;IES 
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nkage employed 
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lassification, for 
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e data structure 
ept of a feature. 
an be added with 
soon as the user 
attempts to extract the perceived features as vector 
polygons, however, he or she enters a different 
world of data linkage. 
1.2 Implied Attributes 
The simplest form of attribution for vector 
objects is based on feature groupings usually 
called levels or layers. A level is reserved for 
each real-world feature to be discriminated in the 
data. Early efforts at automated mapping with 
Intergraph systems, for example, frequently used 
this strategy. When the levels are known only by 
a unique number, the user usually maintains a 
reference document which explains the various 
level assignments. AutoCAD-based systems have 
some advantage insofar as the level names are 
user-defined and therefore self-documenting. 
When two features are geographically 
coincident in this system, they must be 
represented by redundant graphic objects placed in 
each level corresponding to the coincident real- 
world features (See Figure 2). 
The advantage of this system is its 
simplicity. Since there is no external database, 
this is an ideal system of attribution when a single 
attribute (feature level) can be used to discriminate 
all features. Translation to other spatial 
information systems is relatively painless so long 
as the feature level survives translation. 
Intergraph and AutoCAD formats have emerged 
as de facto standards for this system of attribution 
and most GIS software will import and export 
data in these formats preserving the feature level 
  
  
  
  
  
  
  
  
  
Real-World B 
Features A C 
PR nt ri ve us 
Graphic ‘Level 1 i 
Objects Yevel2 0 
EN BEC Lei boo luu. 2. 
User 
Lookup Table 
Level 1 = A 
Level 2 = B 
Level 3 = C 
  
  
  
  
  
  
Figure 2. Implied Attributes 
as a user attribute. 
The disadvantage of this system is the 
potential for asynchronization between coincident 
features on different levels. Coincident features 
are usually copied from one level to another at the 
time of initial automation, but during the lifetime 
of the data, as it receives graphic updates, there is 
always the potential that one feature will be 
changed, while its coincident partner is ignored. 
Procedures must be implemented which link 
coincident features in such a way that if the 
system cannot automatically synchronize the 
geometry between features, then it must at least 
alert the user to the existence of coincident 
partners when a graphic change is attempted. 
These procedures can require low-level application 
programming and/or additional maintenance labor 
costs which erode the savings anticipated by 
choosing the relatively simple system of implied 
attribution. 
1.3 Selective User Linkage 
Many mapping systems store features as 
vector objects with selective linkage to an attribute 
database. Most un-enhanced CAD-based 
applications today fall into this category. A 
selective linkage between graphic objects and their 
attribute records allows the user to attach any 
number of attribute records to a single graphic 
object. Real-world features which are 
geographically coincident, therefore, can find 
representation as distinct attribute records sharing 
a common graphic object (See Figure 3). 
Selective linkage is an efficient method for 
providing intelligence to a limited number of 
intelligent features, especially when they may be 
surrounded by many non-intelligent, or "dumb" 
features. Examples are common in the AM/FM 
world where mapping systems are used to track 
specific "outside plant" assets surrounded by 
dumb representations of geographic landmarks. 
One drawback to the selective-linkage 
approach, however, is that it does require effort, 
usually human labor, to establish and maintain the 
individual links between graphic objects and 
attribute records. This effort can be cost- 
prohibitive if the data are subject to frequent 
updates, especially graphic updates which are 
often multiplied when the spatial relationship 
between features is significant. For this reason, 
the majority of GIS applications today employ a 
system of universal automatic linkage.