Man's physical universe

xanabras

AVIGATION 379

Aeronautical charts are similar to relief maps in the manner in

which they show differences in altitude. Relief is shown by contour

lines, areas of equal altitude between the contour lines being shown in

the same color, while areas of different altitude are shown in different

colors. A contour represents an

imaginary line on the ground, every

point of which is at the same altitude.

With a little practice one

can easily read a contour map or

chart. Figure 168 shows how a

contour is constructed.

For cross-country flying the

course is laid out on the chart and

carefully studied by the pilot before

taking off.

By 1941 the United States Government

had over 25,000 miles of

civil airways, with rotating beacons

at intervals of about 15 miles and

Fig. 168. How contour maps are

made.

with intermediate or emergency landing fields at 50-mile intervals.

Traffic is controlled by means of radio communication on these civil

airways to avoid collisions.

3. Navigation by Dead Reckoning Depends upon the Use of Five

Fundamental Instruments.

Navigation by dead reckoning depends upon the determination of

positions by means of calculating their direction and distance from a

known position, from the course, the direction, speed of the wind, and

the cruising air speed. With the above information the ground speed

and track are easily obtained by simple geometric plotting.

Dead reckoning is used when other methods of navigation are not

possible, and it is usually quite accurate.

Five fundamental instruments are essential for navigation by dead

reckoning.

(i) A clock or watch.

(2) An altimeter. An altimeter is an aneroid barometer that

registers atmospheric pressure on a scale which is calibrated to read in

feet above sea level. The altimeter has to be set just before leaving the

ground in accordance with the atmospheric pressure at that time and

the altitude of the airport. Inasmuch as an increase in temperature

causes air to expand, the altimeter will give readings which are too low

to the extent of about 2 per cent for each 10° F. rise in temperature

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