the production memory by describing operations carried out during the acquisition and transformation process, and it can, on the other hand, prevent a producer from repeating the production step of an already existing dataset, description of structure and role of metadata, in order to allow its interpretation and treatment by a user, especially during transfer steps. These two goals are seldom reached. Metadata is most of the time only documenting information. Moreover, the user must look himself in the metadata set for information that may be useful for him. Indeed,

2 metedata is defined independently of the user needs. For metadata defined by the FGDC, Foresman and al. [9] claims that this set if hard to maintain and to use for a non-experienced user. Now that metadata role and usage are defined, we examine metadata content in the framework of geographic information. Metadata content Metadata definition is subject to a lot standardization work in the context of geographic information (e.g. FGDC, ISO, etc.). These standards precisely define information contained in metadata. The standard of the FGDC [4] presents a classification of metadata in 7 kinds, which are gradually refined. Standards defined by ISO, European Commitee for Standardization (CEN) TC 287 or OpenGIS Consortium (Open GIS (OGIS) is a trademark of the Open GIS Consortium. We use OGIS for Open GIS Consortium) partially agree on the contents and definitions of metadata proposed by the FGDC. Generally speaking, metadata contains the following: Identification. General information related to a dataset is specified. It consists for example in indentifying the producer, keywords, etc. Data quality. Quality description of data in a specified dataset. Quality appreciation of data is based on quality indicators. Positional accuracy, lineage, etc. are included. Spatial Data organization. This information specifies geometry storage description for geographical entities exchanged. Spatial Reference. The description of the reference frame and the means to encode coordinates in the data set. Entity and attribute. This information are devoted to describing exchanged data structure. Data types, atttribute types and relation types are detailed within metadata. This kind of metadata defines the schema of exchanged data. Distribution information. This information is related to data distribution. Producer identification, terms of sale, etc. are included. Metadata reference information. This metadata describes the specifications for the above kinds of metadata (i.e. the standard used), the organization that defines them, their possible usage (i.e. wether they are accessible by all the users), etc. These standards have some characteristics depending upon their possible use. We briefly describe them. FGDC [4]: This standard does not specify the internal organization of metadata (i.e. it does not give a schema that defines metadata structure). Metadata proposed are described above. This information is defined independently of users needs and priorities. CEN TC 287 [8]: This standard, that can be considered as recommendations, defines a schema which gives a structure of metadata. This schema is indenpendant of exchange standards and thus can fit to national standards. ISO [15]: this standard is devoted to defining a common set of metadata terminology, definitions, and extension procedures. This standard is based on [4]. Nevertheless, this document should be developed. Some differences may appear between the ISO and the FGDC recommendations. OpenGIS Consortium (OGC) [5]: the OGC does not exactly define the metadata content. The OGC conforms to the standards defined by the ISO and the FGDC for the definition of metadata and its implementation specifications. However, the OGC specifies a "metadata abstract model". This model defines the possible relationships between metadata and data, as well as metadata organization. The metadata content standardization has promoted the development of catalogues in the context of geographical information. Other domains and communities also use metadata for documenting purposes. It is the case of document retrieval [1] or data exchange between museums [3]. They use a similar approach for their metadata standards proposals. METADATA AND INTEROPERABILITY In addition to this common use within the framework of GIS, metadata can also be used in different contexts. In particular, in the interoperability context, we consider two levels of dialogue between two systems. At the logical level, interoperability consists in proposing exchange formats or exchange protocols. The application level gives informations about the nature of the data to be exchanged. The logical level A lot of producers propose geographical datasets. This data is coded according to a specific format. This format is often related to a particular GIS application. It is therefore optimized for this GIS. Before a data exchange, one must be able to translate these various formats. Interoperability between information systems must promote the dialogue between them. Wegner [24] claims that interoperability is based on several mechanisms: Interface standardization: a standard interface between the various applications is imposed, Interface bridging: each system has its own interface and it is necessary to develop conversion tools between all existing kinds of interfaces. Combined approach: the two approaches can be mixed, imposing a "pivot" standard, in which every kind of interface is converted. In the context of geographical information, the direct communication between systems is still rare. However, the interface bridging approach is comparable to the use of translators which allow data translation from a proprietary format to another proprietary format. Several tools offer this kind of services [18]. The combined approach can consist of using a standard exchange format like DIGEST [10]. It must be complemented, in the context of geographical information; with exchange protocols like Corba. At logical level of data exchange, we think that metadata do not play a significant role. Indeed, proposed

