18 
®< 
= = UV X 
- 
id we 
ObJECT- ORIENTED | DATA. MODELS. IN. GIS 
Gong Jianya Li 
Department of Photogrammetr 
Wuhan Technical Universit 
Wuhan, P. R. China 
Abstract 
Currently, experts in Geographic Information S 
This advanced technique, based upon the defini 
operations in a modular fashion, is character 
paper discusses object-oriented models for geo 
with this concept has been carried 
Key words: Object-oriented data model, Classification, 
Inheritance, Propagation. 
1. INTRODUCTION 
There are three traditional data models: 
hierarchical, network and relational models, 
which all try to deal with the same problems with 
tabular data. The relational data model is the 
most popular DBMS model for GIS. For example, 
ARC/INFO uses INFO, System 9 uses EMPRESS, 
several GISs use Oracle, some PC-based GISs use 
Dbase III. However, the relational model exhibits 
some problems such as efficiency, data semantics, 
model extension, object identity, and program 
interface. It should be pointed out that the 
three traditional data models including the 
relational model do support these vital features 
in various degrees. 
2. THE CONCEPTS OF OBJECT-ORIENTED DATA MODEL 
Object-oriented approach is a new technology in 
software engineering and database. A definition 
of object-orientation is that an entity of 
whatever complexity and structure can be 
represented by exactly one object [Egenhofer and 
Frank 1989a], and every object is a single 
programming entity that combines data and 
procedures or functions that operate on that data 
[Shafer 1988]. The object-oriented data model is 
built upon the four basic concepts of 
abstraction: classification, generalization, 
association, and aggregation. The semantic model 
is enriched by the concepts of inheritance and 
propagation which describe the derivation of 
properties in generalization  hierarchies and 
values in aggregation hierarchies, respectively. 
2.1 Object 
In an object-oriented approach, all conceptual 
entities model objects, for example, & node, an 
arc, a river or a province on a map are objects. 
The object contains a set of data to describe its 
behavior and a set of methods to operate itself. 
That is: 
Object-(ID, S, M) (1) 
Where: ID is an identifier of the object; 
M is a set of methods; 
S is the behavior of the object, it is 
represented by either attributes or identifier of 
other objects. That means that an object can 
include another object in an object-oriented 
approach. 
Deren 
773 
y and Remote Sensing 
y of Surveying and Mapping 
ystem are promoting an object-oriented approach for GISs. 
tion of object types in combination with the corresponding 
ized by clear coding which can be easily maintained. This 
metric and attribute data in GIS. The investigation of a GIS 
out and some approaches and applications are also described. 
Generalization, Assoiation, Aggregation, 
2.2 Classification 
Classification is the mapping of several objects 
(or instances) within a common class (or type) 
[Egenhofer and Frank 1989a]. In the object- 
oriented approach, every object is an instance of 
a class. Each class characterizes the behavior of 
its instances by describing the operators that 
can manipulate these objects. These operations 
are the only means of manipulating objects. 
Classification is referred to as the instance of 
relationship because the individuals are 
instances of the corresponding class. 
Thus, we establish an abstraction class by : 
Classz(CID,CS,CM) (2) 
Where: CID is an identifier of the class, i.e. 
class name; 
CS is the behavior of the class; 
CM is a set method of the class. 
Then 
S€ CS and M = CM when Object Class 
For example, a city GIS includes the classes 
building, street, park, phone-line, etc. A single 
instance, such as the building with the address 
‘King street 51’, is an object of the class 
building. Operations and properties are assigned 
to object types, so for instance the class 
building may have the properties address, holder 
and building date which are specific for all 
buildings. The operations like data retrieval and 
update are also the same for all buildings. 
2.2 Generalization and Inheritance 
2.2.1 Generalization Generalization organizes 
several classes of the objects with common 
properties and operations to form a more general 
superclass. There are two classes: 
Class,7(CID4 , C84, CSp, CM, , CMg) (3) 
Classg=(CID2,CSp,CSc,CMp,CMc) 
Classy and Classy have the same subset CS, of 
behavior and a subset CMA of method. 
CS,C CS] and CSACCS9 as well as CM,C CM; and 
CMAC CM9