Geoscience Reference
In-Depth Information
The Geographic Grid
Great
circles
North Pole
Let's begin the discussion of geographical tools by focusing on
the most basic of geographical themes, location. One way that
location can be determined is by comparing the location of a
place relative to some other place. For example, a geographer
might identify Los Angeles as being south of San Francisco or
observe that Africa is on the eastern side of the Atlantic Ocean.
You have likely made such a comparison yourself, perhaps by
indicating that you live on the west side of town or that your
favorite park is in the eastern part of a state.
Although a determination of relative location can be useful,
such a statement is very general. In most circumstances, geog-
raphers desire to know the absolute location of a place, which is
most effectively done using the geographic grid. If Earth were
flat, then it would be easy to construct a grid similar to a sheet
of graph paper using the cardinal directions: north, south, east,
and west (Figure 2.1). In this imaginary situation, the grid lines
would extend from these four directions and intersect at vari-
ous places on the grid. With this grid in place, it would be easy
to choose a place of interest and then determine the distance
from this reference point to your location by measuring along
the nearest grid lines, using some unit of measurement such as
miles or kilometers.
However, Earth is not flat, but instead has a curved surface.
In order to account for Earth's shape when locating places, the
grid must consist of a series of intersecting circles, with one set
extending north and south and the other set east and west. We
can begin by examining the circles that are oriented in a north-
south direction. These circles converge at the North and South
Poles on their way to the other side of Earth. These circles are
called
great circles
because they are the largest circles that can
be drawn on a sphere (Figure 2.2a). A good analogy of great
(a)
North Pole
t
t
Small
circles
(b)
Figure 2.2 Great and small circles.
(a) Examples of great
circles on Earth. (b) Examples of small circles on Earth.
circles in the Earth's grid system is the set of circles that sepa-
rate orange segments after the fruit is peeled. If you examine
a peeled orange closely, you will see that these great circles
converge on the top and bottom of the fruit, and bisect it into
many segments.
Great circles can be drawn that bisect Earth in myriad ways
(Figure 2.2a). Each of these circles has the center of Earth as its
core, and they bisect Earth into two equal halves. Great circles
are also important because their outlines reflect the shortest dis-
tance between any two points on the planet. This concept is par-
ticularly relevant to international travel because pilots plot their
courses by great circle routes to reach destinations as quickly
as possible and save on fuel costs. If you plan to travel from
New York City to Moscow, for example, you would probably
think that the shortest route would be to fly due east across the
Atlantic Ocean. But, in fact, the great circle route from New
York to Moscow passes north over Greenland.
With respect to Earth's grid system, the most important
group of great circles is the one that converges at the North
and South Poles. Along with this group of circles, another
corresponding set of circles has outlines that extend east and
west. These outlines are always parallel to one another and thus
1000 mi
Figure 2.1 Hypothetical flat land and grid.
If Earth were
shaped in this way, you could determine your location by mea-
suring the distance from the nearest grid lines.
Great circles
Circles that pass through the center of the Earth
and that divide the planet into equal halves.
