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See ISO/DIS 19136 Clause 16.
Many applications require definitions of terms which are used within instance documents as the values of certain properties or as reference information to tie properties to standard information values in some way.  Units of measure and descriptions of measurable phenomena are two particular examples. 
It will often be convenient to use definitions provided by external authorities. These may already be packaged for delivery in various ways, both online and offline. In order that they may be referred to from GML documents it is generally necessary that a URI be available for each definition. Where this is the case then it is usually preferable to refer to these directly. 
Alternatively, it may be convenient or necessary to capture definitions in XML, either embedded within an instance document containing features or as a separate document. The definitions may be transcriptions from an external source, or may be new definitions for a local purpose. In order to support this case, some simple components are provided in GML in the form of 
-	a generic gml:Definition, which may serve as the basis for more specialized definitions
-	a generic gml:Dictionary, which allows a set of definitions or references to definitions to be collected 
These components may be used directly, but also serve as the basis for more specialised definition elements in GML, in particular: coordinate operations, coordinate reference systems, datums, temporal reference systems, and units of measure.  
Note that the GML definition and dictionary components implement a simple nested hierarchy of definitions with identifiers. The latter provide handles which may be used in the description of more complex relationships between terms. However, the GML dictionary components are not intended to provide direct support for complex taxonomies, ontologies or thesauri.  Specialised XML tools are available to satisfy the more sophisticated requirements. 

Schema document information


File path: external/ogc/gml/3.2.1/dictionary.xsd