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Overview of some relevant standards from ISO/TC 211

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General Feature Model

ISO 19101 describes the theory that a data model, being a key part of a language for discourse, is tied to a community of practice, and must be developed and governed by that community.

ISO 19109 describes a general process for developing a community language or "application schema". (Note that in this context the term "schema" is more general than "XML Schema".) It introduces the "General Feature Model" (GFM), using a series of UML (Unified Modelling Language) class diagrams for the details. A feature instance is an identifiable object in the world, or the digital representation of it. Features are classified into feature types on the basis of common sets of characteristics or properties. The properties include
  • attributes
  • associations and relationships
  • operations and behaviours

Discussing the GFM involves several identified "levels of abstraction". The UML for the GFM itself is the "meta-meta model", which is above a model of feature-types for a particular domain at the "meta-level", above the level of actual feature instances.

The GFM modernises GIS information models in some key and useful ways. Traditional GIS systems use a geometry-centric meta-model, in which the items of interest are primarily geometric (points, lines, polygons, etc), with additional attributes, often stored in database tables. Features, on the other hand, are primarily typed by their conceptual significance within the application-domain (e.g. measurement, borehole, geological boundary, mine).

Advantages of this approach are that
  1. semantics, rather than the representation, are primary
  2. the feature instance carries the type of the feature, rather than its package (e.g. the host layer), thus allowing flexible packaging
  3. a feature may have more than one geometry associated with it, either
    • representing different spatial properties of the feature
    • implementing different representations of a property (e.g. at different scales, or using different geometry models such as triangulation and grid for surfaces).

Information models based on the GFM emphasise the conceptual significance of the data, not just its structure. The aim is for the digital representation to reflect the language used by practitioners in the application domain. For example, when discussing issues and decisions in their domain, geoscientists usually talk about "faults", "plutons", "boreholes" and "measurements", not "points", "lines" and "polygons", and certainly not "tables", "tuples", "lists" or "pages" etc. The latter are geometry-centric and representation-centric abstractions, which are necessary at an implementation level, but should be avoided as far as possible when information is being conveyed between practitioners.

Most conventional GIS require the user to work with a geometry-centric data model (points, lines, polygons) loosely coupled to attribute tables. The feature model differs in that it does not consider geometric properties of features to be different to other properties. A feature may have multiple geometry properties, each labelled with a role such as "centroid", "boundary", "trace", "shape-at-1:25000-scale", etc., or allowing multiple representations of the same property.

Coverage Model

Another important meta-meta-model for geographic information (though it may be modelled as a feature-type) is the coverage, described in ISO 19123. A coverage has values from its range available for all locations in its spatio-temporal domain.

Coverages are most often encountered in the representation of "fields", and the domain is often discretised or sampled on a grid.

But the coverage model may be applied to other discrete domains, including point-sets (samples), curve-sets (classified "linear" features), surface patches (classified polygons) and in various dimensionalities (e.g. 1-D logs and time-series). Continuous coverages are usually obtained using a function or algorithm to interpolate between control values in a matching discrete coverage, but may be represented directly as mathematical functions in which the "control" points are implicit.

Note that this use of the term "coverage" is related to mathematical functions. Some GIS implementations used the term "coverage" to mean "theme" or "layer". These usages are broadly consistent if the layer describes a single phenomenon or property.

Feature Type Cataloguing, Maintenance and Governance

When defining an application schema using the GFM, perhaps the key aspect of the schema is the Catalogue of Feature Types for the application domain. ISO 19110 describes a general method for describing the feature-types in a feature catalogue.

Procedures for maintenance and update of definitions of items of interest are described in ISO 19135, along with a register model for hosting.

ISO 19126 brings these principles together in a "profile" of feature-type catalogues hosted in a register, and also extends the model to dictionaries of property types. Extracts from catalogues from several application domains are given.

Note that UML is used as the standard "conceptual schema language" throughout the ISO/TC 211 standards (the specific profile used is defined in ISO 19103). However, there is no standard serialised form for UML: XMI comes close, but is not fully standardised and, when emitted by most tools is generally cluttered with presentation details. Thus UML is not suitable for transfer of feature type definitions and related information, from an online registry, to software that needs to process this automatically.

In ISO 19110 and ISO 19126 a number of alternative syntaxes (SDTS, gofer, DGIWG, IHO S-57, JTC1 SC24) are shown in the examples. Unfortunately not illustrated is the fact that W3C XML Schema (WXS) can fulfill most of the requirements, for static models at least (i.e. no operations or behaviours). In the XMML project we use WXS as the standard format for serialisation of feature-type definitions, property-type definitions, and in some cases also for simple vocabularies expressable as enumerations.

Spatial, Temporal, Coordinate reference systems

Three standards systematically work through consistent models for the spatial and temporal aspects of geographic information.

ISO 19107 presents the spatial schema, including topology; ISO 19111 presents a model for coordinate reference systems for positioning; ISO 19108 presents a schema for temporal geometry, topology and temporal reference systems (calendars, time coordinate systems, ordinal reference systems).

Modelling and Encoding - UML and Geography Markup Language

One standard addresses the issue of modelling idiom. ISO 19103 describes
  1. a profile of UML to be used for application schemas, including some restrictions on the use of optional elements of UML b. some base types to be used in models (Measure, ScopedName, Record, Any, Integer, etc)

Two standards address the issue of encoding geographic information in XML. ISO 19118 presents a general methodology and a number of options, some of which are based on use of WXS as the intermediate conceptual schema language.

ISO 19136 (issued by Open GIS Consortium as GML 3.2.1) is a detailed XML implementation of the GFM, and most of ISO 19123, 19107, 19108, 19111, along with some utility feature types such as observations, and utility components such as units of measure. The GML encoding is described directly using WXS. However, rules for mapping GML to and from UML models are carefully described in Annex E and Annex F of ISO 19136. These rules can be applied providing the UML follows the profile described in ISO 19103.

In particular GML provides a pattern for defining domain-specific feature-types. GML uses WXS to define components in the "gml" namespace. Each domain-specific language based on GML is termed a GML Application Language, and defines components (in particular, Features) in their own namespace, which are derived from or incorporate components drawn from GML. Effectively, the complete WXS representation for the GML Application Language is a formalisation of the Feature Type Catalogue for the application domain, plus some supporting components.

Metadata

Three standards deal with Metadata.

ISO 19115 is a rationalised model for GI metadata, in particular dataset metadata.

ISO 19119 complements 19115 with service metadata

ISO 19139 is an XML encoding rule developed for implementation of 19115.
Topic revision: r19 - 15 Oct 2010, UnknownUser
 

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