dictionaryEntry, DictionaryEntryType Using definitions and dictionaries

176 Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. gml:identifier codeSpace=http:www.opengis.netgml3.2GML Dictionarygml:identifier gml:dictionaryEntry gml:Definition gml:id=term4.1 gml:descriptionconceptual schema for data required by one or more applicationsgml:description gml:identifier codeSpace=http:www.isotc211.org19101application schemagml:identifier gml:Definition gml:dictionaryEntry gml:dictionaryEntry gml:Definition gml:id=term4.2 gml:descriptionapplication schema written in XML Schema in accordance with the rules specified in ISO 19136gml:description gml:identifier codeSpace=http:www.opengis.netgml3.2GML application schemagml:identifier gml:Definition gml:dictionaryEntry gml:dictionaryEntry gml:Definition gml:id=term4.3 gml:descriptionsemantic relationship between two or more classifiers that specifies connections among their instances gml:description gml:identifer codeSpace=http:www.uml.org1.3associationgml:identifier gml:Definition gml:dictionaryEntry gml:dictionaryEntry gml:Definition gml:id=term4.4 gml:descriptionname-value pair contained in an elementgml:description gml:identifer codeSpace=http:www.w3.orgXML1998namespaceattributegml:identifier gml:Definition gml:dictionaryEntry -- … -- gml:Dictionary 16 GML schema — Units, measures and values

16.1 Introduction

Several GML schema components concern or require quantitative values which use a reference scale or units of measure. In 8.2 the types gml:MeasureType, gml:MeasureListType and gml:MeasureOrNilReasonListType are defined to enable GML properties and objects to carry units of measure, in accordance with the following pattern: abc:length uom=―m‖100abc:length The attribute uom means ―unit of measure‖ and holds a gml:UomIdentifier see 8.2.3.6. This clause describes schema components concerning three topics:  a set of components for defining units of measure,  a set of typed measures,  structures for aggregates and lists of measures. Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. 177

16.2 Units schema

16.2.1 Overview

Several GML schema components concern or require a reference scale or units of measure. Units are required for quantities that may occur as values of properties of feature types, as the results of observations, in the range parameters of a coverage, and for measures used in Coordinate Reference System definitions. NOTE The schema document units.xsd defines components to support the definition of units of measure. The units schema is listed in Annex C; it is identified by the following location-independent name using URN syntax: urn:x-ogc:specification:gml:schema-xsd:units:3.2.1 The basic unit definition is an extension of the general gml:Definition element defined in 15.2.1. Three specialized elements for unit definition are further derived from this. This model is based on the SI system of units see ISO 1000, which distinguishes between base units and derived units.  Base units are the preferred units for a set of orthogonal fundamental quantities which define the particular system of units, which may not be derived by combination of other base units.  Derived units are the preferred units for other quantities in the system, which may be defined by algebraic combination of the base units. In some application areas, conventional units are used, which may be converted to the preferred units using a scaling factor or a formula which defines a re-scaling and offset. The set of preferred units for all physical quantity types in a particular system of units is composed of the union of its base units and derived units.

16.2.2 Using unit definitions

Unit definitions are substitutable for the gml:Definition element declared as part of the dictionary model. A dictionary that contains only unit definitions and references to unit definitions is a units dictionary.

16.2.3 unitOfMeasure, UnitOfMeasureType

The element gml:unitOfMeasure is a property element to refer to a unit of measure. It is declared in the schema as follows: element name=unitOfMeasure type=gml:UnitOfMeasureType complexType name=UnitOfMeasureType sequence attribute name=uom type=gml:UomIdentifier use=required complexType This is an empty element which carries a reference to a unit of measure definition see 8.2.3.6. EXAMPLE This element may appear in a data instance as follows: unitOfMeasure uom=m unitOfMeasure uom=http:my.standards.orgunitslengthmetre