databases, is the third semantic level, the relationship between contexts is defining the relationship 
between different GIS applications. The highest level of semantics is the relationship between nodes. 
Figure 3 Context and node Hierarchy 
As shown in Figure 3, a node is a collection of interrelated contexts. Nodes can be divided into sub-nodes. 
A sub-node can have one or more context. Context refers to the assumptions underlying the way in which 
an interoperating agent represents or interprets data. A context is defined by one and only one set of 
semantic specifications. A context can be structured in a hierarchical way. Hence semantic specifications 
of a lower level context are used as building blocks for those at a higher level. A context can have one or 
more sub-contexts. Each context corresponds to one and only one database. A database in turn 
corresponds to one and only one data model. A data model consists of one or more hierarchies. A hierarchy 
is formed by one or more object classes, i.e., intension. A class can have one or more instances, i.e., 
extension. 
4.1 Federated Database Architecture 
[Radwan M. et al., 1996] introduced an 
architecture for sharing data stored in disparate 
geographic information systems. It is four levels 
architecture as shown in Figure 4 
• At the first level, level 1, there are 
schemes which represent the part of 
the database that each member in the 
federation is willing to share. These 
schemes are represented with the 
native language of the host. 
• The second higher level of schemes 
is to abstract the same data posted at 
Data basa* 
Application Database! (DSS DB) 
► A 4 
Shorable data in ! 
O.O. schema 
ì Sharable dala in 
\ O.O. schema 
Sharable data in 
O.O. schema 
i 
L 
i 
! 
Level 4 
Level 3 
Level 2 
Shareable dala 
(Schema l) 
Shareable data 
(Schema 2) 
Shareable dala 
(Schema 3) 
a A a 
fC ^ 
F F 
F ^ 
Elementary • 
Elementary 
Elementary 
DB1 J 
DB2 
DB3 ^ 
Level 1 
Level 0 
Figure 4 federating heterogeneous databases