The Style Layer Descriptor also known as SLD is an OGC specification describing how geographic features can be styled in a Spatial Data Structure (SDI) environment. The OGC SLD specification describes an XML encoding that is independent of any geo-processing component. One immediate benefits of such an SLD encoding is the ability to support concurrently both the styling requirements of the providers of geospatial data as well as the styling requirements of the users of the same data. For example, a Map Server that supports SLD would allow both the data provider organizations as well as any clients of the Map Server who may have other map styling requirements. With a Map Server compliant to the OGC SLD specification a client application can submit a map request along with an SDL document that is either referenced at a URL or encoded into the map request. Sending a map request with an SLD will indicate to the map server that the features requested should be styled according to the requested styles as opposed to using a default style provided by a data provider or any other styles that may be advertized by the data provider.
Within a Spatial Data Infrastructure environment, composed of a large number of OGC compliant Web Map Servers (WMS), one immediate benefit of using the OGC SLD from the client is the ability to generate an integrated and homogeneous cartographic map from a large number of data provider organizations despite the fact that each organization may advertize different map styles.
The following images illustrates the results of map requests using SLD encoded using the USGS topographic map symbols described in the above documents.
Figure 1. Roads and other hydrographic features displayed from the USGS Framework data service (http://frameworkwfs.usgs.gov/framework/wms/wms.cgi) with no styles (default styling is being applied by the Map Server).
Figure 2. Roads and other hydrographic features displayed from the USGS Framework data service (http://frameworkwfs.usgs.gov/framework/wms/wms.cgi) with SLD styles.
Figure 3. Roads and other hydrographic features displayed from the USGS Framework data service (http://frameworkwfs.usgs.gov/framework/wms/wms.cgi) with no styles (default styling is being applied by the Map Server).
Figure 4. Roads and other hydrographic features displayed from the USGS Framework data service (http://frameworkwfs.usgs.gov/framework/wms/wms.cgi) with SLD styles.
Figure 5. Roads and other hydrographic features displayed from the USGS Framework data service (http://frameworkwfs.usgs.gov/framework/wms/wms.cgi) with no style (default styling is being applied by the Map Server).
Figure 6. Roads and other hydrographic features displayed from the USGS Framework data service (http://frameworkwfs.usgs.gov/framework/wms/wms.cgi) with SLD styles.
