Mapping the Universe - Mapping Scientific Frontiers: The Quest for Knowledge Visualization - page 50

Cartography is a process of abstraction, involving selection, classification,
simplification, and symbolization. Each type of abstraction reduces the amount of
specific detail shown on the map. On the other hand, the map reader needs enough
information to be able to understand the map. The most complex of the mapping
abstractions is symbolization. Two major classes of symbols are used for thematic
maps: replicative and abstract. Replicative symbols are designed to look like their
real-world counterparts; they are used only to stand for tangible objects such as
coastlines, trees, houses, and cars. Base-map symbols are replicative in nature,
whereas thematic-overlay symbols may be either replicative or abstract. Abstract
symbols generally take the form of geometric shapes, such as circles, squares, and
triangles. They are traditionally used to represent amounts that vary from place
to place.
Maps and their quantitative symbols are unique mechanisms for the communica-
tion of spatial concepts. Because it is possible to cram a lot of information into one
symbol, the designer often tries for too much of a good thing. Overloaded symbols
are hard to understand; they may send the wrong message or incomprehensible
messages. For example, the proportional circle is the most commonly misused
symbol. It can be sized, segmented, colored, or sectored. It is tempting to include
all of these on one map. Unfortunately, if the map reader cannot see the spatial
distribution clearly and easily, then the map is not communicating. If a thematic
map overloads proportional circles with three or more different data sets, the map
will fail to convey anything useful. A good design guideline is to limit the number of
variables symbolized by proportional point symbols to one, possibly two, but never
three or more.
An isarithmic map, also known as a contour map, is a planimetric graphic repre-
sentation of a three-dimensional volume. Isoline mapping is a system of quantitative
line symbols that attempt to portray the undulating surface of the three-dimensional
volume. Contour is a common example. An isarithmic mapping technique always
implies the existence of a third dimension. The isarithmic technique also requires
that the volume's surface is continuous in nature, rather than discrete or stepped.
Isarithmic mapping has had a long history. Isometric lines showing the depth
of the ocean floor are called isobaths, which were first used in 1584. In 1777, the
isohypse line was proposed by Meusnier as a way of depicting surface features
and was first used in an actual map made by du Carla-Dupain-Triel in 1782. The
isohypse is an isometric line.
The most important perceptional tendency to a viewer as well as the cartographer
is the figure and ground configuration. Our underlying perceptional tendency is to
organize the visual field into two categories: important objects which form figures
and less important ones which form grounds. Gestalt psychologists first introduced
this concept early in the twentieth century. Figures are objects standing out from the
background. Figures are remembered better; grounds are formless and often lost in
perception.
In the three-dimensional world, we see buildings in front of sky and cars in front
of pavements. Texture and differences in texture can produce figures in perception.