ICAG HQ Advanced Flight School: Navigation - Section II Maps and Charts

9. Definitions. -a. Map and chart.-A map is a representation of a sphere or a portion of a sphere on a flat surface. The term "map" is used for areas that are mostly land. Ordinarily, the term "chart" is used for representations of areas that are mostly water; however, the Civil Aeronautics Administration applies the term to publications prepared by them, although many of these charts cover only land areas. In order to represent a sphere or a portion of a sphere on a flat surf ace it must of necessity be distorted. The methods of representing portions of the earth on a map or chart are known as "projections." There are four main types of projections in general use: the Mercator, Lambert conformal (conic), gnomonic, and polyconic. Each type of projection has its use, and no one projection has every feature which is desirable in a map.

b. Sphere.-A sphere is a body bounded by a surf ace all points of which are equidistant from a point called the "center." For the purposes of navigation the earth is usually considered to be a sphere. Actually it is an ellipsoid whose polar diameter is 7,899.7 and equatorial diameter is 7,926.5 statute miles. It is apparent that the earth's deviation from the spherical is slight.

c. Great circle.-A great circle is a circle on the earth's surface whose plane passes through the center of the earth. The great circle equidistant from the two poles is known as the Equator. Great circles passing through the two poles are known as meridians.

d. Small circle.-A small circle is a circle on the earth's surface whose plane does not pass through the center of the earth. Small circles whose planes are parallel to the great circle plane of the Equator are known as parallels of latitude.

e. Rhumb line.-A rhumb line is a line which crosses all meridians at the same angle. It does not necessarily represent the shortest distance between points. Any two points on the earth's surface may be connected by a rhumb line. The rhumb line drawn on the sphere is known as a loxodromic curve. According to the foregoing definition, the meridians, the Equator, and the parallels are rhumb lines, but the meridians and the Equator being also great circles are usually not considered rhumb lines. The parallels are a special type of rhumb line since they intersect every meridian at 900. All other rhumb lines are curves which approach but never reach the poles. The path of an airplane maintaining a constant course is a rhumb line. Rhumb lines appear as straight lines on the Mercator projection. This is true of no other projection.

10. Latitude and the Equator.-Latitude (Lat.) is the angular distance north or south of the Equator, as subtended at the center of the earth, measured from the Equator as a plane of origin. Latitude is measured in degrees, minutes, and seconds of arc, and may have any value from 0' at the Equator to 90' north or south, which would indicate the North or South Pole. North latitude is sometimes designated by the + sign and south latitude by the - sign; it is better practice, however, to use the letters N. or S. to designate the latitude of a point.

11. Longitude and the prime meridian.-Longitude (Long.) is the angular distance, at the axis of the earth, between the plane of a meridian and the plane of the prime meridian of Greenwich, England, measured to the eastward or westward. Longitude is measured in degrees, minutes, and seconds of arc, and may have any value from 0' at Greenwich, England, up to 1801 east or west. The area of the United States is in latitude north (of the Equator) and longitude west (of the prime meridian).

12. Latitude and the nautical mile.-The angular distance between the Equator and the North or the South Pole is 901 or 5,400 minutes. If we use the actual length of a minute of latitude as unit of linear distance, we have 5,400 of these units as the distance between the Equator and either the North or South Pole. This unit of distance is called a nautical mile and is 6,080 feet in length. The statute mile, an arbitrarily selected unit of measure, is 5,280 feet in length. The relationship between latitude and distance, I minute of arc of latitude equaling I nautical mile, makes the nautical mile very useful in navigation and led to its adoption as the standard measure of distance in marine navigation. Although I minute of arc of latitude equals I nautical mile, the same relationship does not exist between arc of longitude and the nautical mile except at the Equator.

13. Longitude, arc and distance.-Although are of latitude may be converted directly into distance as shown above, arc of longitude may be directly converted into nautical miles only at the Equator. There, and there only, 1 minute of arc of longitude equals 1 nautical mile. Methods of determining the linear distance represented by arc of longitude at various other points on the surface of the earth are described in section IX, chapter 2.

14. Conversion, nautical and statute units.-The nautical mile is longer than the statute mile, the ratio being as 115 is to 100. This relationship makes it possible to convert a specified distance, known in one of these units, formulas: to the other unit by use of one of the following

(Distance in) statute miles = nautical miles X 1.15 statute miles

 (Distance in) nautical miles = statute miles / 1.15

15. Mercator, Lambert conformal, gnomonic, and polyconic projections.-a. Mercator.-A map made on the Mercator projection has all meridians of longitude represented by parallel vertical lines and all parallels of latitude shown as parallel horizontal lines. Thus the straight lines representing meridians and parallels all cross each other at right angles. It is used extensively in marine navig tion and in air navigation when the airplane carries a navigator as a member of the crew. Most of the hydrographic office charts for the U. S. Navy and the U. S. Coast and constructed on this projection.

b. Lambert conformal projection. - (1) In the Lambert conformal projection, the meridians of the earth are represented by straight lines converging toward a common point outside the borders of the chart, and the parallels by curved lines which are sections of concentric circles whose center is at the point of intersection of the meridians. Meridians and parallels intersect at right angles.

