Databases Reference
In-Depth Information
terms of internal database concepts in order to be dealt with by the system. To come up with this
dual nature, a visual representation has to be based on a so-called
visual formalism
, a term introduced
by David Harel as follows (
Harel, D.
,
1988
): “The intricate nature of a variety of computer-related
systems and situations can, and in our opinion should, be represented via visual formalisms; visual be-
cause they are to be generated, comprehended, and communicated by humans; and formal, because
they are to be manipulated, maintained, and analyzed by computers.” Visual formalisms include
familiar objects such as tables, diagrams, icons, etc.
In a VQS, the visual representation should effectively convey all and only the database infor-
mation. In other words, a visual representation should be “consistent” with respect to the database
it represents. We say that a representation is consistent if it is “correct and complete” (
Haber et al.
,
1994
). A visual representation is complete if the user can get from it all the database information,
and it is correct if no other information can be derived from it. Note that using a consistent visual
representation to depict the information of interest is crucial in order for the user to correctly grasp
the database information content (see Mackinlay's pioneering work on automatic design of graphical
presentations (
Mackinlay, J.
,
1986
) and a more recent topic (
Card et al.
,
1999
) for many examples of
misleading visual representations).
For example, let us consider the data in Table
2.1
, which refer to towns in Italy, number of
people living in each town, their location with respect to Rome and their distance in kilometers also
with respect to Rome. We may visualize these data through a graph, as the one in Figure
2.1
.
Table 2.1:
Example of database (Note, position
relative to Rome).
To w n
People
# Position
Distance
Rome
4,000,000
0
Milan
1,800,000
North
600
Naples
1,500,000
South-East
200
Pisa
150,000
North-West
350
Pescara
200,000
East
220
In this case, the visualization is neither complete nor correct. It is not complete because not all
attribute values in Table
2.1
have an appropriate representation. There is nothing in the figure to infer
information about the approximate number of people, distance from Rome, or their mutual position.
Moreover, the distribution of the towns in the graph may even convey wrong information about their
position since Naples is very likely to be interpreted as being North of Rome, Milan as being East,
etc., thus the visual representation is also not correct. The example in Figure
2.2
shows another graph,
from which the user can infer all data in Table
2.1
. This representation is complete and correct. Since
we are referring to geographical information, we are taking into account that the observer usually
considers the distribution of data on the plane according to the four cardinal points. There is a
large amount of literature on this topic, starting from (
Mackinlay, J.
,
1986
) up to a variety of other
