auxiliary feature attribute table (See Figure 7). 
Auxiliary table records are present only when 
needed, and entire tables may be omitted when no 
coincident features are present. This requires 
special application programming to establish the 
database relation to the auxiliary feature attribute 
table and to subject auxiliary features to the same 
processing as primary features stored in the 
regular feature attribute table. 
Despite the extra effort required in 
application programming, this approach is 
frequently adopted when there is no prepared 
alternative, or when the occurrence of coincident 
features is relatively infrequent. The advantages 
of the Auxiliary Table method are that the number 
of coincident features which may be linked to a 
graphic object is unlimited, and the wasted space 
of the Reserved-Space method is avoided. The 
disadvantages of the Auxiliary Table method are 
the extra programming requirements. 
2.4 The Abstracted Feature Method 
This method is the most comprehensive 
solution to the real-world feature linkage problem. 
In this method, the system knows that it needs 
both a one-to-one linkage to feature attributes in 
  
Real-World B 
Features A C 
Graphic 
Objects C: > 3 
  
  
  
  
  
1:1 System Linkage 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Feature c 
Attribute Table E 
PE 5 
: À 
> 3 B x l- 
= 
£e 
ge 
c 
User Attributes 
| Auxiliary Feature 
. Attribute Table 
C | X 
i 
  
  
  
  
Figure 7. The Auxiliary Table Method 
order to support topological processing, and one- 
to-many linkage to feature attributes to support 
real-world coincidence. It satisfies both needs by 
separating the user attributes completely from the 
feature attribute table and moving them to a new 
abstracted feature attribute table whose 
relationship with the graphic objects is indirect 
(See Figure 8). 
ESRI introduced this method for its 
coverage data model when it released "Dynamic 
Segmentation" support for lineal features in 
Release 6.0. Real-world features now correspond 
with an abstracted feature class called "routes" 
which have a many-to-many relationship with 
"arcs" which have a one-to-one relationship with 
graphic objects. The concept was extended to 
areal features in Release 7.0 with the introduction 
of a new abstracted feature class called "regions." 
To date, there is no abstracted feature class for 
point features in an ESRI coverage. 
The advantages of this method are the 
same as the Auxiliary Table method without the 
cost of special application programming, provided 
the system already supports abstracted feature 
classes. The disadvantage is that if we turn 
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
  
Real-World B 
Features A C 
Graphic se 
1:1 System Linkage 
Feature = 
Attribute Table S 
2 
X æ 
yz || B 
p 
E 
e 
© 
Abstracted Feature 
Attribute Table 
X «4 - 
y 
User C z M —— 
Attributes 
  
control o 
indirect lir 
graphic ob 
overhead 
OCcurrence 
3. M 
Ta 
considerati 
methodolo, 
left colum 
feature att 
sections 1 : 
algorithms 
on vector s 
in the italic 
follows. 
Raster Co 
Implied A 
Selective | 
Universal 
Linkage... 
Binary Fl: 
Reserved 
Method 
Auxiliary 
Method 
Abstracted 
  
  
  
Figure 8. The Abstracted Feature Method 
148 
Method 
NOTES: