Introduction Change Requests | OGC

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 178 Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved.

16.2.4 UnitDefinition, UnitDefinitionType

A gml:UnitDefinition is a general definition of a unit of measure. This generic element is used only for units for which no relationship with other units or units systems is known. It is declared in the schema as follows: element name=UnitDefinition type=gml:UnitDefinitionType substitutionGroup=gml:Definition complexType name=UnitDefinitionType complexContent extension base=gml:DefinitionType sequence element ref=gml:quantityType minOccurs=0 element ref=gml:quantityTypeReference minOccurs=0 element ref=gml:catalogSymbol minOccurs=0 sequence extension complexContent complexType The content model of gml:UnitDefinition adds three additional properties to gml:Definition described in 15.2.1, gml:quantityType, gml:quantityTypeReference and gml:catalogSymbol. The gml:catalogSymbol property optionally gives the short symbol used for this unit. This element is usually used when the relationship of this unit to other units or units systems is unknown.

16.2.5 quantityType, quantityTypeReference

The gml:quantityType and gml:quantityTypeReference properties indicate the phenomenon to which the units apply. They are declared as follows: element name=quantityType type=gml:StringOrRefType element name=quantityTypeReference type=gml:ReferenceType This element contains an informal description of the phenomenon or type of physical quantity that is measured or observed. EXAMPLE length, angle, time, pressure, or temperature. When the physical quantity is the result of an observation or measurement, this term is known as observable type or measurand. The use of gml:quantityType for references to remote values is deprecated. gml:quantityTypeReference shall be used instead.

16.2.6 catalogSymbol

The catalogSymbol is the preferred lexical symbol used for this unit of measure. It is declared as follows: element name=catalogSymbol type=gml:CodeType The codeSpace attribute in gml:CodeType identifies a namespace for the catalog symbol value, and might reference the external catalog. The string value in gml:CodeType contains the value of a symbol that should be unique within this catalog namespace. This symbol often appears explicitly in the catalog, but it could be a combination of symbols using a specified algebra of units.