Designing and Choosing symbols 147
tonal value since these variables are used to symbolize differences in size or amount. A
particular concern is legibility, especially when a variety of shapes are used. Although
symbol
sizes will not vary on the map, the size and
shape must be large enough
to distinguish the individual symbols. This can be a problem especially with maps
viewed on monitors–if the symbols are too small it is difficult to differentiate between
squares and circles. Complex pictorial symbols with letters within shapes can fill in
if they are reduced.
Most illustration and GIS software have a variety of pictorial or abstract point
symbols available.
Quantitative Point symbols
Quantitative point symbols are of two basic types, dots and proportional figures, but
there is variation within these categories.
Dot Maps
In the most basic form, dot maps use a single dot (“dots” may actually be circles,
squares, triangles, or other shapes)
to represent a given quantity,
such as one dot
represents one house, or one dot represents 500 people. The dot is placed at the loca-
tion of the phenomena for single units or in the center of the distribution for multiple
units. The purpose of dot maps is to show the distribution of the phenomena; they
are not created to determine quantities, that is, the individual dots cannot be counted
to find totals. These maps are called
simple dot maps or sometimes
dasymetric dot
maps (Figure 8.2). Because the dots are placed in the area of greatest density, the map
shows areas of concentration and sparseness. They can be very effective maps.
Making a simple dot map requires two kinds of data: quantity and location.
Quantities can be determined from censuses or other tallies,
and location can be
PICTORIAL
ASSOCIATIVE
ABSTRACT/
GEOMETRIC
DYNAMIC
STATIC
