G. R. Putnam

“ The convenience of mariners should govern in the selection and arrangement of the information to be shown on charts, though they may be made useful for other purposes so long as this convenience is not lessened. The needs and preferences of navigators alone, however, differ so much that a reasonable chart must be somewhat of a compromise between conflicting views. For certain classes of navigation a boldly drawn chart showing only the dangers and a few other soundings and some landmarks might be useful. ”

“ It is evident that it is impossible to represent on a chart of any practicable scale all the features that exist on the corresponding area of the earth's surface. It is essential, therefore, that a selection be made of the classes of facts that are to be shown, as well as of the detail that is to be used for each class. ”

“ The hydrography, or the survey of the water area, is of prime importance for the chart, but in the order of prosecution of the work it is convenient but not essential that it come after sufficient points have been located by the triangulation and topography. A hydrographic sheet is prepared on which all the points are plotted which will be useful. A system of sounding lines is then run over the entire area to be surveyed, locating the position of the sounding boat at intervals by sextant [Pg 41-49] angles on survey signals or by angles from the shore. ”

“ Because of the facility with which they may be taken compass bearings are much used for inshore navigation, but in point of reliability they are inferior to some of the other methods.
A single or "danger" bearing of an object is often a valuable guide in avoiding a danger. For example, a reef may lie to the westward of a line drawn South 10° East from a lighthouse; in approaching a vessel will pass safely to the eastward of the reef if the lighthouse is not allowed to bear any to the northward of North 10° West. ”

“ On British charts the nautical mile scale is divided into tenths (that is, cables of 100 fathoms or 600 feet length) ; on the American charts into quarters and eighths. Where the scale covers more than one mile the fractional divisions are shown only for the left-hand mile and the zero of the scale is placed between this and the full mile scale, so that with dividers the full miles and fraction may readily be taken off. The nautical mile in the United States is taken to be the length of a minute of arc of a great circle on a sphere whose surface equals that of the earth ”

“ On a mercator chart an island in latitude 60° would appear four times as large as an island of the same actual area at the equator, but this distortion of areas, while it gives erroneous impressions on charts of great extent in latitude, does not seriously affect the use of the chart for nautical purposes. Areas may also be correctly measured on a mercator map by taking each projection quadrilateral separately, subdividing it if necessary, and using the published tables of areas of quadrilaterals in different latitudes. ”

“ A compass bearing of a single object, as a lighthouse or a tangent to a point of land, laid down on the chart, shows that the vessel is somewhere on that line, and when combined with other information, as with a Sumner line or the course by dead reckoning or the distance by a vertical angle, will give a position whose correctness of course depends on the accuracy of the data used. Bearings of two objects not in the same direction give two lines on the chart whose intersection is the position. ”

“ As the depth of water varies with the tide, it is necessary for charting purposes to adopt some standard plane to which the soundings are referred. Practically all countries have adopted for this purpose a low stage of the tide, as this is obviously on the side of safety; in most cases an extreme low water is used, so that the actual depths will seldom, owing to the tide, be less than those shown on the chart. ”

“ The surface of the earth being curved, there is no possible system of projection by which it can be represented on a flat sheet of paper in an ideally satisfactory way. Numerous methods of projecting the earth's surface upon a plane have been proposed and many of them are actually used for various purposes. In general each projection has qualities which are valuable for certain uses, and deficiencies which make it less valuable in other ways. Only four of the different projections need be mentioned here as of special interest in chart construction.
Mercator projection. ”

“ When a sounding is made without the lead reaching bottom, the depth obtained is sometimes [Pg 118] shown on the chart by a short line and zero above the figure, indicating that at the depth stated, bottom was not obtained (no bottom) . There are a few important symbols shown in the water area of charts. The sunken rock symbol indicates a dangerous area, or a danger having a moderate depth of water over it, or a rock the least water over which is not known; ordinarily on the United States charts the least depth will be stated when known, and the symbol omitted. ”

“ From the measured length of the base and the angles the lengths of the sides of the triangles are computed, and from these lengths and the latitude and longitude of one point the latitudes and longitudes of all the other points are obtained. When several astronomically determined points are connected by such a triangulation a complication arises from what is known as "deflection of the plumb line," which is the angular amount by which the actual sea-level surface of the earth departs from the symmetrical figure of revolution, owing to the variations in the density of the earth's outer layers. ”