3 1: Schema correlation Metadata Identification Spatial Reference Quality Transfer Schema S T Data E C A F R E T I N User Schema S U Dataset 2: Data exchange User GIS Logical level Figure 1. Data transfer standards only contain a reference to the format used and its version. In metadata, there is no description of the formats upon which the "translators" could rely. It is up to the user to set up the necessary tools in order to make the systems communicate. The application level If, at logical level, information exchange is a communication problem between heterogeneous systems, the application level deals with data organization and semantics. The schema describes exchanged data organization. However, geographical data users have different needs. Therefore, their perception of the real world is different. The consequence is that each user can structure its information system according to its needs. Each information system schema is then different. To allow communication between these systems, one must relate data of various schemata, and solve semantic conficts that can appear. In figure 1, the transfer schema ST and the user schema SU are different. However, one must correlate them. This kind of work is very close to schema integration, in database systems. Nevertheless, schema merging technique, that is used for schema integration, does not seem to be useful here because the user is mainly interested in data retrieval. The goal is to look for data in a distant system, by mean of a data transfer, in order to integrate them into a GIS, according to the user-defined schema. Thus the goal is not to unify various viewpoints. On the other hand, we think that building a unified schema seems difficult to achieve if several data sources are used. Indeed, every couple of schemata should be unified, and as Nyerges [20] remarked, some elements of one of the two shemata can stay outside unification, if the user want it to (ex.: the user is not interested in buildings: it does not want to integrate them into its GIS, and this object type is not correlated with the GIS schema) In schema correlation processes, we think that semantics problems are the most difficult to solve. The reasons for a semantic difference have been presented by Batini and al. [2]. In the specific framework of geographical data, Spaccapietra and al. [23] classifies the different kinds of conflicts appearing in the case of geographical data integration. Several alternatives have been proposed in order to solve semantics problems. The most significative ones are: Dictionaries. For Hammer and MacLeod [13], each database has its own local dictionary, called "local lexicon", that contains the semantic description of every kind of object in the database. Between two systems that communicate, a "semantic dictionary" allows comparisons between local dictionaries Ontologies and metadata. Kashyap and Sheth [16] use metadata in order to describe the organization of each data repository. Specific ontologies are used in order to describe each domain. These ontologies help defining the application context. Comparing ontologies, one can solve semantic differences. With these two approaches, in order the communication between different systems to be possible, it is necessary to use a common description language to describe dictionaries or ontologies. We consider that this problem belongs to the logical level. In the more specific framework of geographical information, some work has been undertaken in order to facilitate data exchange. The OGIS [7] proposes a similar approach as the one presented above. Each "geospatial information community" has specific data, which is described in a "catalogue". Data exchange between comunities is based on the use of "semantic translator". Each translator has translation rules from an entity to another one. These translation rules are built up from data description made by each community. This

4 Figure 2. Classification of metadata in 3 levels description is called "project world" [6] and contains meta-information that is necessary to understand and to interpret data. We consider that meta-information content is similar to that of metadata. As for traditional database systems, data exchange problem is linked to the resolution of difficulties arising from logical communication between systems and to the problems of exchanged data semantics and organization. As this data is spatially-referenced, data exchange mechanisms must take into account possible modifications of the spatial reference system and objects geometry. We claim that metadata which is defined in the first section gives this information, and thus it can be used by these mechanisms. However, current standards do not use a common metadata description language. Therefore it is hard to exchange information between different communities. This problem is not adressed in this paper. The metadata standards we presented focus on the contents definition of metadata. However, if one wants to use them more easily within the framework of data exchange, we claim that it is necessary to structure them. That is why we propose in the next section an organization of metadata that must ease communication between systems. A CLASSIFICATION OF METADATA We saw in the first section that metadata defined for geographical information have mainly a documenting role. The information which is given to the user is related to the dataset structure and to its contents. While the data exchange process, we claim that metadata must be organized according to a three-level architecture. These three levels allow description of the studied domain (first level), characteristics of exchanged data (second level) and the geographical information (third level). This architecture is illustrated in figure 2. The first level Metadata describes the