quantityType, quantityTypeReference Units schema

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: