Mapping the Universe - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 52

Fig. 2.3 The visual hierarchy. Objects on the map that are most important intellectually are
rendered with the greatest contrast to their surroundings. Less important elements are placed lower
in the hierarchy by reducing their edge contrasts. The side view in this drawing further illustrates
this hierarchical concept

forms of perceptual organization. Objects that stand out against their backgrounds
are referred to as figures in perception, and their formless backgrounds as grounds.
The segregation of the visual field into figures and grounds is a kind of automatic
perceptual mechanism. With careful attention to graphic detail, all the elements can
be organized in the map space so that the emerging figure and ground segregation
produces a totally harmonious design. Later chapters in the topic include examples
of how figure-ground perception plays a role in describing scientific paradigms.
Cartographers have developed several techniques to represent the spherical sur-
face of the Earth. These techniques are known as map projections. Map projections
commonly use three types of geometric surfaces: cylinder, cone, and plane. A few
projections, however, cannot be categorized as such, or are combinations of these.
The three classifications are used for a wide variety of projections, including some
that are not geometrically constructed.

2.1.1 Thematic Maps

All thematic maps consist of a base map and a thematic overlay that depicts the
distribution pattern of a specific phenomenon. Different types of phenomena or
data require different mapping techniques. Qualitative and quantitative maps can
be distinguished as follows.
Qualitative maps show a variety of different phenomena across different regions.
For example, an agriculture map of Virginia would show that tobacco is the
dominant commercial product of Southside, beef cattle the dominant commercial
product of the Valley of Virginia, and so forth. Quantitative maps, on the other
hand, focus on a particular phenomenon and display numerical data associated
with the phenomenon. The nature of the phenomena, either continuous or discrete,
determines the best mapping method. For example, spatially continuous phenomena
like rainfall amounts are mapped using isolines; total counts of population may be
mapped using dots or graduated symbols; mean income on a county-by-county basis
would use area symbols.