Swiss Cadastral Core Data Model

Data modelling, interoperability, spatial data infrastructure, OpenGIS and GML/XML are important for data access and eGovernment developments in cadastre and land administration. Geo-referenced information also has a crucial role in many other land-related decision making processes. The author describes creation of the data-description language INTERLIS and a Swiss-developed data model for cadastral surveying.

Integration and sharing of geo-referenced data is becoming more and more crucial, and there is an increasing need for efficient and reliable data-exchange standards. In order to provide long-term security these have on the one hand to be independent of any specific hard or software systems, while on the other, for reasons of flexibility, they also need to provide a model-based approach.

Data Exchange
In Switzerland in 1985 a bi-level approach to any future data-exchange standard in cadastral surveying was proposed. A data-description language to describe data models in an orderly and precise way was proposed and developed, while the respective authorities had to come to a decision on the data model itself. A detailed concept was commissioned and led to the creation of the data-description language INTERLIS and a data model for cadastral surveying. The model-based concept, as opposed to format-based exchange standards, allows data exchange without information loss and thus provides a system-independent data exchange mechanism for protection of the high financial investment in data acquisition. The principle of method and system independence was also the basis for introduction of tendering for data-acquisition projects. Through description and definition of the data model it became possible to define the final product rather than method or system.

Data Modelling
The INTERLIS language has been developed in such a way that it can easily be read by human beings. The interpretation of INTERLIS models is nevertheless precise and unambiguous. The language is textual and well complements the graphical description language Unified Modelling Language (UML), but goes well beyond that. The concept also includes a transfer service, which generates a transfer file from the conceptual model. Properties and relations of real-world objects are described with clearly defined expressions and terms, and there is clear distinction between description of real-world objects and their graphical representations. The language is based on object-oriented principles and, while coordinates, lines and areas are basic constructs, there are many others to describe other properties of the objects so that non-geographic data and applications can be addressed. Data modelling in conjunction with system-independent interface services is called a ‘model-based approach’ or ‘model-driven architecture’. Models can be defined on the basis of a common concept and standard, which is crucial from the perspective of semantic interoperability.
The core data model for cadastral surveying data was defined using the data-description language INTERLIS and became an integral part of the new legal basis for digital cadastral surveying introduced in 1993. The data model consists of eight information layers; distinguishing between different information layers served a mainly data management purpose, namely to be able to assign distinct stakeholders and to separate and distinguish data acquisition for each layer. Two new ordinances were introduced in 1993: the Ordinance for Cadastral Surveying and the Technical Ordinance for Cadastral Surveying. The objective was to renovate the cadastral surveying system and to introduce the digital data format. Due to the versatility of data in digital form, the 'raison d'être' of cadastral surveying data was extended from a solely legal purpose to multi-purpose, serving not only land registration but also "any kind of land information system". The data-description language INTERLIS is a crucial element for this extension of purpose because it is well suited as a basis for definition of data models in any domain.

Information Layers
The cadastral map in digital format consists of eight information layers. By definition, the two layers ‘land cover’ and ‘ownership’ cover the whole territory in a complete way, i.e. without overlaps or gaps, while other layers have different structural definitions. Buildings are part of the ‘land cover’ layer. The separation of data into eight information layers has the advantage that the layers can be acquired independently of each other. Each of the information layers is object-oriented and defined by an entity-relationship diagram representing the data model and basis for translation into INTERLIS data-exchange format.
The new ordinances introduced in 1993 required that cadastral surveying data not serve only for land registration but for any kind of land information system. The idea behind this stipulation was to integrate and share spatial information of public interest and to avoid double data acquisition. Spatial data can only be integrated and shared within one information system when the data shares a common geodetic reference framework and a common data modelling concept. Data ownership and responsibility can still be retained without interfering in stakeholder interests and data acquisition and maintenance processes. In this respect data modelling plays a similar role as the geodetic reference framework. Neither are an absolute necessity for data integration and sharing, but they both provide the required ease of use for land information systems to work. This concept is at the core of spatial data infrastructures, which are very important in sharing information and, ultimately, in the set-up of eGovernment services.

