ISPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS’’, Bangkok, May 23-25, 2001
228
Geographical information interoperability is arisen from the needs
of integrating and sharing the huge amount of geo-data from
various sources. Open GIS Consortium (OGC) is the most active
organization that develops industry-wide agreed standards for
interoperable geoprocessing. OGC’s goal is to develop a unified
framework for geo-spatial information communities. Hence, the
communities adopt common mean to represent the Earth digitally;
to implement geo-spatial data manipulation; and to solve
institutional non-interopability problems [2]. Important standards
to the Spatial Information Marketplace include;
• OpenGIS Simple Features Specification is the fundamental
standard that deals with the representation and
manipulation for features on the earth surface.
Implementation specifications have been developed for
OLE/COM, COBRA and SQL [5], [6], [7].
• Catalog service and information community [8]
• OpenGIS Web Map Server (WMS) Interface
Implementation Specification - a standard for publishing
maps on the Internet [9].
• OGC Web Feature Server (WFS) is a feature-based version
of WMS. This specification will be developed in the second
phase of WMS.
• WFS Filter interface specification - standard querying
language for WFS. This specification will be developed in
the second phase of WMS.
• Geography Markup Language (GML) - standard
description of geographical features for data transportation
and storage, this is an extension of Extended Markup
Language (XML) [10].
The standards are contributed and agreed by major GIS software
vendors in the industry, for examples, ESRI, Intergraph, Autodesk
and Oracle. Details of those standards can be found from the
OGC's Web site (http://www.openqis.org).
4. PUTTING THEM TOGETHER: ARCHITECTURE OF
SPATIAL INFORMATION MARKETPLACES
Referring to the available technologies of geographical
information interoperation, this section proposes an integral
architecture of the Spatial Information Marketplace. The proposed
marketplace consists of host of the marketplace, service provider,
data communication between providers and customers, and
wrapper at each service provider.
Figure 1a. Parallel access, 1b. Cascading access,
Data communication and data sharing among customers and
services providers are critical to the Spatial Information
Marketplaces. Hence, servers of various service providers must
be efficiently connected. With referring to the OCG's standards,
several possible architectures of the Spatial Information
Marketplace are proposed [11]. Those are, direct connection,
cascading server and mediating server (Figure 1).
In the architecture of parallel access (Figure 1a), the software
module that co-ordinates all the service providers is executed on
the client-side. Computational loading for the client is extremely
heavy, moreover, there is no server acts as the host of the
marketplace. Consequently, the parallel access is not the
anticipate architecture for the marketplaces. The other
architecture, which is suggested by OGC in the WMT is
cascading access [9] (Figure 1b). The architecture of cascading
access treats map server as the client of another map server. In
the client point of view, this approach integrates resources of
multiple servers to one location on the Internet. This architecture
is not enough for the adequate to the marketplace, since the client,
responds for communicating service providers with different
interfaces, input and output data formats and comminuting
language. Hence, workload at the client side is extremely heavy.
However, to accomplish a complex task, this approach can be
applied to co-operate different service providers in the
marketplace. Among the three architectures, mediator server is a
more appropriate architecture for the marketplace. Mediator has
been suggested to manipulate huge volumes of data,
heterogeneities among data resources and mismatch between
data values [12] (Figure 1c). To reduce computational load on the
client side, mediator is used to co-ordinate all service providers to
accomplish a service required by customer. The mediator
ensures not only parallel access to all services providers, but also
data compatibility between data sets received from different
providers.
Figure 2 illustrates the proposed architecture of Spatial
Information Marketplace, customers firstly interact with the
service providers through the catalogue service. The catalogue
stores descriptive information of services to customer.
Implementation of the catalogue shall follow the OGC’s catalogue
implementation specification. Service providers establish their
services according to the standardized OCG WMS or WFS server.
GML should be the media for exchanging of geographical
information between these service providers and the Spatial
Information Marketplaces.
On the client side, customer equips with a simple ActiveX viewer
and Internet browser to access the Spatial Information
Marketplace Server. The Server provides either GML or
Scaleable Vector Graph (SVG) data set to the customer. SVG is a
standard graphic format developed by World Wide Web
Consortium (W3C), the format is another extension to XML for
exchanging two-dimensional graph on the Internet [13]. With a
Map Style Sheet, SVG can use as a visualizing form of GML. If
the customer interests in the graphic result of the service, the
Spatial Information Marketplace server generates SVG to user
with the Map Style Sheet requested by the customer. If the
customer requires the result for further processing, GML is
delivered to the customer. A special type of customer is the other
Internet-based Marketplace, which requires Gl and GlService in
its own business using the format of SVG.
The proposed spatial information marketplace benefits from the
existing standards developed by OGC. The standards offer
fundamental spatial data exchange formats and specification on
Internet map server. However, the marketplace is a new concept
to the OGC’s standards, specifications on service and acquiring
methods are required to develop. This is impossible to limit the
type of services available in the marketplace, hence, there is no
way to standardize all types of services. Rather, the marketplace
demands a common data model for service that is more flexible to
represent disparate types of services. Moreover, a common
requesting language is required to access those services from
different providers [4], [14].
