Global Positioning System Reference
In-Depth Information
(i.e., the implementations can be provided by several companies). One of
these propositions is named “Web services” (OGC 2005). These propositions
allow accesses to geographical data without taking into account physical
aspects of communications (i.e., with an URL). They also propose a set of
parameters, methods and communication rules to simplify accesses and
manipulations as soon as clients and servers respect them.
The Web Map Service (WMS) (OGC 2006) and the Web Feature Service
(WFS) (OGC 2010b), used in this chapter, are OGC standards for web
services. WMS deals with the dynamic production of maps as images,
built with geo-referenced data. WFS are designed to provide an access
and a manipulation tool of geographical data within a map. End-users'
interactions are reduced to a selection of an object on a map. This selection
raises the GetFeatureInfo operation of WMS or the GetFeature operation
of WFS and as a result these operations provide data associated with a
geographical map or object. Information may be alphanumerical such as
for example a town name or graphical such as for example the segments
defi ning a border of a town.
Designing a database schema requires analyzing the information
system and enumerating data which should be handled by the Data
Base Management System (DBMS). The implementation of a database
schema is divided into three main levels according to the ANSI/X3/
SPARC recommendations (ANSI-SPARC) and (Brodie and Schmidt 1982).
The most important level is the intermediate one. This level models all
available information to be managed by a DBMS. The lower level defi nes the
physical storage system depending on a set of parameters (e.g., operating
system, update rate, and number of users). The upper level defi nes the
specifi c views of a database schema, each of which is adapted to a specifi c
application. Some data available in the DBMS are presented, some are
hidden and others are re-organized depending on users' needs. As an
example a tourist application may not be concerned by information about
the number of television sets in a specifi c area but a marketing application
is concerned by this information. In the fi rst application, this information
will be hidden and application users are not aware that this data is available
in the information system. In the second case this data will be presented
(may be as aggregated fi gures). Depending on the physical storage some
data may be easy to obtain, some may require the resolution of a complex
query and this process may take time.
In our case, we are in the context of distributed applications all over the
world. Database administrators could not know in advance the different
kinds of applications (i.e., their needs) that will query their databases.
Therefore, due to the number of potential applications and users, providing
the defi nition of external schemas as defi ned in the ANSI/SPARC (ANSI-
SPARC) and (Brodie and Schmidt 1982) decomposition is not realistic at all.
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