Legibility relates to the ease with which one can form a cognitive map of an environment. Comparison of various cities with regard to legibility. Urban planners have tried to design cities with an eye to improving their legibility. These elements enhance human ability to impose a pattern, an organizing structure which assists learning. Use of simplifying frameworks which allow humans to cope with vast quantities of information.
Visual imagery appears to have fewer limitations space-wise in comparison to linguistic codes.
Landmarks tend to facilitate the use of imagery. When landmarks are unique and freestanding, they tend to produce rich codes. E.g., Eiffel Tower in Paris. There is nothing like it in the world, let alone Paris. You can't confuse it with anything else.