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The concept of "unit" represents one of the most fundamental categories of metadata. The classic example of data entropy is the case in which a reported numeric value loses meaning due to lack of associated units. Much of Ecology is driven by measurement, and most measurements are inherently comparative. Good data description requires a representation of the basis for comparison, i.e., the unit. In modeling units, the authors of EML drew inspiration from the NIST Reference on Constants, Units, and Uncertainty. This document defines a unit as "a particular physical quantity, defined and adopted by convention, with which other particular quantities of the same kind are compared to express their value." The authors of the EML 2 specification (hereafter "the authors") decided to make the unit element required, wherever possible. The units are defined in the STMML language in a document that is shipped with each release of EML. See the accompanying STMML file, eml-unitDictionary.xml, for precise, quantitative definitions of each of these units and their relationships to base SI units which modules use these types (and which could but dont yet?) anything else?

any module that needs units no Standard Units The enumerated list of standard units, mainly S unitsI The unitDictionary is the standard set of units included with the EML distribution, mainly from the SI standard. These unit names should be used in the standardUnit field to describe an attribute. See the accompanying STMML file, eml-unitDictionary.xml, for precise, quantitative definitions of each of these units and their relationships to base SI units. The standard Unit Dictionary is built from a union of simpleTypes. This construct allows unit types to be used individually as appropriate in EML content (e.g., LengthUnitType for distances ) Length Units The list of units which are of length type, or have a parentSI of meter SI The LengthUnitType is the enumerated list of units which are of length type, or have a parentSI of meter. These unit names can be used where ever content should be restricted to a length, such as a distance or altitude. The units are defined in the STMML language in a document that is shipped with each release of EML called eml-unitDictionary.xml. See this file for precise, quantitative definitions of each of these units and their relationships to base SI units. Mass Units The list of units which are of mass type, or have a parentSI of kilogram The MassUnitType is the enumerated list of units which are of mass type, or have a parentSI of kilogram. These unit names can be used where ever content should be restricted to a mass, such as an amount. The units are defined in the STMML language in a document that is shipped with each release of EML called eml-unitDictionary.xml. See this file for precise, quantitative definitions of each of these units and their relationships to base SI units. Other Standard Units The enumerated list of standard units that are not included in the any other named simpleType The unitDictionary is the standard set of units included with the EML distribution, mainly from the SI standard. These unit names can be used in the standardUnit field to describe an attribute. The units are defined in the STMML language in a document that is shipped with each release of EML. See the accompanying STMML file eml-unitDictionary.xml for precise, quantitative definitions of each of these units and their relationships to base SI units. The standard Unit Dictionary is built from a union of simpleTypes. This Type enumerates the units which are not in other Type definitions, but are to be included as standard. Angle Units An example of a group of units which could be added. These units are a derived typein the group of derived SI units with a special name or symbol. The AngleUnitType is the enumerated list of angle units. For example, plane angle (radian, rad) and solid angle (steradian, sr) are actually dimensionless, and their symbols used as appropriate (e.g, sr in photometry). These unit names could be used where ever content should be restricted.