An aeronautical chart is a pictorial representation of a portion of the earth’s surface upon which lines and symbols in a variety of colors represent features or details seen on the earth’s surface. In addition to ground image, many additional symbols and notes are added to indicate navigational aids (NAVAID) and data necessary for air navigation. Properly used, a chart is a vital adjunct to navigation; improperly used, it may become a hazard. Without it, modern navigation would never have reached its present state of development. Because of their great importance, the navigator must be thoroughly familiar with the wide variety of aeronautical charts and understand their many uses.
Aeronautical charts are produced on many different types of projections. Since the demand for variety in charts is so great and the properties of the projections vary greatly, there is no one projection satisfying all navigation needs. The projection that most nearly answers all of the navigator’s problems is the Lambert conformal, and this projection is the one most widely used for aeronautical charts. An aeronautical chart of some projection and scale can be obtained for any portion of the earth.
Maps made by a given country traditionally use the datum created by that country. There may be as many as a thousand of these various datums in use throughout the world. Inherent problems result from over a hundred countries using widely different methods and standards to measure coordinate systems. When added to the effects of local variations in topography and the gravity field, systems are created that differ substantially from each other. These individualized datums are classified as local or regional.
The Department of Defense adopted a datum in 1987 called World Geodetic System 84 (WGS 84). This global datum is a system that models the entire planet, instead of one small piece. WGS 84 is used by NGA for production of almost all new maps and charts. The purpose of such a system is to minimize the confusion created by the proliferation of local datums. As long as all coordinates are stated in WGS 84, combat interoperability problems are minimized. In addition, WGS 84 positions may be computed from global positioning system (GPS) equipment to an extreme level of precision by NGA surveyors, well under half a meter anywhere in the world. Widespread use of WGS 84 virtually eliminates problems due to different datums.
It is important to realize that every coordinate is related to a specific datum. A latitude and longitude extracted from a WGS 84 chart is still a WGS 84 coordinate, and an MGRS point pulled from that same chart is also WGS 84. However, a ground survey of that same point could have established a local datum coordinate that is different from the map derived one by as much as a half mile. Always use the same datum throughout a mission, or serious positional errors are possible.
Obviously, charts are much smaller than the area they represent. The ratio between any given unit of length on a chart and the true distance it represents on the earth is the scale of the chart. The scale varies, and may vary greatly from one part of the chart to another. Charts are made to various scales for different purposes. If a chart is to show the whole world and yet not be too large, it must be drawn to small scale. If a chart is to show much detail, it must be drawn to a large scale; then it shows a smaller area than does a chart of the same size drawn to a small scale. Remember, large area, small scale; small area, large scale.
The scale of a chart may be given by a simple statement, such as 1 inch equals 10 miles. This means a distance of 10 miles on the earth’s surface is shown 1 inch long on the chart. On aeronautical charts, the scale is indicated in one of two ways: representative fraction or graphic scale.
The scale may be given as a representative fraction, such as 1:500,000 or 1/500,000. This means one of any unit on the chart represents 500,000 of the same unit on the earth. For example, 1 inch on the chart represents 500,000 inches on the earth. A representative fraction can be converted into a statement of miles to the inch. Thus, if the scale is 1:1,000,000, 1 inch on the chart stands for 1,000,000 inches or 1,000,000 divided by (6,076 × 12) equaling about 13.7 NM. Similarly, if the scale is 1:500,000, 1 inch on the chart represents about 6.86 NM. Thus, the larger the denominator of the representative fraction, the smaller the scale.
The graphic scale may be shown by a graduated line. It usually is found printed along the border of a chart. Take a measurement on the chart and compare it with the graphic scale of miles. The number of miles the measurement represents on the earth may be read directly from the graphic scale on the chart. The distance between parallels of latitude also provides a convenient scale for distance measurement. One degree of latitude always equals 60 NM and 1 minute of latitude always equals 1 NM.
