STYLED LAYER DESCRIPTOR version 0.7.3 (2002-09-21) A StyledLayerDescriptor is a sequence of styled layers, represented at the first level by NamedLayer and UserLayer elements. RUN-TIME ENVIRONMENT VARIABLES An "EnvironmentVariables" element includes an XML blob which can be used to declare the values to use for global environment variables during the execution of the styling operations. These variables may be refered to in styles and symbols in the same way as feature properties. SLD LIBRARIES The UseSLDLibrary tag specifies that an external SLD document should be used as a "library" of named layers and styles to augment the set of named layers and styles that are available for use inside of a WMS. In the event of name collisions, the SLD library takes precedence over the ones internal to the WMS. Any number of libraries may be specified in an SLD and each successive library takes precedence over the former ones in the case of name collisions. LAYERS AND STYLES A NamedLayer is a layer of data that has a name advertised by a WMS. A NamedStyle is used to refer to a style that has a name in a WMS. A UserLayer allows a user-defined layer to be built from WFS and WCS data. A RemoteOWS gives a reference to a remote WFS/WCS/other-OWS server. A Service refers to the type of a remote OWS server. LayerFeatureConstraints define what features & feature types are referenced in a layer. A FeatureTypeConstraint identifies a specific feature type and supplies fitlering. An Extent gives feature/coverage/raster/matrix dimension extent. A UserStyle allows user-defined styling and is semantically equivalent to a WMS named style.