Chapter - 08

Charts

When the broad subject of navigation is considered carefully it will be agreed that the chart is perhaps the most important item of piloting equipment. Without a chart, we could seldom know from which point we depart, and we would not know where we were when we got there. In other words, without some visual means of representing the surface of the earth, navigation would be hopeless, except along a known course. There are dozens of different types of charts, and cartography is an art in itself. However, the mariner is principally interested in Mercator charts, which are used about 98 per cent of the time, and on which are located shore lines, prominent landmarks, lights, buoys, radio stations, and other aids to navigation, together with depths of the water.

Constructing A Mercator Chart

The construction of the Mercator chart is not difficult. However, it is not necessary for the skilled navigator to construct a chart in these days, and even should this knowledge be useful, there is a very simple way of constructing a Mercator chart which is sufficiently accurate for navigation. Referring to Fig. 1, the Mercator chart may be constructed as follows:

1. Draw a horizontal line near the bottom of the chart for the lower latitude parallel, say 40° N.

2. Draw a vertical line near the left of the page for the west meridian, say 82° W, and draw other parallel meridians at any selected scale.

3. Draw a line from the lower left corner, A, making an angle equal to the mid-latitude (402°) with the horizontal line, and cutting the second meridian from the left at B. The distance AB measured on the west meridian, AC, is the latitude corresponding to the longitude for latitude 4O2°.
 
4. From C, draw the mid-latitude at an angle equal to the mid-latitude of the next highest latitude parallel, 42°, and continue this process as needed, and number the longitude lines as required.

As a test of this simple method of constructing a Mercator chart, see how this construction compares with an ordinary Mercator chart.

Fig. 1 Practical construction of a Mercator chart

The method is absolutely accurate for a spherical earth, and for practical purposes of navigation, the earth may be considered round. It is, in fact, as nearly spherical as an eight inch globe could be constructed without excessive cost.

Other Projections

In the past, principal use has been made of the Mercator, polyconic and gnomonic projections. The principle of the Mercator projection is illustrated in Fig. 2, although actually the Mercator chart is not a projection, but only a mathematical proportion. The polyconic chart is a development of the earth's surface on a series of cones, with the point of projection at the center of the earth, against a series of cones tangent to the earth's surface, in the desired latitude. These charts are very little used at present, and are being replaced by Lambert charts which are the development of the earth's surface on one cone cutting two parallels of latitude with the point of projection at the center of the earth as shown in

by careful study of Fig. 2, the parallels of latitude on the Lambert chart are concentric circles, and the meridians of longitude are converging radii of these circles.

Fig. 2. Great Circle Charts

Heretofore considerable use was made of the great circle chart based on the gnomonic projection which is a development of the earth's surface on a plane tangent to the earth at one point, and with the point of projection at the center of the earth. Great circle charts have the one advantage that a straight line on the great circle chart is the shortest distance between any two points on that line The disadvantage is that there is excessive distortion of all areas except those near the point of tangency of the plane on which the chart is projected. A somewhat better chart, based on the principle of the great circle chart in that it is a projection on a plane tangent to the earth at one point, is the stereographic projection illustrated in Fig. 2. The difference is that the point of projection in the stereographic projection is at the opposite pole instead of at the center of the earth. This reduces the distortion and produces an excellent chart.

It may be seen by careful study of Fig. 2, that the Mercator chart, and the stereographic chart are special cases of the Lambert chart. In the case of the Mercator chart, the cone of projection of the Lambert chart may be considered as being extended into a cylinder, and that in the case of the Stereographic chart, the cone has been flattened until its apex is in the plane of the projection. For practical navigation, these three projections, the Mercator, Lambert, and Stereographic are all that are required. It is predicted that we will in the future use more of Lambert and less of polyconic and gnomonic charts.

Who Issues Charts

Three departments of the Government issue charts, as follows: The Coast and Geodetic Survey of the Department of Commerce publishes from its surveys charts which are suited to the purposes of navigation, commerce, and public defense. The Hydrographic Office in the Navy Department has charge of the duplication of charts and plans issued by other nations, and the publication of charts by the Navy of coasts not under the jurisdiction of the United States; the Corps of Engineers in the War Department issues charts of the Great Lakes.

