derivationUnitTerms, DerivationUnitTermType Units schema

180 Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. element name=derivationUnitTerm type=gml:DerivationUnitTermType complexType name=DerivationUnitTermType complexContent extension base=gml:UnitOfMeasureType attribute name=exponent type=integer extension complexContent complexType This unit term references another unit of measure uom and provides an integer exponent applied to that unit in defining the compound unit. The exponent may be positive or negative, but not zero.

16.2.10 ConventionalUnit, ConventionalUnitType

Conventional units that are neither base units nor defined by direct combination of base units are used in many application domains. For example electronVolt for energy, feet and nautical miles for length. In most cases there is a known, usually linear, conversion to a preferred unit which is either a base unit or derived by direct combination of base units. The gml:ConventionalUnit element is declared as follows: element name=ConventionalUnit type=gml:ConventionalUnitType substitutionGroup=gml:UnitDefinition complexType name=ConventionalUnitType complexContent extension base=gml:UnitDefinitionType sequence choice element ref=gml:conversionToPreferredUnit element ref=gml:roughConversionToPreferredUnit choice element ref=gml:derivationUnitTerm minOccurs=0 maxOccurs=unbounded sequence extension complexContent complexType The gml:ConventionalUnit extends gml:UnitDefinition with a property that describes a conversion to a preferred unit for this physical quantity. When the conversion is exact, the element gml:conversionToPreferredUnit should be used, or when the conversion is not exact the element gml:roughConversionToPreferredUnit is available. Both of these elements have the same content model. The gml:derivationUnitTerm property defined above is included to allow a user to optionally record how this unit may be derived from other more primitive units. 16.2.11 conversionToPreferredUnit, roughConversionToPreferredUnit, ConversionToPreferredUnitType, FormulaType The elements gml:conversionToPreferredUnit and gml:roughConversionToPreferredUnit represent parameters used to convert conventional units to preferred units for this physical quantity type. A preferred unit is either a Base Unit or a Derived Unit that is selected for all values of one physical quantity type. These conversions are declared in the schema as follows: element name=conversionToPreferredUnit type=gml:ConversionToPreferredUnitType element name=roughConversionToPreferredUnit type=gml:ConversionToPreferredUnitType complexType name=ConversionToPreferredUnitType complexContent extension base=gml:UnitOfMeasureType Copyright © 2007 Open Geospatial Consortium, Inc. All Rights Reserved. 181 choice element name=factor type=double element name=formula type=gml:FormulaType choice extension complexContent complexType The inherited attribute uom references the preferred unit that this conversion applies to. The conversion of a unit to the preferred unit for this physical quantity type is specified by an arithmetic conversion scaling andor offset. The content model extends gml:UnitOfMeasureType, which has a mandatory attribute uom which identifies the preferred unit for the physical quantity type that this conversion applies to. The conversion is specified by a choice of  gml:factor, which defines the scale factor, or  gml:formula, which defines a formula by which a value using the conventional unit of measure can be converted to obtain the corresponding value using the preferred unit of measure. The model for the formula is given as follows: complexType name=FormulaType sequence element name=a type=double minOccurs=0 element name=b type=double element name=c type=double element name=d type=double minOccurs=0 sequence complexType This formula defines the parameters of a simple formula by which a value using the conventional unit of measure can be converted to the corresponding value using the preferred unit of measure. The formula element contains elements a, b, c and d, whose values use the XML Schema type double. These values are used in the formula y = a + bx c + dx, where x is a value using this unit, and y is the corresponding value using the base unit. The elements a and d are optional, and if values are not provided, those parameters are considered to be zero. If values are not provided for both a and d, the formula is equivalent to a fraction with numerator and denominator parameters.

16.2.12 Example of units dictionary informative

This dictionary contains definitions corresponding to all the base and derived units defined by in the SI system [SI], plus a selection of conventional units to illustrate the usage of these components. gml:Dictionary gml:id=unitsDictionary gml:descriptionA dictionary of units of measuregml:description gml:identifier codeSpace=http:www.opengeospatial.orginitiatives?iid=79OWS-1.2 Unitsgml:identifier gml:dictionaryEntry gml:Dictionary gml:id=SIBaseUnits gml:descriptionThe Base Units from the SI units system.gml:description gml:identifier codeSpace=http:www.opengeospatial.orginitiatives?iid=79OWS-1.2 SI Base Unitsgml:identifier gml:dictionaryEntry gml:BaseUnit gml:id=m gml:descriptionThe metre is the length of the path travelled by light in vacuum during a time interval of 1299 792 458 of a second.gml:description gml:identifier codeSpace=http:www.bipm.fren3_SIbase_units.htmlmetregml:identifier gml:name xml:lang=enUSmetergml:name gml:quantityTypelengthgml:quantityType