Cartograms and Diagrams 191
maintained on Plate 10.1, the shapes are badly distorted and in some cases unrecog-
nizable.
Advantages and Disadvantages.
Research, especially by the late Borden Dent,
has shown that a contiguous cartogram that approximates shape with straight line
segments is probably the most useful and the least confusing to the reader. His stud-
ies showed that of the qualities exhibited by a conventional geographic base map—
shape, orientation, and contiguity—shape is the essential factor to preserve on a car-
togram.
Shape aids the reader in retrieving information from the map; if individual states
cannot be recognized and compared with either a conventional map or a mental map
of the geographic space, then the cartogram can have no effect. Dent felt a value-
by-area cartogram should not be used if the general shape of the enumeration areas
cannot be maintained. It might also be noted that if the shapes of the enumeration
areas are unfamiliar to the reader, the cartogram loses its effectiveness. Labeling or
including an inset with a conventional map might aid the reader, but more standard
representations, such as census tracts or minor civil divisions, should be considered
for unfamiliar areas.
Cartograms that are drawn with curved outlines and are truly contiguous must
distort shapes excessively and are therefore more difficult for the reader to interpret
unless only a few very familiar areas are used. True contiguity
appears to be less
WA
OR
MT
WY
UT
CO
NM
AZ
CA
ID
NV
Number of Deaths
25
10
3
Western
United States
TOTAL DEATHS
fIgURe 10.2.
Hand-drawn cartogram.
192
nOnTRaDiTiOnaL MaPPing
important than shape for reader acceptance and recognition. However, probably the
most
commonly seen cartograms,
especially on the Internet, do not preserve shape.
nOnCOnTigUOUs CaRTOgRaMs
Traditional noncontiguous value-by-area cartograms show shapes correctly and either
enlarge or reduce enumeration areas according to the variable represented; adjoining
areas do not touch (Figure 10.3). Although there is usually an effort to place the vari-
ous areas in roughly their correct positions relative to one another, they are separated
by empty spaces. Often, as in Figure 10.3, a true outline of the area is placed around
the enumeration areas. This gives the reader a frame of reference. It allows the reader
to see the region in its entirety in true geographical space and gives an idea of the
gaps, which are considered important in this type of cartogram.
Advantages and Disadvantages. Judy Olson (1976) lists three properties of the
noncontiguous cartogram that make it a useful device:
1. [The gaps between the enumeration units are] meaningful representations of
discrepancies of values.
2. The representation and manipulations involve only the discrete units for which
information is available and only the lines that can be accurately relocated on
the original map appear on the noncontiguous cartogram.
3. Because only sizes of units change, not their shapes, recognition of the units
represented is relatively uncomplicated for the reader.