Charts Of The Great Lakes

The charts of the Great Lakes and other inland waterways are published by the War Department Corps of Engineers, with headquarters at United States Lake Survey Office, Detroit, Michigan. Their charts differ somewhat from the Coast and Geodetic Survey charts. They show the courses and distances between various points and harbors on the chart. The buoy system is in general similar to the coastwise charts, although the symbols used to indicate lights are not identical. Depths of water are shown on tinted areas up to 30 feet. Depths greater than this are given in fathoms on untinted areas. Useful information concerning the heights of water, tabulation of magnetic variation, lists of dry docks, and other data are printed on them. All lighted aids are emphasized by a small red circle printed over them which causes them to stand out very clearly on the chart. On Coast and Geodetic Survey charts, the colored spot is orange.

Four Series Of Charts

There are four series of charts on the Atlantic, Gulf, Pacific, and Philippine Island Coasts, the first series consisting of sailing charts, which embrace long stretches of coasts—for instance, from the Bay of Fundy to Cape Hatteras. These are intended to serve for offshore navigation, or between distant points on the coast, as for example, Portland, Me., to Norfolk, Va. They are prepared for the use of the navigator in fixing his position as he approaches the coast from the open ocean, or when sailing between distant coast ports. They show the offshore soundings, the principal lights and outer buoys and landmarks visible at a great distance.

The second series is known as the general charts of the coast. They are on a scale three times as large as those of the first series, and embrace more limited areas, such as the Gulf of Maine, etc. They are intended for coastwise navigation when the vessel's course is mostly within sight of land, and her position can be fixed by landmarks, lights, buoys, and soundings.

The third series comprises the coast charts, which are constructed on a scale five times as large as that of the second series. One inch on these charts represents about one nautical mile, or one and one-seventh statute miles. They are intended for close coastwise navigation, for entering bays and harbors, and for navigating the large inland waterways.

The fourth series embraces the harbor charts, which are constructed on large scales intended to meet the needs of local navigation.

Special Caution Required On Some Coasts

With respect to a well-surveyed coast, only a fractional part of the soundings obtained are shown on the chart, a sufficient number being selected to clearly indicate the contour of the bottom. When the bottom is uneven, the soundings will be found grouped closely together, and when the slopes are gradual fewer soundings are given. Each sounding represents an actual measure of depth and location at the time the survey was made. Shores and shoals where sand and mud prevail, and especially bar harbors and the entrances of bays and rivers exposed to strong tidal currents and a heavy sea, are subject to continual change of a greater or less extent, and important ones may have taken place since the date of the last survey. In localities which are noted for frequent and radical changes, such as the entrance to a number of estuaries on the Atlantic, Gulf, and Pacific coasts, notes are printed on the charts calling attention to the fact.

It should also be remembered that in coral regions and where rocks abound it is always possible that a survey with lead and line, however detailed, may have failed to find every small obstruction. For these reasons when navigating such waters the customary sailing lines and channels should be followed, and those areas avoided where the irregular and sudden changes in depth indicate conditions which are associated with pinnacle rocks or coral heads.

Chart Readings

Chart reading aims to give such explanation concerning the various symbols and standards as will establish easily remembered relations between these graphic representations and the physical features which they represent. Briefly stated, the standards governing charts are the following:

The "shore line" is the boundary between water and land at high water. This boundary is shown by a continuous line wherever data are sufficient to plot the same with any degree of accuracy; otherwise a dashed line is used, indicating "approximate" delineation.

Vertical lettering is used for any feature dry at high water and not affected by the movement of the waters. (See the designation of Towers and Bell in the illustration at the right.)

Leaning lettering is used to describe such features as are parts of the hydrography. (See the manner in which hrd s is used to indicate hard sand in the same illustration.)

STANDARD SYMBOLS USED ON CHARTS

Very often, on smaller scale charts, a small reef can not be distinguished from a small islet; the proper name for either might be " Rock." Following the standard of lettering the feature in doubt is an islet if its name is in vertical letters, but is a reef if lettered in leaning characters.

