Web Services Composition and Geographic Information - Geographical Information Systems: Trends and Technologies - page 116

operations. Additionally, the OGC standard lets users decide whether to
store geographic information directly in GML or use some other storage
format and convert it to GML only for data transportation purposes.
As for the third aspect, the only key point is the defi nition of the feature
concept. According to the ISO19101 Reference Model, a feature 'is an
abstraction of real world phenomena'; if such an abstraction is associated
with an Earth location, the term geographic feature is used. Features are
fundamental objects in GML (Burggraf 2006) and are described through
a list of geometric and non-geometric properties. The former represent
the basic geometry primitives and their aggregates, such as points, lines,
curves, and polygons, while the latter describe the objects semantics and
are expressed in the standard XML form. In GML a feature is represented by
an XML element whose individual children elements describe a property.
Furthermore, a feature can be defi ned as the result of the composition
of other features. Therefore, the use of GML in conjunction with OGC
services allows the implementation of infrastructures for sharing geospatial
information in a globally accessible manner, independently of the different
proprietary formats used (Burggraf 2006).
Web Map Services
A Web Map Service (WMS) provides an HTTP based interface for requesting
georeferenced map images from one or more distributed geospatial
databases (de la Beaujardiere 2006). Basically, the requester specifies
elements of interest, such as an area, and the response consists of a map
image referred to that area, returned in a picture format. Appropriate MIME
types (e.g., “image/png”) usually accompany the response objects.
According to the OGC standard, a generic WMS can be classifi ed as
a Basic or a Queryable WMS. A Basic WMS supports the GetCapabilities
and the GetMap operations, while a Queryable WMS also supports the
GetFeatureInfo operation, whose goal is to provide more information about
the features contained in the pictures of a map. Textual output (usually
XML documents) is also available and can be used to provide for an error
description or response to information requests about the features shown
on a map. A WMS has to support the HTTP GET method and may support
the HTTP POST method.
Finally, the WMS specifi cation represents another difference between
an OGC service and a W3C service, namely the former can return binary
documents (as well as XML documents), the latter always relies on pure
XML documents (although, as described later, there are several ways to
encode binary documents in a SOAP message, such as images and PDF
documents).