The earth’s graticule and Projections 93
distance (latitude) from the equator.
Parallel and latitude are not synonyms; a paral-
lel is a specific line, and latitude is a distance.
An infinite number of parallels can be drawn, since degrees can be divided into
minutes, seconds, and fractions of seconds. This allows
us to locate any place on
earth precisely with respect to the equator. If the earth were truly spherical, the dis-
tance in miles between each degree of latitude would be the same. However, as we
have seen, the earth is not a perfect sphere, and thus there is some variation in the
length of a degree of latitude.
For most small-scale mapping, this variation is too
small to be significant.
The system of parallels is only one-half of the earth’s reference system. There are
360° along each parallel, and to pinpoint a location, we must also specify where it lies
on the parallel. Unfortunately, there is no fixed point or line on the earth comparable
to the poles or the equator that can be used as a convenient origin for measurement
along the parallels. For many years each country used a true north–south line pass-
ing through its capital or some other significant location. Distances were measured
from this line in degrees and called
longitude. When maps of only one country were
used there were few problems; but with faster travel around the earth and the use of a
variety of maps, this system became cumbersome. Many atlases in the United States
during the 19th century contained maps on which the longitude from Washington
was printed along one border and the longitude from London along the opposite bor-
der to eliminate tedious comparisons and calculations.
In 1884, the International Meridian Conference established
the line through the
transit instrument at the Royal Observatory at Greenwich, England, as the starting
line for east–west measurement; this line is called the
prime meridian. The angular
distance east or west of the prime meridian to some other point, measured from 0° to
180°, is the longitude. If a series of lines is drawn at right angles to the parallels and
connecting the poles (by passing planes through the poles), the lines are true north–
south lines and are arcs of great circles. These lines are called
meridians. A meridian
is a line that joins all points having the same longitude (Figure 6.2). Note that each
meridian is one-half of a great circle. Also note that like parallel and latitude, merid-
ian and longitude are not synonymous. Meridians are lines, longitude is angular dis-
tance. The grid formed by parallels and meridians is the
graticule.