necessary elements for a model to be usable. These elements form the concepts of a particular studied domain: the ontology. An ontology defines terms and semantics necessary to interpret these concepts (entities and relations), according to the notions usually accepted within a community. Metadata defines, for each studied domain, a scale that allows to grasp data semantics. For this reason, it represents one of the elementary parts of semantic interoperability between domains. Semantic interoperability between domains can exist only by way of ontology confrontation. It is necessary to establish connections between different ontologies. For this purpose, it is necessary to use a common language in order to describe these ontologies (this is part of the logical level defined above). The problems arising from this abstraction level are nowadays chiefly a matter of artificial intelligence techniques [21]. At present, metadata does not contain ontologies. The domain description is coded in a lexicon. The second level At this level, the domain is predefined. Thus, an ontology has been chosen. We consider two kinds of metadata: Metadata associated with the process of geographic data production (called "Protocol" in figure 2). This metadata describes the production specifications chosed by the producer. Example: a dead-end road is included only if its length is greater than 500 meters. Successive transformations undergone by the data are recorded. This kind of metadata is usually included in lineage quality information. Example: reference to the original aerial

5 photography, reference to the treatments applied, etc. Metadata concerning data organization within the current production (called "schema" in figure 2). For a dataset, the schema defines the geographical entities structure, their attributes, and the semantic relationships between them. This schema is imposed by the producer or can be the result of an agreement between the producer and the user. It can also be subject to standardization. Metadata concerning data organisation also contain the way to represent spatial objects, i.e. how the topology is captured (e.g. spaghetti, etc.). This metadata is called "denotative metadata" by Günther and Voisard [12]. It gives general informations on a set of digital geographic data (called "Digital file"), and is generally valid for several data sets, independently of their content. The third level Metadata represents informations on data contained in the file, but which is perceived at different levels of granularity: global, partial or individual. We defined these three levels in the figure by "Set level", "Category level" and "Object level". These three levels of information are present within a dataset. Example: Information related to the projection are valid for the whole data contained in a dataset. On the other hand, positional accuracy can be estimated to 2 meters for a dataset, while it can be 0,5 meters for a subset formed by objects of which type is building, i.e. can be related to a category of objects. This information is, for the biggest part, concerning quality of exchanged data and is only valid for data contained in a specific dataset. This kind of metadata is called "annotative metadata" [12]. We think that such a hierarchical organization of metadata eases the retrieving and integration processes of external data. For information retrieval, the user often relies on catalogues. According to the needs and the domain knowledge, users do not use metadata in a similar way. The user is not a specialist of the studied domain: Firstly, it gets all the basic notions that allow understanding data semantic (i.e. the signification of the exchanged data). It uses metadata of level 1. Secondly, it is interested in the exchange contents (i.e. which entities are present). This information comes from metadata of level 2. Metadata of level 3 gives information about specific properties of each dataset. The user often uses metadata of level 3 before that of level 2, in order to determine first the available data before worrying about its characteristics. The user know the studied domain: For this kind of user, metadata gives the possibility to compare different datasets. It uses metadata of level 2 and 3. The metadata analysis can be more thorough that for a non-specialist user. This metadata is used for data integration. It describes the dataset contents. We isolate two cases of data integration. Usage of metadata is not identical: Casual integration. Characteristics of the user database are different from exchanged data. Firstly, the user schema is linked to the exchanged data schema. Metadata of level 2 (schema) is used because it describes the organization of data to integrate. Secondly, metadata related to the production process (level 2: protocol) help deciding wether linked entities represent the same part of real world. At this stage, the user can identify the geometric transformations that data to integrate must undergo. Finally, data description can be used in order to isolate a subset of data to integrate, according to several criteria as quality. Metadata of level 3 is then used. Regular integration: the user knows the product he receives. The integration procedure is defined for the first integration (i.e. operations are identical to those of casual integration). In this configuration, metadata of level 1 and 2 do not bring any new information (unless the product has changed). The three-level metadata organization we proposed must help geographical data interpretation and retrieval. Nevertheless, our proposal is subject to further work. The organization must take into account the technical skills differences between users. Moreover, the levels are organized downward: the user must master the concepts defined in metadata of level 1 in order to interpret correctly metadata of level 2 (level 1 gives metainformation on level 2). We must define an interface between these different