Global Positioning System Reference
In-Depth Information
FIGURE 1.4.
Types of tides around the world. Semidiurnal tides occur twice daily, with
more or less equal amplitude. Diurnal tides happen once per day; mixed tides occur
twice a day but with markedly different amplitudes.
Adapted from an image by Michael
Pidwirny, University of British Columbia.
easy-to-understand example of the influence of local factors is the funnel-
ing e√ect caused by a large bay that narrows significantly: we can expect
that such a configuration of coastline might lead to high tides, and this is
certainly the case for the Bay of Fundy, between the provinces of Nova
Scotia and New Brunswick on the east coast of Canada (fig. 1.5). The tides
there are as high as 16 m (53 ft), and these may be the largest tidal fluctua-
tions anywhere in the world—perhaps 5 or 10 times larger than the aver-
age.
8
Despite the variability of tide heights around the world, once data has
been gathered for a given coastal area it is possible to calculate quite
accurately what the future tides in that region will be like. Data is obtained
from thousands of tide gauges—measuring stations—installed at harbors
all over the world. Predictions are made of high and low tide times and
water levels, and are published weeks in advance for the benefit of mari-
ners, fishermen, and other coastal workers (fig. 1.6).
9
Our understanding of tides, and our ability to predict them in all parts of
the world, has increased dramatically over the centuries. Pytheas, the an-
cient Greek navigator whom we meet later, is said to have recorded the
8. The tidal flow in and out of the Bay of Fundy amounts to a hundred billion tons of
water, twice daily. The sheer volume of tides is enough to depress coastlines an average of
15-20 cm.
9. See, for example, Maloney (2006, chap. 17) for the use of tide tables by mariners and
for the influence of tides on maritime navigation accuracy.
