Map reading is a critical skill for navigators in many aircraft, but it takes time to become proficient. Keep a good DR, work from chart to ground, and remember the effect varying conditions have on what is seen outside a window.
Introduction to Map Reading
Map reading is the determination of aircraft position by matching natural or built-up features with their corresponding symbols on a chart. It is one of the more basic aids to dead reckoning (DR) and certainly the earliest used form of aerospace navigation. The degree of success in map reading depends upon a navigator’s proficiency in chart interpretation, ability to estimate distance, and the availability of landmarks.
Checkpoints
Checkpoints are landmarks or geographic coordinates used to fix the position of the aircraft. By comparing the aircraft position to that of the checkpoint, the navigator fixes the aircraft’s location. Arrival over checkpoints at planned times is a confirmation of the wind predication and indicates reliability of the predicted track and groundspeed. If the aircraft passes near but not over a checkpoint, the anticipated track was not made good. If checkpoints are crossed but not at the predicted time, the anticipated ground speed (GS) was in error. Prudent navigators are quick to observe and evaluate the difference between an anticipated position and an actual position. They must make corrections to maintain their intended course as soon as possible because small errors can be cumulative and may eventually result in becoming lost.
Before fixing each position, navigators should look for several related details around each checkpoint to make sure it has been positively identified. For example, if the checkpoint is a small town, there may be a lake to the north, a road intersection to the south, and a bridge to the east.
Generally, it is better to select a feature on the chart and then seek it on the ground rather than to work from the ground to the chart. The chart does not show all the detail that is on the ground, and one could easily become confused. Checkpoints should be features, or groups of features, that stand out from the background and are easily identifiable. In open areas, any town or road intersection can be used; however, these same features in densely populated areas are difficult to distinguish. Figures 6-1 and 6-2 compare various chart and corresponding photo areas and list the features to look for when identifying landmarks as checkpoints.
Figure 6-1. Landmarks as checkpoints, populated areas. [click image to enlarge]Figure 6-2. Landmarks as checkpoints, coastal areas. [click image to enlarge]
Chart Selection
Use a chart for map reading that provides sufficient natural and built-up features to accurately position the aircraft. The Operational Navigation Chart (ONC), with a scale of 1:1,000,000, has excellent cultural and relief portrayal. For increased detail, a Sectional Aeronautical, with a scale of 1:500,000, or suitable scaled USGS topographic charts with a scale of 1:10,000, may be used.
Map Reading Procedures (Part One)
When in flight, orient the chart so that north on the chart is toward true north (TN). The course line on the chart is then aligned with the intended course of the aircraft so that landmarks on the ground appear in the same relative position as the features on the chart. Obtain the approximate position of the aircraft by DR. Select an identifiable landmark on the chart at or near the DR position. It is important to work from the chart to the ground. Identify the landmark selected and fix the position of the aircraft. The importance of a good DR cannot be over emphasized. When there is any uncertainty of position, every possible detail should be checked before identifying a checkpoint.
The relative positions of roads, railroads, airfields, and bridges make good checkpoints. Intersections and bends in roads, railroads, and rivers are equally good. When a landmark is a large feature, such as a major metropolitan area, select a small prominent checkpoint within the large landmark to fix the position of the aircraft. When a landmark is not available as a reference at a scheduled turning point, make the turn on the estimated time of arrival (ETA). Extend the DR position to the next landmark and fix the position of the aircraft to make sure the desired course and GS are being maintained. Remember, the desired magnetic course on any given leg corrected for drift is the magnetic heading which parallels course. This helps to keep from getting any farther off course.
Map Reading While Flying At Low Altitudes
When flying at lower altitudes, additional difficulties may be encountered. Turbulence increases the difficulty of reading instruments. Depending on the aircraft’s altitude above ground level (AGL), the circle of visibility can be greatly reduced, and those objects that are visible pass by so rapidly only the largest landmarks can be easily identified.
In low altitude navigation, flight planning is especially important as there is little time for inflight computations. An important part of good flight planning is proper chart preparation. Normally radius-of-turn procedures are used when drawing the chart, but depending on your tactics, point to point is also an option. Time elapsed marks and distance remaining marks along the course line of each leg gives navigators a running DR with the aid of a stopwatch.
In low altitude flight, one should be particularly alert to possible danger from obstructions. Hills and mountains are easily avoided if the visibility is good. Radio and television towers, which may extend as much as 1,000 feet or more into the air, often from elevated ground, are less conspicuous. All such obstructions may or may not be shown on the aeronautical charts. Flights need to be above the highest elevation listed for that grid square on the sectional chart to ensure obstruction clearance. Maximum elevation figures (MEF) are explained on the inside panel (left side) of the sectional chart.
Map Reading at Night
At night, unlighted landmarks may be difficult or impossible to see. Lights can be confusing because they appear closer than they really are. Fixing on points other than those directly beneath the aircraft is very difficult. Objects may be more easily seen by scanning or looking at them indirectly to eliminate the eye’s visual blindspot. Preserve night vision by working with red or green light, being aware that red light can detract from the chart color. Moonlight makes it possible to see prominent landmarks like land-water contrast. Reflected moonlight often causes a river or lake to stand out brightly for a moment, but this condition is usually too brief for accurate fixing. Roads and railroads may be seen after the eyes are accustomed to the darkness. Lighted landmarks, such as cities and towns, stand out more clearly at night than in daytime. Large cities can often be recognized by their distinctive shapes. Many small towns are dark at night and are not visible to the unaided eye. Some airfields have distinctive light patterns and may be used as checkpoints. Military fields use a double white and single green rotating beacon, while civilian fields use a single white and single green rotating beacon. Busy highways are discernible because of automobile headlights.
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. [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.
