Full text: Proceedings of the Symposium on Global and Environmental Monitoring (Part 1)

616 
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
	        
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