ABBREVIATIONS FOR USE WITH HYDROGRAPHIC SYMBOLS Abbreviations Relating To Bottoms

Cl cloy, Co. coral, G. grovel, M. mud, Oz. ooze, f. pebbles, Sstm}, Sh. shells, Sp. specís, SI. stones, brk. broken, col. colcareout, crs. coarse, dec decayed, dk. d¤rlc,fíy. flinty, fne. fine, grd. ground, g)y. griWy, lird. nord, )rg. forge. If. fight rky. rocky, ror. rotten, sff. soft, sml. small, spk. speckled, s(f. stiff, sir. srreoky, vol. volcanic, bk. black, br. brown, bu. blue, gn. green, ay. gray, rd. red, wh. while, yl. yellow, lilt, sticky, Crs. grass

General Abbreviations

Bn. beacon, Rk. rock, Wk. wreck, N R.S. novol radio station, N.R.C naval rod¡o direction Under (radio compass) sjatton, P.D.posih¯on doubtful, P.Apposition opproximore, E. D. existence doubtful

Abbreviations Relating To Lights

F. fixed, Fl. flashing, Occ occulting, Alt. alternating, Gp. group,

R. red, W. while, G. green, B. Uue,SEC.iector, (U) ur>watched,

ev. every, m. mites, min. minules, sec. seconds, vis. visible.

Ok. quick, I Qk. interrupted <juick, S·L. snorl-long.

OBSC. obscured, AERO, aeronautical í¡ghl

Abbreviations Relating To Fog Signals

(F.BJ log bell, (F. D) fog diaphone. (f. G) fog gun, (F HI fog horn, ÍF.S¡ log siren, (F.TJ fog Irumpef, (F.VVJ fog whistle, ÍS.BÌ submarine fog belL

Abbreviations Relating To Buoys

C can, N nun, S spar, H.S. norizonla( slripes, B. fc/ocf·, H. red, W.ukile,V.S. vertical stripes, G green, Y.yellow,Ch. checkered, R£f. reflector

Depths

Depths of water are shown very clearly by numbers which indicate the depth in feet or fathoms as the case may be, below mean low water. It is well to examine the chart to determine whether the soundings are in feet or fathoms, or even a combination of the two. Some charts are made with depths under thirty feet given in feet, while greater depths are given in fathoms. Further, these charts would have the depths in feet on tinted areas so that they can be quickly distinguished. Contour lines which run through all depths of the same dimensions are shown on most charts. A line consisting of a particular characteristic dotted line will run through all depths and can readily be followed.

Dangers And Aids To Navigation

Reefs, ledges, sunken rocks, rocks awash, and foul ground arc marked by symbols. Discolored water, ripples, current, and weeds are noted by symbol or lettering.

Aids to navigation are shown by symbols and by abbreviations, or by as much descriptive text as the scale of the chart may admit.

To render these symbols distinct it is necessary to greatly exaggerate these aids in size, as compared with the scale of the chart; therefore certain parts of the symbols have been agreed upon to indicate the exact position of such aids, as follows:

The center of the base line of any symbol presenting a horizontal line, namely, mooring buoys, beacons, light vessels.

How Buoys Are Shown

All buoys, excepting mooring buoys, are shown by compressed diamond-shapes and a small dot, denoting the anchor ring. This dot indicates the proper position. To avoid interference with other features upon the chart it is often found necessary to show the diamond-shape at various bearings to the anchor ring, so that at times the symbol might be upside down. Since the buoys are also shown with such superimposed marks, as drums, cones, and balls attention should be given to the fact that the anchor ring does not touch the diamond shape, while the distinguishing marks are joined to the top of the buoy-symbol. For example: Numerous soundings close together might compel the buoy to be shown so that the top of the symbol bears in the opposite direction from the actual position; the isolated dot is the "position" part of the symbol, the opposite ring (connected with the buoy by a staff) is the distinctive mark.

