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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