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INFORMATION EXCHANGE
An information exchange system addresses
the problem at the application
entity / GIS construct level. It converts
forestry stands to forestry stands,
municipal roads to municipal roads.
Considerations for developing an effective
information exchange system should
include the following issues:
• To achieve the application entity
match, the exchange system must
know how each entity is
represented in each system. As
most GIS systems do not provide
meta-information, an information
exchange system must provide the
ability to produce and maintain
data dictionaries.
• Each GIS data set refers either
explicitly or implicitly to a
coordinate system, projection,
spheroid and datum. The
exchange system should address
this.
• Each GIS data set has an implicit
or explicit data dictionary. For
example, roads may be stored on
level 5 and differentiated by
colour on System A, while they
are stored by feature code and
differentiated by line style on
System B. An exchange system
should convert roads to roads.
• Each GIS data set will use models
to represent GIS constructs.
Some systems provide more than
one model, permitting the user to
choose the most effective one for
a specific entity. An exchange
system should convert between
models.
The above specifies a very powerful
system. There are some limitations that an
exchange system cannot overcome. These,
most obvious when converting from one
system to a second system and then
converting the data back, include:
• Resolution of data cannot be
improved in an exchange system.
If the incoming data is stored at a
one metre resolution, the results of
the exchange will be to the
nearest metre - even if it is
specified in inches. This problem
is even more apparent when
looking at classified data. If one
system divides data into classes at
intervals of 20, and a second
system at intervals of 30, there is
informational incompatibility.
This is most evident when data is
converted from one system to
another and back.
• Interrelationships may be lost if
they cannot be stored in an
intermediate system. For
example, the label for a polygon
is definitely associated with the
polygon. It should be possible
(even automatic) that, if the
information about the polygon
changes, the label reflects this.
Yet, in some systems, there is not
an inherent means of connecting
a polygon label with polygon
attributes. If system A stored that
interrelationship and system B did
not, then it would be lost on
conversion from system A to
system B and could not likely be
replaced were the data transferred
back.
• In cases where application entities
are not modeled by the same GIS
construct, conversion may not be
possible. For example, as rivers