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.