INTERLIS and GML
At the time of its introduction the data-modelling concept could call upon only few international standards available to support it. One was the data-description language EXPRESS which, however, does not seem to have overcome initial problems in practical implementation of geo-information. Due to several minor but important restrictions, INTERLIS had to be extended and complemented in 2003. It became INTERLIS2, which offers new possibilities such as incremental updating, definition and transfer of graphical representations, data views, and the description of units and coordinate systems. Some of the language terms have also been altered for better readability and compatibility. Instead of the specific INTERLIS file transfer format .itf, it is possible with INTERLIS2 to encode data for transfer directly with the ‘eXtensible Markup Language’ XML. Open, accessible program libraries will support the full integration of INTERLIS2 into user systems. INTERLIS2 does not replace the initial INTERLIS description language; it rather provides complementary features that facilitate new possibilities. The most important changes are:

• New extensions such as data types, constraints, data views, graphical descriptions, description of units, descriptions of coordinate systems, and user-specific extensions, such as line geometries.
  
• Incremental updating, which requires that both the primary and secondary data bases support the transfer format (XML) and that new object identifiers be introduced.
  
• XML; this ensures that the national standard is compatible with internationally accepted standards.

Both INTERLIS2 and GML offer constructs for the description of spatial objects including their properties and mutual relations. The modelling method is very similar in both languages. Table 1 gives an overview of strengths and weaknesses of both languages.
INTERLIS2 and GML3 are both model-based and object-oriented and they both support domain-specific application schemas and data transfer with XML.

Future
The Cadastre in Switzerland will continue to use INTERLIS2 for the description of data models, mainly for the following reasons:

• There is a well-established industry to deal with INTERLIS2 while GML, although an accepted international standard is still undergoing rapid development.
  
• Many software tools are available for INTERLIS2 (parser, compiler, checker).
  
• Data described with INTERLIS2 are compatible with the .xml format.
  
• The compiler for INTERLIS2 has been extended and now provides a basis for the generation of GML application schemas and XSL style sheets for data transformation.
  
• Data services can easily be set up on the basis of OGC.
  
• INTERLIS2 supports incremental updating.
  
• There are control tools in INTERLIS2 that do not as yet exist for XML.
  
• INTERLIS2 can describe graphic representation, which is not possible in GML.

The GML and XML developments will of course be closely monitored. The suggested continuation with INTERLIS2 provides the required stability for the industry. The Swiss geo-information community has a tool that provides the required services, and it enjoys the privilege of not having to rush into a new standard.

Office Interface
Land registration and cadastral surveying offices operate separately in Switzerland. In spite of this institutional separation, there is a close co-operation between the two organisations and data is readily shared. While cadastral surveying has defined the digital format of its own data, land registries have been slower to adapt. Over the last few years, however, digital data have also become standard in land registration, and since 2001 there has been an ongoing project to gain access to land registration data through an information system.
Several and different software and database systems are in use in land registration and cadastral surveying. The transfer of data, however, had to be independent from any specific system, and it therefore was decided to take the same data modelling approach as in cadastral surveying. The data model was described in INTERLIS, also benefiting from further advantages, such as incremental updating. It was important to have a future-oriented solution.

Concluding Remarks
The introduction of the new data-modelling concept for the description of cadastral surveying data in 1993 triggered the development of SDI in Switzerland. INTERLIS has initiated the definition of more than a hundred other spatial data domains since 1995, enabling the use of the same data-exchange mechanisms as in cadastral surveying. Since 2000 a new agency (COSIG) has come into operation to foster the co-ordination, acquisition and use of spatial data within the federal administration. COSIG promotes the INTERLIS concept for the definition and handling of all spatial data. This concept is also at the core of the new eGovernment initiative, which attempts to bring digital spatial data closer to users. INTERLIS has become the accepted approach within the Swiss geodata community for modelling, checking and exchange of data.