Web Services Composition and Geographic Information - Geographical Information Systems: Trends and Technologies - page 123

While the last four points have been previously mentioned and
discussed, the technical choices carried out for the fi rst point deserve some
further considerations. From a high level perspective, geographic services are
loosely arranged in four tiers as shown in Fig. 5. These tiers are geographic
data independent although each of them includes specifi c services that deal
with geographic data. Each tier can offer its functionality both to other tiers
and also directly to fi nal clients. In fact, a complete separation in four tiers
is not always required and clients can bypass the tiers that are not needed.
Such a characteristic is useful especially when a complete separation could
be ineffi cient for the purposes of the fi nal system.
The fi rst tier, the Information Management Services includes services
that provide access to geographic datasets and are usually used to retrieve
a subset of such data useful for the client. WMS and WFS are typical
examples of services that belong to this layer. The Processing Services
tier contains services designed to process data. Services in this tier can
either use other services of the same tier or services from the Information
Management Services layer. The Application Services tier encompasses
those services useful to provide support functions to fi nal clients of the
system, in particular web browser clients.
The development of a SII is a non-trivial task exhibiting different
challenges that, currently, can be overtaken only by adding some form of
customization to the various services functionalities, thus proving the lack
of fl exibility and generality of current standards.
A complete example that demonstrates how the use of the various
OGC standards can constitute a possible basis to perform distributed
geoprocessing and that investigates the diffi culties arising during the
Clients
Application Services
Processing Services
Information Management Services
Fig. 5. OGC Proposed Architecture-Service tiers subdivision.