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ur groups:
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Given a
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the servers
jon devel-
1d restruc-
e.
e Developing a user interface tailored to the
application.
e Displaying the results of his or her analy-
sis in different visualization modes: graph-
ics, photo-imagery, text, animated modeling,
video, etc.
e Checking the data back onto the server, ei-
ther as a new version or as an updated and
revised form of an existing one.
This mode of operation raises a number of
questions.
e Given the different data types, what
database management system can retain, in
a consistent manner, all the data of a server?
e How should updates be reflected in the
stored data? In particular, is there a storage
efficient method for managing revised copies
from which past versions can be reconstruct.
e Given the information on the server, can a
user request data over an area for only a
selected set of features? What concurrency
control mechanism would be most efficient
for multiple data access under such mode of
operation?
e In the case of data held on special devices,
e.g., CD-ROM, can information be concur-
rently accessed by multiple user?
e What is the optimum data capacity for a
cost-effective on-line data distribution given
users’ access profiles?
These and similar related questions are what
the Delta-X project is concerned with. Such a
model of operation is not exclusive to GIS. Sim-
ilar problems have been addressed in purely cor-
porate database environments. One approach
to managing heterogeneous databases is the use
of multi-databases management systems [3, 11,
13, 17]. Except for, sound (i.e., speech, voice
and music), GIS data requirements are similar
to those of multimedia computing. Similar prob-
lems are encountered in a network of multi-media
databases [6].
On a wide area network of GIS data servers
that service thousands of clients, each data server
must be configured for very high rate and long
505
lived transaction since typical operations involve
the retrieval and restructuring of data. This may
take some considerable amount of time to com-
plete. Asin the case of multi-media database, the
potential for high volume data transfers requires
very high network bandwidths, good data com-
pression and decompression techniques with no
information loss, large storage capacity as well as
high performance I/O bandwidth between main
memory and secondary storage.
Storage and delivery of data to client worksta-
tions constitute only one aspect a series of ser-
vices in Delta-X. Other services include adminis-
trative support, secure and reliable data capture,
delivery of data to a large customer base, visu-
alization of results of remote computations. We
describe the basic ideas of Delta-X, its function-
alities and its current implementation status in
the subsequent sections. A number of projects
and researchs are being pursued elsewhere to ad-
dress similar problems. These include the Geo-
DASDBS [16], the SEQUIOA project [19] and the
Papyrus project [12].
3 A Federated Spatial Informa-
tion System
3.1 Basic Configuration
To establish the mode of operation in network
of GIS workstations as discussed in the preced-
ing sections, a federated spatial information man-
agement system development (Delta-X) was ini-
tiated in the GIS Division, Natural Resources,
Canada. Delta-X is best characterized as a fed-
erated multi-database system [3, 17] that is con-
strained for:
e remote read-only data access,
e remote computations,
e remote display and
e limited database management system func-
tions.
The global model is materialized, using a set
of consistent transformation rules, onto local
databases. The local database management sys-
tems (LDBMS) that can be relational, object-
oriented or a simple file system. Similarly,
the schema definitions of a local database may
