Mapping Associations - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 89

placed at the center of such triangles. Edges are drawn to show the boundaries of
large districts. Features such as signposts, history and backtracking mechanisms
were also considered in their city image metaphor, but they were not fully
implemented.
Legibility of a city helps people traveling in the city. The more spatial knowledge
we have of a city, the easier we can find our way in it. Thorndyke and Hayes-Roth
distinguished three levels of such spatial knowledge (Thorndyke and Hayes-Roth
1982 ) as landmark knowledge, procedural knowledge, and survey knowledge.
Landmark knowledge is the most basic awareness of specific locations in a city
or a way-finding environment. If all we know about London is the Big Ben and
the Trafalgar Square, then our ability to navigate through London would be rather
limited. Procedural knowledge , also known as route knowledge, allows a traveler to
follow a particular route between a source and a destination. Procedural knowledge
connects isolated landmark knowledge into larger, more complex structures. Now
we should know at least one route leading from the Big Ben to the Trafalgar Square.
At the level of Survey knowledge we have fully connected topological information
about a city. Survey knowledge is essential in performing way-finding tasks. A good
example of survey knowledge is the Knowledge of London examination that
everyone applying for a taxi license must have. The official Transport of London
says to each applicant:
You must have a thorough knowledge of London, including the location of
streets, squares, clubs, hospitals, hotels, theatres, government and public buildings,
railway stations, police stations, courts, diplomatic buildings, important places of
worship, cemeteries, crematoria, parks and open spaces, sports and leisure centers,
places of learning, restaurants and historic buildings; in fact everything you need to
know to be able to take passengers to their destinations by the most direct routes.
You may be licensed either for the whole of London or for one or more
of the 16 suburban sectors. The “All London” license requires you to have
a detailed knowledge of the 25,000 streets within a six-mile radius of Charing Cross
with a more general knowledge of the major arterial routes throughout the rest of
London. If you wish to work as a taxi driver in central London or at Heathrow
Airport you need an “All London” license.
We will briefly introduce the famous traveling salesman problem (TSP) in
Chap. 4 . The salesman needs to figure out a tour of a number of cities such that he
visits each city for once only and the overall distance of the tour must be minimal.
If the salesman is in London, it looks his best bid is to take a taxi. Figure 3.2 shows
the coverage of London taxi drivers' survey knowledge.
The most sound survey knowledge is acquired directly from first-hand navigation
experience in an environment - London's taxi drivers have certainly demonstrated
their first-hand navigation experience in London. Alternatively, we can develop our
survey knowledge by reading maps. However, survey knowledge acquired in this
way tends to be orientation-specific, which means that the navigator may need
to rotate the mental representation of the space to match the environment. This
concern led Marchon Levine to explore how this phenomenon should be taken into