We said in Exhibit 1 that a map is always selective. In other words, the mapmaker determines what is, and equally importantly, what is not included in the representation. This is the first important sense in which maps are conventional. What is on the map is determined not simply by what is in the environment but also by the human agent that produced it. Furthermore, we saw in Exhibit 1 that maps employ non-iconic signs and symbols. These are as arbitrary as the letters of the alphabet and are therefore largely conventional. Of course, many elements of maps are at least partly iconic, portraying certain visual features of the landscape represented, but even these images partake to a significant degree of the conventions of the artist (see Imagining natur*e, pp. 35-8, and Beasts and other illusions*, pp. 24-39).
The historian of geology Martin Rudwick has discussed the inherent conventionality of maps in the context of his argument that geology could not become a fully developed science before the development of visual diagrams:
. . . a geological map . . . is a document presented in a visual language; and like any ordinary verbal language this embodies a complex set of tacit rules and conventions that have to be learned by practice . . . [Therefore there also has to be] a social community which tacitly accepts these rules and shares an understanding of these conventions.
Martin Rudwick, 'The emergence of a visual language for geological science, 1760-1840', 1976, p. 151
Mercator's projection maintains compass direction between places as straight lines. This is achieved by making the distance between lines of latitude greater towards the poles and thus introducing considerable distortion of area and shape. The boxed areas indicate that Mercator had an understanding of the epistemological status of a map that differs from our own. Some of the boxes contain more information about how to use the map than we now deem necessary; how to measure distances, for example; and the box In the north-east corner of the map is about Prester John, the mythical king of Africa.