GML Schema

GML provides objects for describing geography, including features, coordinate reference systems, geometry, topology, time, units of measure and generalized values. A GML document is an XML-encoded text file and by itself, does not “do” anything.

There are GML schema for three categories of objects.

1. A geographic feature is an abstraction of a real world phenomenon; it is a geographic feature if it is associated with a location relative to the Earth. A digital representation of the real world can then be thought of as a set of features. The state of a feature is defined by a set of properties, where each property has a name, type and value.
   
   
2. A coverage can represent one feature or a collection of features to show spatial relationships between earth phenomena as well as their spatial distribution.
   
   
3. An observation models the act of observing, or recognizing and noting a fact or occurrence often involving measurement with instruments. An observation is considered to be a GML feature with a time at which the observation took place, and with a value for the observation.

The GML framework is very extensible: you can create and define feature types and feature relationships specifically for your area of specialization.