1.3.6. Styled Layer Descriptor
The OpenGIS Styled Layer Descriptor (SLD) Implementation Specification is
an encoding that extends the Web Map Service specification to allow user-defined
symbolization of feature data. It allows users (or other systems) to determine
which features or layers are rendered with which colours or symbols. (OGC)
An example: if a water-bodies layer on a WMS server portrays the water
features with the default colour of black, the adding SLD support on the server
allows the WMS client to specify that it wants the water features returned as
blue. (Geoconnections.org)
The importance of visual portrayal of geographic data
cannot be overemphasised. It is needed for geospatial consumers (either humans
or machines) to control the visual portrayal of the data with which they work.
The current OpenGIS Web Map Service (WMS) specification supports the ability
for
an information provider to specify very basic styling options by advertising
a
preset collection of visual portrayals for each available data set. However,
while a WMS currently (in 2006) can provide the client with a choice of style
options, the WMS can only tell the client the name of each style. It cannot tell
the client what portrayal will look like on the map. More importantly, the
client has no way of defining its own styling rules. The ability for a human
or
machine client to define these rules requires a styling language that the client
and server can both understand. This language is called the Styled Layer
Descriptor (SLD). (OGC)
The SLD gives the WMS client some control over the visual appearance
of the map, as well as the existing control over the combination of layers and
viewing geometry. (Geoconnections.org)
Following you see a request result of a web map service without SLD (above) and with SLD (below).
Web Map Service without SLD (UMN MapServer)Web Map Service with SLD (UMN MapServer)If you are interested in how to use the Styled Layer Descriptor consult the following literature (please read first the chapters Web Map Service and Web Feature Service):