Open GIS Consortium is developing specifications to
standardize the message interfaces. These include: Web Map
Service (WMS) for maps encoded as images; Web Feature
Service (WFS) for geographic objects or vector data; Web
Coverage Service (WCS) for continuous data; Sensor
Collection Service (SCS) for live access to observations from
sensors; Geography Markup Language (GML): XML
components for encoding geographic objects for transport [12].
The OGC WMS specification offers a standard client-server
interaction protocol that each map server implements as a
common interface for accepting requests and returning
responses. The same client is able to access to all available
OGC web map servers over the Internet. The client through the
common interface accesses each map server. In a distributed
OGC WMS, a WMS server can also run as a WMS client that
requests capabilities and maps from other WMS servers.
OGC WMS is an interoperable web mapping system. It
provides common interfaces to connect with the client
application and dynamically process geo-referenced data such
as geographic maps and features coded using Geography
Markup Language (GML) documents. The OGC Web Map
Service specification defines a set of functions that clients may
use to interact with WMS providers (servers). Any client
making requests that conform to the specification can interact
with any server that implements the WMS service. In effect,
this creates an interoperable, distributed web mapping systems.
An OGC web map server implements three functions:
GetCapabilities, GetMap, and GetFeatureInfo. The
GetCapabilities function provides the client with a map
server's service metadata, specifying its capabilities. The
GetMap function specifies map request parameters that enable
the client to request an image map. Finally, the GetFeaturelnfo
function allows the client to request more information about
features at a specific location in the map.