units.xsd

See ISO/DIS 17.2.
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. 
The basic unit definition is an extension of the general gml:Definition element defined in 16.2.1.  Three specialized elements for unit definition are further derived from this. 
This model is based on the SI system of units [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.  
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.