levels in order to associate a subset of metadata that gives information about a metadata item of lower level. This is the subject of further work. CONCLUSION Beyond the general definition of metadata as "data about data", it is important to define the contents of metadata and to think about this definition like it has been already done for data itself. Its widespread use is conditioned by data structure relevance. We proposed in this paper a classification of metadata which aims at helping a geographical data user. As standards and formats define metadata, its importance may increase, particularly within the framework of data exchange, data update and communication between distributed systems. REFERENCES 1. The Dublin Core Metadata Element Set. URL: 2. Batini, C., Lenzerini, M. and Navathe S. B. A comparative analysis of methodologies for database schema integration. ACM Computing Surveys, Vol. 18, Nr. 4, December 1986, pp Bearman, D. and Perkins J. Standards Framework for the Computer Interchange of Museum Information. SPECTRA, Vol. 20, Nr. 2 and 3, Federal Geospatial Data Committee. 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6 5. Open GIS Consortium. The OpenGIS Specification Model - Topic 11: Metadata, Document Number: r2. 6. Open GIS Consortium. The OpenGIS Specification Model - Topic 5: The OpenGIS Feature, Document Number: Open GIS Consortium. The OpenGIS Guide - Introduction to Interoperable Geoprocessing, Part I of the OGIS, Comité Européen de Normalisation. Geographic Information - Data Description - Metadata, DRAFT. 9. Foresman, T. W., Wiggins H. V. and Porter D. L. Metadata Myth: Misunderstanding the Implications of Federal Metadata Standards, in First IEEE Metadata Conference, Silver Spring Maryland, URL: wiggins/foresman_final.html 10. Digital Geographic Information Working Group. The Digital Geographic Information Exchange Standard. Department of National Defence, Canada, 1.2a edition, Günther, O., Lessing, H. and Swoboda, W. UDK: A European environmental data catalogue in Proc. Third International Conf. on Integrating GIS and Environmental Modeling, URL : ROM/sf_papers/guenther_oliver/my_paper.html. 18. Lazar, B. Break Through Spatial Data Translation Obstacles. GIS World, Vol. 11, Nr. 6, June 1998, pp MEGRIN. Geographical Data Description Directory Version GDDD Data Model. Project Number INF Nyerges, T. L. Schema Integration Analysis for the Development of GIS Databases. Int. Journal of Geographical Information Systems, Vol. 3, Nr.2, 1989 pp Noy, N. F. and Hafner, C. D. The state of art in ontology design. IA Magazine, Fall, 1997, pp Shklar, L., Behrens, C. and Au, E. New approaches to cataloguing, querying and browsing geospatial metadata. In Second IEEE Metadata Conference, September URL: pers/lshklar/lshklar.html 23. Spaccapietra, S., Parent, Ch. and Devogele, T. Analysis of Discrepancies in Spatial Data Representation in Cooperative Database Systems for Advanced Systems (CODAS), Kyoto (Japon), Wegner, P. Interoperability. ACM Computing Surveys, Vol. 28, Nr. 1, 1996, pp Günther O. and Voisard A. Metadata in Geographic and Environmental Data Managment in W. Klas, A. Sheth (eds.) Managing Multimedia Data: Using Metadata to Integrate and Apply Digital Data. McGraw Hill, Hammer, J. and McLeod, D. An approach to resolving semantic heterogeneity in a federation of autonomous heterogeneous database systems. International Journal of Intelligent and Cooperative Information Systems, Vol. 2, Nr. 1, 1993, pp International Organization for Strandardization. Collections of terms form iso/tc 211 Geographic information/geomatics and guidelines for terminology, Working draft. 15. International Organization for Strandardization. Geographic Information-Metadata. ISO Standard , Second Draft. 16. Kashyap, V., and Sheth, A. Semantic Heterogeneity in Global Information Systems: The Role of Metadata, Context and Ontologies in Cooperative Information Systems: Current Trends and Directions. Academic Press, Klas, W. and Sheth, A. Metadata for digital media. SIGMOD Record (ACM Special Interest Group on Management of Data), Vol. 23, Nr. 4, 1994, pp. 19-??.

7 Thérèse LIBOUREL Thérèse Libourel is a senior lecturer in Computer Science at Conservatoire National des Arts et Métiers, and is currently working at Laboratoire d Informatique de Robotique et de Microélectronique de Montpellier. She is a member of research groups GDR I 3 and GDR CASSINI. She has been responsible for the success of several GIS Projects at National (for CNET, the french national telecom research center) and Regional (POLLEN project) Government. Among its interests are object oriented design, reuse of software components, object oriented databases and evolution, time and GIS. Thérèse joined CNAM in Thérèse has a Ph.D. on Computer Science from the University of Montpellier 2. L.I.R.M.M. 161 rue Ada Montpellier Cedex 5 France Tel: +(33) Fax: +(33) URL: Laurent SPÉRY spery@lirmm.fr Laurent Spéry is a Ph.D student in geography at University of Avignon, and is currently working at Laboratoire d Informatique de Robotique et de Microélectronique de Montpellier. He is a member of research groups GDR I 3 and GDR CASSINI. Among its interests are data updating, data transfer, interoperability and metadata. Laurent has a surveyor engineering diploma from the Conservatoire National des Arts et Métiers. L.I.R.M.M. 161 rue Ada Montpellier Cedex 5 France Tel: +(33) Fax: +(33) URL: Stéphane COULONDRE coulondre@lirmm.fr Stéphane Coulondre is a Ph.D student in Computer Science at University of Montpellier 2, and is currently working at Laboratoire d Informatique de Robotique et de Microélectronique de Montpellier. He is a member of research group GDR I 3. Among its interests are object oriented databases, spatio-temporal databases and semi-structured data management. Stéphane has a postgraduate diploma on Computer Science from the University of Montpellier 2. L.I.R.M.M. 161 rue Ada Montpellier Cedex 5 France Tel: +(33) Fax: +(33) URL: