1.4 Universal System Linkage 
Systems which provide universal attribute 
linkage offer the user an all-or-nothing decision 
about which features should receive attribute 
intelligence. Features must be separated into 
specific feature classes depending upon topological 
behavior (point, line, or polygon), then the user 
may have the option of making all features in a 
feature class intelligent or not. If the user chooses 
to make a feature class intelligent, the system 
automatically creates and maintains a one-to-one 
attribute linkage for every graphic object in the 
specified feature class. ESRI's "coverage" data 
model as maintained by its ARC/INFO software 
is an example of a system with universal 1:1 
linkage (See Figure 4). 
The overwhelming advantage of a 
universal linkage system is that it facilitates 
automated processing on entire classes of objects 
with the assumption that they all have a minimum 
set of standardized properties. These properties 
are put to good use as spatial descriptors and 
topological pointers. In an ESRI coverage, for 
example, spatial descriptors include length, area 
and perimeter attributes. Topological pointers 
include from-node, to-node, left-polygon and 
right-polygon attributes. By keeping these spatial 
descriptors and topological pointers in a user- 
accessible external database, the user can easily 
add other attributes while allowing the system to 
carry the burden of maintaining the linkage to 
  
  
  
  
  
   
  
  
  
Real-World B 
Features A C 
Graphic sed TEE C EU 
Objects oo OO QC 
_ 1:Many User Link age 
User 
Attribute Table 
A 
B 
  
  
  
  
  
  
  
  
  
  
  
Figure 3. Selective User Linkage 
146 
graphic objects, even during extensive graphic 
editing. 
For all its advantages, however, a 
universal linkage system does introduce some 
complications when it comes to the handling of 
coincident features. No longer can the user 
simply point to a specific graphic object and attach 
double attribute records. The system will not 
accept it because there can be only one set of 
spatial descriptors and topological pointers per 
graphic object, and all user attributes must be 
appended as an extension to the system attributes. 
Special handling requirements for 
coincident features, therefore, are of particular 
interest to those users of GIS data who need to 
allow the system to maintain a universal one-to- 
one linkage between graphic objects and feature 
attribute table records. 
2. REAL-WORLD FEATURE LINKAGE 
There are at least four workarounds to 
achieving one-to-many relationships between 
graphic objects and real-world features while at 
the same time allowing the system to maintain the 
one-to-one attribute linkage necessary for 
topological processing. These workarounds are 
distinguished by the way in which user attributes 
can imply feature coincidence. We will refer to 
these workarounds as the Binary Flag Method, the 
Reserved Space Method, the Auxiliary Table 
Method and the Abstracted Feature Method. 
  
Real-World B 
Features A C 
Objects — } doas 
  
  
  
  
  
  
  
  
D 1:1 System Linkage 
Fe, Feomire. | 
| Attribute Table 
_ 2 A | 
Led B C 
  
  
  
  
  
System Attributes ! User Attributes 
  
  
  
  
Figure 4. Universal System Linkage 
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