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3. OBJECT-ORIENTED LOGICAL DATA MODEL IN 
GIS 
The conceptual data model is abstracted from natural 
features or their images. The model needs to be translated to 
logical data model so that it can be represented in computer. 
For GIS, attribute data are usually expressed by relational data 
models. Similar to relational data models, object-oriented 
data models might be designed for attribute data in GIS, 
because object-oriented data model can contain and extend 
relational data model. Here the emphasis is on geometric 
data organization since it is more complicated. 
A very important concept in GIS is the topological relationship 
between spatial objects. Two basic relations for most GIS 
applications are connection relation and association or 
aggregation relation. 
Thare are five geometric classes: COMPLEX, SURFACE, 
LINE, NODE-POINT, and one LOCATION type. They are 
organizated via aggregation semantic model. An object- 
oriented data model in GIS is shown in figure 3. 
Object-oriented data model is different from relational data 
model in that the former not only supports variable length 
attributes, but also can directly deal with SET OF OBJECTS in 
an attribute field. In this case, a single structure table represents 
a type of spatial object, which is defined as a class. Therefore, 
the semantics of the object-oriented data model is more 
natural and clearer than the relational model. 
A partition should be established above the Spatial Objects. All 
objects belong to the partition. Several partitions are organized 
into a project. In order to satisfy different requirements for 
application, display and mapping, thematic layers can be 
custom-designed. Each theme might contain several feature 
classes. Here thematic layer is not like coverage, with the 
former organized only in logical, not physical. sense. 
  
  
  
  
  
  
  
  
Figure 3 An object-oriented spatial data model 
4. OBJECT-ORIENTED 
MANAGEMENT ENGINE 
SPATIAL DATA 
An object-oriented spatial database management engine called 
GeoDB developed by Research Center for GIS at Wuhan 
Technical University of Surveying and Mapping is designed 
based on the above spatial data logical model. The engine is 
actually a class library of spatial data. The database 
programming language is an extended variant of C++. It uses 
the same compiler tools and context as C++. 
301 
The system-defined classes constitute an engine of database 
management. The user can model the application domain by 
using these classes. The system-defined classes mirror the 
spatial object model semantics. SPATIAL OBJECT is the root 
of an object lattice. The class SPATIAL OBJECT is a 
superclass in C++, where there is no attribute, but it includes 
many functions such as attribute data structure definition, insert 
select, delete and update attribute records, index and join 
, 
tables, and other operations like a general database 
management system. These operations are responsible for 
dealing with attribute data in GIS. 
International Archives of Photogrammetry and Remote Sensing. Vol. XXXI, Part B4. Vienna 1996