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landmark with the distance between two other points as measured on the chart. Another method, shown
in Figure 6.3, is to estimate the angle between the aircraft subpoint and the line of sight. The distance in
NM from the landmark to the subpoint of the aircraft depends on the sighting angle:
AFPAM11-216 1 MARCH 2001 171
Figure 6.3. Estimating Distances.
6.7.1. (60o) horizontal distance = absolute altitude of aircraft X 1.7.
6.7.2. (45o) horizontal distance = absolute altitude of aircraft.
6.7.3. (30o) horizontal distance = absolute altitude of aircraft X .6.
6.8. Seasonal Changes. Seasonal changes can conceal landmarks or change their appearance. Small
lakes and rivers may dry up during the summer. Their outlines may change considerably during the wet
season. Snow can cover up almost all of the normally used landmarks. When flying in the winter, it is
often necessary to rely on more prominent checkpoints, such as river bends, hills, or larger towns.
However, due to the size of these checkpoints, course control can be somewhat degraded.
6.9. Map Reading in High Latitudes. Map reading in high latitudes is considerably more difficult than
map reading in the lower latitudes. The nature of the terrain is drastically different, charts are less
detailed and less precise, seasonal changes may alter the terrain appearance or hide it completely from
view, and there are fewer cultural features.
6.9.1. In high latitudes, navigators find few distinguishable features from which to determine a position.
Built-up features are practically nonexistent. The few which do exist are closely grouped, offering little
help to the navigator flying long navigation legs. Natural features which do exist are in limited variety
and are difficult to distinguish from each other. Lakes seem endless in number and identical in
appearance. The countless inlets are extremely difficult to identify, particularly in winter. What appears
172 AFPAM11-216 1 MARCH 2001
to be land may in reality be floating ice, the shape of which can change from day to day. Recognizable,
reliable checkpoints are few and far between.
6.9.2. Map reading in high latitudes is further complicated by inadequate charting. Some polar areas are
yet to be thoroughly surveyed. The charts portray the appearance of general locales, but many individual
terrain features are merely approximated or omitted entirely. In place of detailed outlines of lakes, for
example, charts often carry the brief annotation—many lakes. Fixing is possible, but requires extended
effort and keen judgment on your part.
6.9.3. When snow blankets the terrain from horizon to horizon, navigation by map reading becomes
acutely difficult. Coastal ice becomes indistinguishable from the land, coastal contours appear radically
changed, and many inlets, streams, and lakes disappear. Blowing snow may extend to heights of 200 to
300 feet and may continue for several days, but visibility is usually excellent in the absence of
interfering clouds or ice crystal haze. However, when snow obliterates surface features and the sky is
covered with a uniform layer of clouds so that no shadows are cast, the horizon disappears, causing earth
and sky to blend together. This forms an unbroken expanse of white called whiteout. In this complete
lack of contrast, distance and height above ground are virtually impossible to estimate. Whiteout is
particularly prevalent in northern Alaska during late winter and spring. The continuous darkness of night
presents another hazard; nevertheless, surface features are often visible because the snow is an excellent
reflector of light from the moon, the stars, and the aurora.
6.10. Contour Map Reading. Use of contours is the most common method of showing relief features
on a chart. Contours are lines that, at certain intervals, connect points of equal elevation. To understand
contours better, think of the zero contour line to be sea level. If the sea were to rise 10 feet, the new
shoreline would be the 10-foot contour line. Similarly, successive 10-foot contour lines could be easily
determined. Contour lines are closer together where the slope is steep and farther apart where the slope
is gentle. Within the limits of the contour intervals, the height of points and the angle of slope can also
be determined from the chart. Refer to Figure 1.32 for an illustration of the use of contour lines.
6.10.1. Contour intervals are determined by the scale of the chart, the amount of relief, and the accuracy
of the survey. These intervals may range from 1 foot on a large-scale chart through 2,000 feet or greater
on a smaller scale chart. Contours may be annotated in feet or meters. Contours may be shown on charts
in varying colors and are frequently labeled with figures of elevation. To further accentuate the terrain, a
　
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