196
nOnTRaDiTiOnaL MaPPing
otherwise, time must be averaged in both directions, and this is inaccurate. The fig-
ure shows the route in two directional segments; segment A–B going westward is
longer than the same segment going eastward because it is uphill on the westward
journey and downhill on the eastward journey. Such charts can be a useful addition
to hiking trail maps.
These cartograms are simple to construct; the most time-consuming aspect is the
fieldwork because seldom does one find charts showing travel time. The route must
be traversed, preferably more than once, to determine the average travel time. A scale
is chosen, for example, 1 inch represents 1 hour, and usually the route itself is highly
generalized.
The
center point cartogram (Figure 10.8) shows times from a center point and
the cartogram can be read only that way. Direction is important; times to and from
the center point might differ significantly. Times cannot be determined between any
other points on the map; they are all related to the center. The time scale can be for
average times or only apply to a specific time of day if freeway routes are the subject.
A center point cartogram shows the same information as an isochronic (lines of equal
time) map, but the isochrones are converted to concentric circles.
The first step in creating a center point cartogram is construction of an isochronic
map. Travel times must be determined from the center point to all relevant points.
An isochronic line connects all points the same time away from the center (Figure
10.9). These lines are then converted to concentric circles that distort the geographic
base by removing some locations from the center and pulling others closer. On some
of these maps the distorted base is shown; on others only the concentric circles are
shown with place-names labeled to act as orienting points for the reader. Another
variation of the distance-by-time cartogram simply uses lines radiating from the cen-
ter point to the other points of interest. Again, the geographic base is distorted.
SACRAMENTO
Redding
Alturas
Eureka
Reno
Carson City
Francisco
San
Oakland
Fresno
Monterey Bakersfield
Los Angeles
Santa
Barbara
San Diego
2 hrs.
DRIVING
TIMES FROM SACRAMENTO
fIgURe 10.8.
Center point cartogram.
Cartograms and Diagrams 197
