(b) The serious drift caused by the wind. This may take aircraft anything up to forty or more miles off the course in each hour's flying, according to the direction and strength of the wind. In cloudy weather, or at night, a change in the wind can alter the drift without the knowledge of the navigator. Hence, special precautions must be taken to observe the drift at all possible times.
(c) The absence of need for extreme accuracy of navigation in the air, since a ten or even twenty mile error from the destination in a long journey is permissible. Another favorable point is that rocks, reefs and shoals need not be avoided. This permits the aërial navigator to use short cuts and approximations in calculation, which would be criminal in marine navigation.
There are three methods of aërial navigation—"Dead Reckoning," Astronomical Observation, and Directional Wireless Telegraphy. None should be used alone; for although accuracy may be obtained with any single method, it is highly advisable to check each by means of the others.
As in the case of marine navigation, a reliable compass, either of the magnetic or gyroscopic type, is essential for aërial navigation, as well as an accurate and reliable chronometer. Suitable charts must be provided, showing all parts of the route to be covered. When the magnetic compass is used, such charts should show the variation between True and Magnetic North at different points on the route.
NAVIGATION BY "DEAD RECKONING"
"Dead Reckoning" is the simplest method of navigation; and, under favorable conditions, it gives a high degree of accuracy. A minimum of observation is required, but careful calculation is essential.
The "Dead Reckoning" position of an aëroplane or airship at any time is calculated from its known speed and direction over the surface of the earth or ocean, and its known course as indicated by the magnetic or gyroscopic compass.
To determine the direction of movement of an aëroplane or airship, as apart from the direction in which it is headed, an instrument known as a Drift Indicator, or Drift Bearing Plate, is used.
One form of Drift Indicator consists of a simple dial, with the center cut away and a wire stretched diametrically across it. The outer edge of the dial is divided into degrees, in a similar manner to that of the compass. It is mounted in such a way that an observer can, by looking through the center of the disc, see the ground or ocean below him. The disc is then turned until objects on the ground—or white-caps, icebergs, ships, or other objects visible on the surface of the ocean—are seen to move parallel with the wire, without in any way deviating from it. The angle which the wire then makes with the direction in which the nose of the aëroplane or airship is pointing gives the angle of drift.