EXCHANGE OF DATA 
by Seren Riff Alexandersen 
National Survey and Cadastre - Denmark 
Why do we have to exchange data, can't we use one big common system ? 
The historical background for using GIS in Denmark will be gone through. The different national as well as international 
standards for exchanging CAD-data and GIS-data will be expounded. 
Finally the Danish standard for exchanging geographical related information - the so-called DSFL-format - will be 
explained, and so will the possibility of updating data without losing the connection between the graphic-data and the 
attribute-data. 
KEY WORDS: Data Exchange, GIS/LIS, Standards. 
Why exchange of data? 
The fewest users of geographic information systems collect 
all the necessary data themselves. Usually, "the map" is 
bought as semi-manufactured articles from e.g. photogram- 
metric firms, chartered surveyors and Kort- og Matrikelstyr- 
elsen (National Survey and Cadastre-Denmark). These data 
are merged into your own data to a proper geographic 
information system, used later on within the organization. 
In order to "defend own values" it might be of importance 
to "sell" data to others. 
Consequently, it is a question of being able to receive data 
from others as well as being able to deliver data to others. 
If we all used one and the same central geographic 
information system, such a sequence of operation would 
not cause great problems, but we are indeed Danes. We 
cannot run away from the Danish mentality implying "we 
will do it ourselves", "we can do it ourselves" and "we are 
better than others". A typical example is that three of our 
regional natural gas companies build up different systems 
in spite of the fact that they have to solve the same task 
with equipment from the same supplier - that is Danish. 
If we want to keep this independence to build up our own 
Systems covering our own needs, it is necessary that we, at 
the same time, bind ourselves to exchange data with others 
in a standardized way. The advantage of standards for 
exchange of data is partly that the same system supplier 
can deliver and maintain the necessary edp-programmes 
and partly that you are free to exchange data with other 
systems as long as they observe the same standards. The 
previously mentioned Danish mentality is probably the 
reason why we in Denmark started early to deal with the 
question of standards for exchange of digital map related 
information, and that, already in 1982, we got a Danish 
standard in this field, the one we colloquially call the 
DSFL-format. 
What does the term "exchange" cover? 
When you talk about a standard for exchange of geograp- 
hic information, the standardization normally comprises the 
following three items: 
- the medium of transport 
The exchange of digital geographic information is 
normally not time critical, for which reason you 
often choose to use a text format stored on 
magnetic tape or discettes depending on the 
amount of data. Even if we stick to international 
standards, we often have problems with the Danish 
letters "ae", "o" and "À". 
In future, it will be of current interest to be able 
to exchange digital geographic information via edp- 
net. Abroad tests have been made with online 
updating of decentralized geographic information 
systems. 
- description of syntax 
To make it possible to exchange the coherence of 
data, it is necessary that the data arrive in a fixed 
succession, i.e. follow a certain syntax. If e.g. you 
want to exchange a set of co-ordinates in system 
34, the syntax will be that the y-value shall arrive 
before the x-value. In a standard for exchange of 
geographic information you lay down the coherence 
to be exchanged and describe the syntax belonging 
to it. 
At present international efforts are being made to 
avoid these "rigid" syntax rules and instead to 
exchange information about the data coherence 
simultaneously with the exchange of data. 
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