Map Reading Procedures (Part Two)

Estimating Distance

A landmark often falls right or left of course and the navigator must estimate the distance to it. While the ability to estimate distance from a landmark rests largely in skill and experience, the following methods may be of assistance. One method is to compare the distance to a landmark with the distance between two other points as measured on the chart. Another method is to estimate the angle between the aircraft subpoint and the line of sight. [Figure 6-3] The distance in NM from the landmark to the subpoint of the aircraft depends on the sighting angle:

(60°) horizontal distance = absolute altitude of aircraft × 1.7
(45°) horizontal distance = absolute altitude of aircraft
(30°) horizontal distance = absolute altitude of aircraft × .6

Figure 6-3. Estimating distances.

Figure 6-3. Estimating distances. [click image to enlarge]

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.


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.

In high latitudes, navigators find few distinguishable features from which to determine a position. Built-up features are practically nonexistent. The few that do exist are closely grouped, offering little help to the navigator flying long navigation legs. Natural features that 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 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.

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.

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.

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.

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 gradient system of coloring is also employed. The lighter colors are used to show lower areas while a gradual increase in density (darkness) is used to portray the higher terrain.