Spatial objects can be divided into point feature, line feature, 
surface feature and complex feature. Four corresponding classes 
in CH are declared as subclasses of the class SPATIAL 
OBJECT. At the same time, they are supercalsses of all features 
in GIS (see Figure 2). Generally, the feature classes are defined 
by the user. Except SPATIAL OBJECT, NODE-POINT, LINE, 
SURFACE, and COMPLEX, the classes ARC, 
ANNOTATION, PARTITION, PROJECT and type 
LOCATION are system-defined. The class PARTITION and the 
class PROJECT contain some matedata attributes and spatial 
operations like spatial index, attribute merge in different 
partitions etc.. 
System-defined classes have  system-defined geometric 
attributes and some operations, which are inherited by the 
user-defined subclasses. 
Objects are identified by an object identifier (OID). An objec 
identifier is generated by the system and represented by the 
system-defined attribute OID which the user can refer to. The 
object identifier is not reused even when the object is deleted. 
Geographic information systems using object identifier have the 
advantage that only the topology of modified objects need to be 
rebuilt when spatial data are edited. While a GIS based on 
records must re-build topology for all objects in a coverage 
when an arc or a polygon is modified. 
The system-defined classes containing attributes and methods, 
are an extended subset of C++, which form an engine for spatial 
database management. Actually they are functions and 
attributes of Application Programming Interface (API) based on 
C++. The user can define new classes, and add new attributes 
and operations for user-defined classes. If he declares that the 
new class is a subclass of system-defined classes, all attributes 
and operations from its superclasses, including superclass of 
superclass, for example class SPATIAL OBJECT, are 
inherited. The engine provides sufficient system-defined classes 
with their attributes and operations to make possible a wide 
variety of applications in GIS. 
In GeoDB, spatial topological relations and complex objects 
are represented by object identifiers. Nested relations make it 
possible to represent complex objects through clustering. There 
are seven geometric classes in Figure 3, which are COMPLEX, 
SURFACE, LINE, NODE-POINT, ANNOTATION, ARC, and 
one LOCATION type. Each spatial object has an identifier, 
which contains class identifier, so the inspection of an object 
identifier of an object can determine the class to which the 
object belongs without looking up the content of the object. 
While the type LOCATION contains only coordinates not 
identifier. In this model, classes COMPLEX, SURFACE, LINE, 
ARC, and NODE-POINT are feature classes which have 
302 
attributes. Class ANNOTATION is a secondary class and each 
annotation is related to its object. 
In object-oriented logical data model, an object class can be 
constructed into a table, in which we can explicitly declare its 
relations with other classes. Unlike relational data model, a 
table can not represent an entity type and the relations. The 
object manipulation functions support associative access of 
object and direct access of complex object without explicit 
joints. 
The GeoDB provides some main functions of commercial 
database management system like page buffer, object buffer, 
data security, integrity control, multiple user access and 
concurrency management, object index, spatial index, and etc. 
The system architecture is shown in figure 6. 
The data management subsystem provides nested relations and 
supports variable-length records so that it can efficiently store 
objects, which have several kinds of non-formal structures such 
as variable-length attributes, set of object identifiers and multi- 
value attributes. A tuple represents an object. When 
information that an object contains can not fix its length, a 
variable-length tuple need be set to store the variable-length 
data. The data management subsystem provides two data types, 
fixed-length and variable-length fields. 
The data management subsystem in GeoDB can store both 
spatial data and non-spatial data. It has the same storage 
management functions as commercial DBMS. However, in 
order to interface with some other general DBMS like Oracle, 
Sybase, Ingres, Paradox and etc., a gateway in GeoDB will be 
developed as a general-purpose extended relational database 
system to support the object-oriented spatial data model and 
provide the operations on top of RDBMS. (We can adopt the 
ODBC as API of the gateway). It replaces the data management 
in GeoDB for data storage. The object management, object 
buffer, object index, spatial index, object manipulation, schema 
definition, and spatial topological relations building and 
maintainenance are still accomplished by the object-oriented 
spatial data management engine. 
5. OBJECT-ORIENTED GEOGRAPHIC INFORMATION 
SYSTEM 
An object-oriented geographic information system based upon 
the object-oriented spatial database management engine 
(GeoDB) has been developed by the Research Center for 
Geographic Information System at Wuhan Technical University 
of Surveying and Mapping in China. The system is named 
GeoStar. It is developed with C++ and includes the modules 
shown in Figure 4. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996 
  
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