unitOfMeasure, UnitOfMeasureType Units schema

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. Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. 179 EXAMPLE The symbol cm might indicate that it is the m symbol combined with the c prefix.

16.2.7 BaseUnit, BaseUnitType, unitsSystem

A base unit is a unit of measure that cannot be derived by combination of other base units within a particular system of units. For example, in the SI system of units, the base units are metre, kilogram, second, Ampere, Kelvin, mole, and candela, for the physical quantity types length, mass, time interval, electric current, thermodynamic temperature, amount of substance and luminous intensity, respectively. This is supported using the gml:BaseUnit element which is declared as follows: element name=BaseUnit type=gml:BaseUnitType substitutionGroup=gml:UnitDefinition complexType name=BaseUnitType complexContent extension base=gml:UnitDefinitionType sequence element name=unitsSystem type=gml:ReferenceType sequence extension complexContent complexType gml:BaseUnit extends generic gml:UnitDefinition with the property gml:unitsSystem, which carries a reference to the units system to which this base unit is asserted to belong.

16.2.8 DerivedUnit, DerivedUnitType

Derived units are defined by combination of other units. Derived units are used for quantities other than those corresponding to the base units, such as hertz s –1 for frequency, Newton kg.ms 2 for force. Derived units based directly on base units are usually preferred for quantities other than the fundamental quantities within a system. If a derived unit is not the preferred unit, the gml:ConventionalUnit element see 16.2.10 should be used instead. The gml:DerivedUnit element is declared as follows: element name=DerivedUnit type=gml:DerivedUnitType substitutionGroup=gml:UnitDefinition complexType name=DerivedUnitType complexContent extension base=gml:UnitDefinitionType sequence element ref=gml:derivationUnitTerm maxOccurs=unbounded sequence extension complexContent complexType The gml:DerivedUnit extends gml:UnitDefinition with the property gml:derivationUnitTerms.

16.2.9 derivationUnitTerms, DerivationUnitTermType

A set of gml:derivationUnitTerm elements describes a derived unit of measure. Each element carries an integer exponent. The terms are combined by raising each referenced unit to the power of its exponent and forming the product. The element gml:derivationUnitTerm is declared as follows: