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the first level using object oriented modelling. 
• The third level is a global schema which provides a uniform perspective of the information 
posted at the second level. 
• The federated schemes are supporting different nodes. 
• Another fourth level of schemes is representing the application specific views which can be 
partly retrieved form the federation (e.g. application database). 
Referring to Figure 4, the severs at level 3 provide link between nodes and sub-nodes. This means that the 
semantics of the global schema in those servers are restricting the semantics of the data to be shared 
between those nodes. 
4.2 What is Semantic Specifications 
In the above definition we loosely used the term specifications. Specifications, or semantic specifications 
hereafter are divided into metadata definitions and semantics. The metadata in our concept will take FGDC 
as its standards [FGDC metadata standards, 1994]. Instead of having the metadata defined for the whole 
federation at one place, i.e., the corresponding global server, the semantic specifications will be 
constructed hierarchically. This will allow us to store the semantics where it is needed. In other words, the 
specifications of nodes will be stored in the global servers, those for contexts, hierarchies and classes will 
be stored in the underlying databases. Semantic specifications are defined as follows. More on semantic 
specifications is given in [Bisr Y., 1996] 
Semantic Specifications = < Metadata, Object Behavior> 
Metadata = identification information, Data Quality Information, Spatial Data Organization information, 
Spatial Reference information, Entity and Attribute Information, Distribution Information> 
Object Behavior = <Role in the database, Functional Dependency:^ 
4.3 Testbed for Data sharing 
In this section, the architecture and methodology that will be implemented for developing the testbed is 
explained. The architecture is implemented in a Local Area Network. An FGIS driver component will be 
added to the global server as well as to each user who wishes to join the federation. The role of the driver 
is to firstly provide a platform and operating system independent interface; secondly to provide a 
transparent access to services from remote GISs. FGDC specifications is used to implement the metadata 
of the global server and those of the component databases. 
At the global server, the context hierarchy, Figure 5, is implemented and accessed via an inference engine. 
This engine is embedded in the query optimizer and search engine. Following our hierarchical approach 
and the difficulty of implementing a global schema for all possible GIS applications, not all schemes and 
their metadata are implemented in the global server. Thus the global schema includes a general description 
of database contents of each GIS as well as the semantic relationship between them. The metadata will be 
at the same level as that of the schema, e.g., transportation, agriculture, earth resources, etc. The query 
optimizer is enriched with the semantic operators [Bishr, Y. 1996].