Object - Component GIS: The new
Dr. David J. Maguire
Director of Products, ESRI
380 New York Street
Redlands, California, USA, 92373
Tel. 909-793-2853
Fax. 909-793-5953
Email dmaguire@esri.com
Data models
The heart of any AM/FM/GIS is the data model which describes how facilities and
processes in the real world are represented digitally in a computer. In comparison to
other spatial applications, AM/FM/GIS systems tend to have data models which are
highly structured and formalized. For example, in a typical water utility data model there
is a clearly defined set of fittings (e.g., valves, hydrants, meters and back flow
preventers), line types (e.g., canals, mains, and minor lines) and devices (e.g., aerators,
clear wells, chemical injectors and generators) to be modeled. There is also very specific
collection of rules and process methods which need to be encoded in the data model (e.g.,
valid valve types, add connectivity rules, delete processes, and draw methods).
Figure 1: AM/FM/GIS Data Modeling.
Over the last 30 years several different data models have been tried in AM/FM/GIS with
varying degrees of success (Maguire, Goodchild and Rhind 199 1; Maguire and Raper
1992). Many data models (e.g., the CAD, Image and TIN models) are niche or
application specific. In database-oriented AM/FM/GIS the gee-relational model has been
widely and successfully employed for over a decade. In this model the geometry and
topology of geographic features are stored in files, with the attributes in a database
management system (DBMS). The link between the files and database is managed by
AM/FM/GIS software. This model is geometry-centric, modeling the world as a
collection of points, lines and polygons, with operations on the geometry encoded as
separate procedures (scripts). The gee-relational data model has served AM/FM/GIS
the
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users well for many years because of its flexibility, extensibility and good performance.
However, this approach has limitations for modeling the richness of geographic objects,
and it can not be easily extended to support user- or domain-specific features. Also,
recent advances now allow geometry and topology to be stored in commercial DBMS.
Figure 2: AM/FM/GIS data models and their applications.
To try to address these problems object-based data models were introduced. These allow
the state and behavior of objects to be modeled in an integrated way in an AM/FM/GIS
(Worboys, Hearnshaw and Maguire 1990). The state of an object can be thought of as
the properties or attribute data of an object (e.g., for an electrical recloser [a type of
isolation device or circuit breaker] it could be the coil size, interrupting capacity, current
rating in kVA and nominal status). The behavior is the methods or operations which can
be performed on an object (for a recloser it could be create, delete and draw methods, as
well as network connectivity rules).