(2) The scale error of any chart is small, and distances may be measured directly by means of the graphic scales printed on the border. If the entire United States is shown in a single chart, the maximum scale error for nearly 90 percent of the chart is about 1/2 of 1 percent-an error quite negligible in practice.

c. Gnomonic.-A map or chart made with this projection has all meridians shown as converging straight lines and the parallels of latitude shown as curved lines with the exception of the Equator which appears as a straight line. Charts made on this projection are sometimes called "great circle charts" because a straight line drawn on such a chart will indicate great circles, which is the shortest distance between points. Courses drawn on the gnomonic chart are transferred to another kind of chart or map for actual use in navigation.

d. Polyconic.-A map made on this projection has the central meridian shown as a straight line and all other meridians shown as curved lines. The parallels are arcs of circles, each with a different radius. This type of projection is not used for navigation maps in the United States but is used for Geological Survey, engineer, and many military maps. Maps based on the polyconic are sometimes the only ones available when navigating over land areas outside the United States.

e. Reference.-A more complete description of these and other projections is given in section III, chapter 2.

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(5) Maps in general may be thought of as containing information which is subject to comparatively little change even over a considerable period of time. By way of contrast, the aeronautical charts include 25,000 miles of airways equipped with beacon lights, radio ranges, teletype service, and other related features. Over such an extensive system it is obvious that many changes must occur. New airways are being established and old routes are being rebuilt for more efficient operation; improved equipment is being installed; and aids are even being provided for the navigation of air routes across the oceans. The frequent correction of these charts to show the changes as they occur is a most important function of the Government and is imperative for safety in all forms of cross-country flying.

17. Chart reading.-a. Importance-(I) An aeronautical chart is a small scale representation of a portion of the earth and its culture, presenting to the trained eye a description of the charted region more nearly perfect than could be obtained from the pages of a book. It depicts the landmarks and other information found of value by pilots long familiar with the region. Consequently, any time spent in learning to read and interpret its detailed information will be well repaid; that this is beginning to be appreciated is evidenced by the growing demand for these charts.

(2) In charting the details of the terrain and the system of aids to navigation, many conventional symbols are employed. Some of these have been in use for many years and their significance is generally understood. Others have been adopted recently and therefore are not as well known. The following description of these symbols and their significance has been prepared as an aid to chart reading. It applies primarily to sectional charts, since the scale of that series permits the charting of fairly complete information. On the smaller scale charts many details must be omitted, but with few exceptions those that can be included are shown by the same symbols.

b. Features.-Features shown on these charts may be divided into two groups:

(1) Those necessary to a clear and accurate topographic representation of the region, such as

(a) Water, including streams, lakes, canals', swamps, and other bodies of water.

(b) Cultural, such as towns, cities, roads, railroads, and other works of man.

(c) Relief, including mountains, hills, valleys, and other inequalities of the land surface.

(2) Aeronautical data and information of interest chiefly for use in air navigation.

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20. Other aeronautical charts of the United States-a. Radio direction-finding charts (fig. 14).- (1) These charts show the area of the United States on six sheets and use a scale of one to two million, or about 32 miles to the inch. They are designed especially for use in the plotting of radio bearings. Their smaller scale and wider extent make it possible to plot bearings from radio stations which would frequently be outside the limits of the local chart when using either of the larger scale series of charts. The method of plotting bearings on these charts which are constructed on the Lambert conformal projection is treated in detail in section X, chapter 2.

(2) Specially designed compass roses (fig. 15), oriented to magnetic north, are used on the radio direction finding charts. These roses are graduated to read both from magnetic south and from magnetic north. The outer figures are ordinarily used and are therefore larger; they are sometimes used to plot reciprocal bearings (the radio compass bearing observed at the plane plus or minus 180'), and for that reason read from 0 at magnetic south. For certain other problems a rose reading from 0 at magnetic north is more convenient, and for such problems the inner (smaller) figures are also available. These magnetic compass roses should be used to plot bearings taken by an airplane only when the airplane is relatively close to the transmitting station, i. e., close enough so that the difference in variation existing at the airplane and at the station is negligible. Otherwise a correction is necessary to account for this difference in variation. These roses should not be confused with the conventional compass roses appearing on the sectional and regional charts, nor used in the same manner. (See see. VIII.)

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d. Magnetic chart (No. 3077).-This chart shows the entire area of the United States on one sheet, with a scale of one to seven and one-half million, or about 115 miles to the inch. Lines of equal magnetic variation at 1° intervals are shown on this chart.