GLOBAL SERVER 
The sharable data schemes, posted by the local databases in the 
federation, are integrated to form a global conceptual schema 
(GCS) to serve watershed management. The GCS is held by the 
GS and is intended to support, among others, DUFLOW . 
The operations indicated for the GS in section 2.1 require the 
implementation of methods and rules. The rules are needed to 
select, upon the receipt of the Client request, the servers where 
the required data are located and the procedures to get the data 
using the information contained in the metadata. The methods, 
and the corresponding messages, are needed for: requesting 
information from the LS, aggregating data, and sending data to 
the client. 
MULTILEVEL SERVER 
The conceptual model developed for the integration of the 
management levels is implemented as a schema in the Multilevel 
Server. It is composed of methods, and the corresponding 
messages, to enable the communication between the decision 
making levels and the transfer of data, knowledge and decisions 
amongst them. There are methods for communication, retrieval 
of data, sending decisions. 
6. CONCLUSIONS 
The main objectives of this research were to develop a federated 
database system able to support watershed management at 
different levels of decision making and to develop a model that 
enables the interaction between these levels for environmental 
management. 
Object-Oriented modelling was selected for the development of 
the servers of the federation. Its powerful constructs: 
classification, generalisation, association, and aggregation, 
together with the other features provided by Object-Oriented 
concepts, like inheritance, encapsulation, operations, and 
message passing, a powerful with great 
expressiveness and semantic contents. In the present research, it 
was realised how an object oriented shell on top of the local 
DBMS facilitates the federation of heterogeneous databases. 
Peculiarities of the data structures provided by the local DBMS 
are encapsulated, and in this way are hidden from the user who 
would be provided, additionally, with the more powerful 
constructs of Object-Oriented model for his/her developments. 
renders tool 
The implemented prototype gives an indication of the 
advantages that the client-server computing can offer in the GIS 
environment. Furthermore the prototype showed that multi- 
disciplinary environmental applications can be supported 
through the FDBS architecture. 
The use of the information provided by the Federated Database 
would prove to be more useful if, as it has been shown, all 
management levels can profit from it by means of easy access 
and use, and facilities for interchange of information, data, 
knowledge, and decisions. In this context we are currently 
670 
developing a mechanism for semantic data sharing across 
heterogeneous databases. 
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