94 THE GEOGRAPHIC
AND CARTOGRAPHIC FRAMEWORK
1 minute of arc on a great circle. We determine the number of minutes separating the
two points by comparing the distance to the same distance on the equator of a globe.
If it is necessary to have this distance in statute miles or kilometers, it can easily be
converted (see Table 5.2).
Direction
Direction is a difficult concept when applied to a sphere.
Direction is the position of
one point on the earth relative to another point. It is usually measured by the angle
between a reference line and the shortest line that can be drawn from the point of
observation and the point of interest. Since the shortest distance on a sphere is a great
circle, then the direction of a point on the earth is the angle between the reference
line (a meridian) and an arc of the great circle running between the observer and the
point. Direction is stated as
azimuth. Azimuth is the angle measured clockwise from
the reference line and has a value between 0° and 360° (Figure 6.3).
Compass directions such as north, south, east, west, southwest, and so on are
commonly used when speaking of direction. A line of constant compass direction,
that is, a line that cuts all meridians at the same line, is called a
loxodrome or
rhumb
line. An aircraft following a constant compass course
other than north or south
would trace a spiral toward a pole, but would theoretically never reach it. This is a
loxodromic curve (Figure 6.4).
In this book “loxodrome” or “line of constant compass direction”
refers to
these directions, and the term “azimuth” is used synonymously with “direction.” It
is important to distinguish between these terms because, as we shall see later, some
projections show lines of constant compass
direction as straight lines, and others
show great circles as straight lines. These are mutually exclusive except for the merid-
ians and the equator.
PRIME
MERIDIAN
WEST
TO 180°
EAST
TO
180°
150°E
180°
120°E
90°E
60°E
30°E
0°
150°W
120°W
90°W
60°W
30°W
FIGURE 6.2.
Meridians are lines of equal longitude.
The earth’s graticule and Projections 95