The buoy symbol is shown "open"—in outline—for buoys of any color other than black; black buoys are shown by "solid" shape. If the buoy system shown upon the chart consists or the black and one other color only, the explanation under the title will ascribe such color to the "open" symbol. Thus upon one chart it may be found to denote "red buoy" while upon another chart it may be stated as "white" or "green"; the meaning of the "open" symbol varies, the meaning of the "solid" symbol is always the same—"black."

Upon any chart containing buoys of various colors besides black the color will be found stated by abbreviation or in full alongside each symbol or in the form of a note on the chart.

The buoy symbol, with anchor dot surrounded by rays, denotes a "lighted" buoy; surmounted by a crescent (points downward) denotes a "whistling" buoy; surmounted by a half disk with dot above the same denotes a "bell" buoy.

A line drawn between the upper and lower points of the diamond-shape (longer axis) denotes "vertical stripes"; a line drawn between the side points (shorter axis) denotes "horizontal stripes"; both lines drawn denote "checkered" buoy.

Note: The most recent sheets summarizing symbols and abbreviations used on charts show the compressed diamond shape and dot as a standard buoy symbol, differentiating several specific types of buoy by lettering or abbreviations alongside the symbol as follows: bell, BELL; gong, GONG; whistle, WHIS; can, C; nun, N; spherical, SP; spar, S. The newest charts use a heavy black dot for a light, emphasized by a solid magenta spot of color instead of an open circle.

Caution In Using Buoys

While buoys are valuable aids, the mariner should always employ a certain amount of caution in being guided by them. It is manifestly impossible to rely on buoys always maintaining their exact position, or, indeed, of finding them at all. Heavy seas, strong currents, ice, or collisions with passing vessels may drag them from their positions or cause them to disappear entirely, and they are especially uncertain in unfrequented waters, or those of nations which do not keep a good lookout on their aids to navigation. Buoys should therefore be regarded as warnings and not as infallible navigation marks, especially when in exposed places; and a ship's position should always, when possible, be checked by bearings or angles of fixed objects on shore. The lights shown by lighted buoys can not be implicitly relied upon.

The Compass Rose

The top of the chart is generally north. In addition a compass rose is printed in several places on the chart giving both the true north and the magnetic north with the variation between them at the time the chart was made. As this is a variable quantity the annual rate of increase or decrease is noted from which the variation at any time can be quickly found.

Magnetic Variation

Upon charts of small scale and greater territory, coast charts, and ocean charts, "variation lines" are given because the magnetic conditions differ greatly in the various localities represented upon one chart.

Identification Of Lights

Before coming within range of a light the navigator should acquaint himself with its characteristics (see page 87), so that when the light is sighted it will be recognized. The charts, sailing directions, and light lists give information as to the color, character, and range of visibility of the various lights. Care should be taken to note all of these and compare them when the light is seen. If the light is of the flashing, revolving, or intermittent variety, the duration of its period should be noted to identify it. If a fixed light, a method that may be employed to make sure that it is not a vessel's light is to descend several feet immediately after sighting it and observe if it disappears from view. A navigation light will usually do so while a vessel's light will not. The reason for this is that navigation lights are, as a rule, sufficiently powerful to be seen at the farthest point to which the ray can reach without being interrupted by the earth's curvature; they are therefore seen the moment the ray reaches the observer's eye on deck, but are cut off if the light is lowered. A vessel's light, on the other hand, is of limited intensity and does not carry beyond a point within which it is visible at all heights.

Care must be taken to avoid being deceived on first sighting a light. The glare of a powerful light is often seen beyond the distance of visibility of its direct rays by the reflection downward from particles of mist in the air. The same mist may cause a white light to have a reddish tinge, or it may obscure a light except within short distances. A fixed light when first picked up may appear flashing, as it is seen on the crest of a wave and lost in the hollow.

Some lights are made to show different colors in different sectors within their range. In such lights one color is generally used on bearings whence the approach is clear and another covers areas where dangers are to be found. By consulting the chart or books the explanation of the color of the ray in which you find yourself is found.

Charts For Instruction Purposes

The U. S. Coast and Geodetic Survey has made available for instruction purposes copies of U. S. Coast and Geodetic Survey Chart

No. 1210-Tr at ten cents a copy. This is a 1/80,000 full size chart, Martha's Vineyard to Block Island.

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