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Title
The 3rd ISPRS Workshop on Dynamic and Multi-Dimensional GIS & the 10th Annual Conference of CPGIS on Geoinformatics
Author
Chen, Jun

ISPRS, Vol.34, Part 2W2, “Dynamic and Multi-Dimensional GIS”, Bangkok, May 23-25, 2001
4. Federated Spatial databases
4.1 the Integration of Spatial Data
In the field of database theory, data or database
integration is an often-discussed interoperability
topic. Some times users want to access data from
various distinct databases. They used to create a
new database and migrate all data from other
databases to the new database. This is an approach
of re-enginneer databases. But the method has its
obvious limitations. Sheth and Larson (1990) provide
the concept of multi database systems (MDBS). A
MDBS is homogeneous if the parts are managed by
the same type of software and is heterogeneous if
managed by different kinds of software. A MDBS is a
federated database system (FDBS) if the autonomy
of the component is preserved (ANDREJ
VCKOVSKI, 1998).
Some of the current commercial software packages
can create vector database, DEM database and
image database. But it is difficult to integrate the
three heterogeneous spatial databases. This paper
will introduce a method that integrates the three
databases based on OPENGIS, COM.
The Open GIS Consortium, Inc. vision statement
states that “OGC envisions the full integration of
geospatial data and geoprocessing resources into
mainstream computing and the widespread use of
interoperable geoprocessing software and geospatial
data products throughout the information
infrastructure.” (OGC, 1999). The Open Geodata
Interoperability Specification (OGIS) provides a
framework to create software that enables users to
access and process geographic data from a variety
of sources across a generic computing interface
Component Object Model (COM) is software
architecture. It allows the components that made by
different software vendors to be combined into an
integrated application. COM has the following
characters:
•Defines a binary standard for component
interoperability:
•Is programming language-independent:
•Is provided on multiple platforms:
•Provides for robust evolution of component-
based applications and systems;
•Is extensible.
•A base interface providing:
•A way for components to dynamically discover
the interfaces implemented by other components.
•Reference counting to allow components to
track their own lifetime and delete themselves when
appropriate.
•A mechanism to uniquely identify components
and their interfaces.
•A “component loader" to set up component
interactions and additionally in the cross-process and
cross-network cases to help manage component
interactions.
•Basic interoperability is provided by COM's
use of vtables to define a binary interface standard
for method calling between components.
4.2 Data Model of Federated Spatial Databases
The data model of federated (integrated) spatial
databases is based on the technology of COM and
OPENGIS (see Figure4).
•GeoDatabase is a COM component. It has an
interface named IGeoDatabase. Users access the
component through interface pointers. The
component includes some fundamental spatial
methods and properties, such as the operations of
spatial objects and features.
•GeoOracle is a COM component. It supports
IGeoOracle interface. Through IGeoOracle interface
users can operate the vector database, image
database and DEM database that are saved in
Oracle spatial database. The IGeoOracle interface is
inherited from IGeoDatabase interface using the
technology of COM aggregation.
•GeoGeostar, GeoArcInfo, same as GeoOracle,
are COM components. They deal with their self data
sources and are inherited from GeoDatabase.
•GeoDatabasse is a base class. According to
COM polymorphism, users can use the component to
operate all data sources. In order to open a kind of
database, only_the database name and database
type are needed. Users do not need to care for the
inherited components and the database that are
using, such as GeoOracle component or GeoGeostar
component. But if users want to develop a certain
database, such as Oracle database, he can use the
GeoOracle component directly.
•This data model has extensible. If users have
a new data source, a new component can be created
and be inserted to the component group. The new
component is also inherited from GeoDatabase
component. So the GeoDatabase component can
invoke it dynamically.
•The Geometry component which is based on
OPENGIS, describes the detailed Geometry object,
Spatial References and feature collections. Figure 5
describes the Geometry object model. IGeometry is a
base interface. It derives from IUnknown interface,
which is a special interface of COM. All component
objects are required to implement the IUnknown
interface, and conveniently, all other COM and OLE
interfaces derive from IUnknown.