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See ISO/DIS 19136 8.2. W3C XML Schema provides a set of built-in "simple" types which define methods for representing values as literals without internal markup. These are described in W3C XML Schema Part 2:2001. Because GML is an XML encoding in which instances are described using XML Schema, these simple types shall be used as far as possible and practical for the representation of data types. W3C XML Schema also provides methods for defining - new simple types by restriction and combination of the built-in types, and - complex types, with simple content, but which also have XML attributes. In many places where a suitable built-in simple type is not available, simple content types derived using the XML Schema mechanisms are used for the representation of data types in GML. A set of these simple content types that are required by several GML components are defined in the basicTypes schema, as well as some elements based on them. These are primarily based around components needed to record amounts, counts, flags and terms, together with support for exceptions or null values.
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