Types of Charts
Aeronautical charts are differentiated on a functional basis by the type of information they contain. Navigation charts are grouped into three major types: general purpose, special purpose, and plotting. The name of the chart is a reasonable indication of its intended use. A Minimal Flight Planning Chart is primarily used in minimal flight planning techniques; a Jet Navigation Chart has properties making it adaptable to the speed, altitude, and instrumentation of jet aircraft. In addition to the specific type of information contained, charts vary according to the amount of information displayed. Charts designed to facilitate the planning of long distance flights carry less detail than those required for navigation en route. Local charts present great detail.
Standard Chart Symbols
Symbols are used for easy identification of information portrayed on aeronautical charts. While these symbols may vary slightly between various projections, the amount of variance is slight and, once the basic symbol is understood, variations of it are easy to identify. A chart legend is the key to explaining the meaning of the relief, culture, hydrography, vegetation, and aeronautical symbols. [Figure 1-31]
Chart relief shows the physical features related to the differences in elevation of land surface. These include features, such as mountains, hills, plateaus, plains, depressions, etc. Standard symbols and shading techniques are used in relief portrayal on charts; these include contours, spot elevations and variations in tint, and shading to represent shadows.
A contour line is a line connecting points of equal elevation. Figure 1-32 shows the relationship between contour lines and terrain. Notice on steep slopes the contours are close together and on gentle slopes they are farther apart. The interval of the contour lines usually depends upon the scale of the chart and the terrain depicted. In Figure 1-32, the contour interval is 1,000 feet. Depression contours are regular contour lines with spurs or ticks added on the down slope side.
Spot elevations are the height of a particular point of terrain above an established datum, usually sea level.
The relief indicating contours is further emphasized on charts by a system of gradient tints. They are used to designate areas within certain elevation ranges by different color tints.
Perhaps the most obvious portrayal of relief is supplied by graduated shading applied to the southeastern side of elevated terrain and the northwestern side of depressions. This shading simulates the shadows cast by elevated features, lending a sharply defined, three-dimensional effect.
All structural developments appearing on the terrain are known as cultural features. Three main factors govern the amount of detail given to cultural features: the scale of the chart, the use of the chart, and the geographical area covered. Populated places, roads, railroads, installations, dams, bridges, and mines are some of the many kinds of cultural features portrayed on aeronautical charts. The true representative size and shape of larger cities and towns are shown. Standardized coded symbols and type sizes are used to represent smaller population centers. Some symbols denoting cultural features are usually keyed in a chart legend. However, some charts use pictorial symbols which are self-explanatory.
In this category, aeronautical charts depict oceans, coast lines, lakes, rivers, streams, swamps, reefs, and numerous other hydrographic features. Open water may be portrayed by tinting or vignetting, or may be left blank.
Vegetation is not shown on most small scale charts. Forests and wooded areas in certain parts of the world are portrayed on some medium scale charts. On some large scale charts, park areas, orchards, hedgerows, and vineyards are shown. Portrayal may be by solid tint, vignette, or supplemented vignette.
In the aeronautical category, coded chart symbols denote airfields, radio aids to navigation, commercial broadcasting stations, Air Defense Identification Zones (ADIZ), compulsory corridors, restricted airspace, warning notes, lines of magnetic variation, and special navigational grids. Some aeronautical information is subject to frequent change.
For economy of production, charts are retained in stock for various periods of time. To keep the charts current, only the stable kinds of information are printed on navigation charts.
NGA produces and distributes all aeronautical charts and Flight Information Publication (FLIP) documents. A summary of the typical charts is in Figure 1-33. Requisitions should indicate item identification and terminology for each item requested as listed in the catalog. List aeronautical charts by series in numerical or alphabetical sequence; list FLIP documents by type (en route, planning, terminal), title, and geographic area of coverage. Contact NGA or its squadrons and detachments for technical assistance in preparing statements of requirements. Addresses are listed in the NGA Catalog of Maps, Charts, and Related Products.