“ The tidal information becomes important and must be considered in navigation or in anchoring in waters where the available depth at low water approximates the draft of the vessel. In the general use of coast charts it is also important to observe the effect of the stage of tide on the appearance of many features. Rocks rising some feet above low water may be entirely submerged at high water. In some areas the aspect may be radically changed between high and low water by the baring of extensive shoals or reefs. ”

“ As shown below, only a small part of the coast line of the world is thoroughly surveyed. In the extensive ocean areas which are dotted with islands or reefs, a large amount of work is required for their sufficient charting, although many doubtful areas have been cleared up in recent years. Even the parts that are known to be of depths so great as to be free from navigational dangers should be sounded over sufficiently to develop the general configuration of the ocean bottom. ”

“ The ordinary method of sounding is to cast a lead from a boat and read the depth when the lead touches bottom and the line is vertical, and make note of the nature of the bottom. There is a systematic spacing between the casts of the lead and between the lines passed over by the boat, depending on the depth of water and character of the bottom. For soundings in deeper water various forms of sounding machines are used, with weight attached to a wire. For very great depths a small steel wire is employed and the weight is detached and left on the bottom. ”

“ In this country it is more commonly done by carrying the direction with a parallel ruler either from or to a compass rose printed on the chart. Distances are measured or laid down on a mercator chart by using the latitude border scale for the middle latitude. On polyconic and other larger scale charts distances are measured from the scales printed on the chart. It should be remarked that in general where special accuracy is required distances should be computed and not scaled from any chart, because of the error due to the distortion of paper in printing. ”

“ Depth curves if clearly shown are a great aid in interpreting the hydrography and making plain the shoals and passages. The system of curves should always be understood when using a chart, and it may sometimes aid the navigator to trace out with a pencil an additional curve, if needed, beyond the draft of his vessel. The abbreviations used for the bottom characteristics are explained either on the chart or on the sheet of chart symbols, and give information which is useful in anchoring, and may be helpful in identifying a position by soundings. ”

“ Ships engaged in international commerce must enter foreign ports. As the information is constantly changing and charts are being corrected or improved, it is sometimes desirable for the navigator to consult the local foreign charts, and it may often be necessary for him to carry in his chart room the charts of several different countries. There are therefore important advantages in international uniformity in chart publication. ”

“ The principal methods used for locating the position of a vessel are by astronomical observations, by dead reckoning, by compass bearings, by ranges, by horizontal angles, by soundings, by vertical angles, and by sound. ”

“ Some characteristics of the mercator projection are these: The meridians and parallels are all straight lines and perpendicular to each other; there is no convergence of the meridians; the minute of longitude is a constant distance on the map; the minute of latitude increases in length from the equator toward the poles but locally retains its true proportion to the minute of longitude; areas and distances increase in scale with the latitude so that a given scale is strictly correct only for one latitude ”

“ Even by passing over an extensive region a man unaided by instruments will obtain only a rather crude notion of the relations, which he could clearly see on a good map. The importance among the human arts of the making of maps is indicated by the references to them in very early historical records, and by the skill in map drawing shown by some of the primitive peoples of to-day. ”

“ The sailing along Libya from the Canopic mouth in Egypt to the Pillars of Hercules ... takes 74 days if one coast round the bays.... From the cape of Hermæa extend great reefs, that is, from Libya towards Europe, not rising above the sea; it washes over them at times.... From Thymiateria one sails to cape Soloes, which juts far into the sea. But all this district of Libya is very famous and very sacred.... This whole coasting from the Pillars of Hercules to Cerne Island takes twelve days. The parts beyond the isle of Cerne are no longer navigable because of shoals, mud, and sea-weed. ”

“ The gnomonic chart is therefore useful in supplementing the mercator chart, supplying its deficiencies as to convenience in marking out great circle courses. The great circle course can be derived not only more easily and quickly from the gnomonic chart than by computation, but the chart is also to be preferred because the course marked out on it will show at once if any obstruction, as an island or danger, is met or too high a latitude is reached. ”

“ It is important that objects which may be useful in navigation as landmarks, whether natural or artificial, be plainly shown and described, if necessary to their identification, and that they should not be obscured by details of lesser importance. ”

“ EARLY CHART OF NEW YORK HARBOR, 1737.
[Pg 16-17]
Extension of maritime surveys. Of the total area of the earth's surface, 51,886,000 square miles is land and 145,054,000 square miles is sea. The oceans thus occupy nearly three-fourths of the whole surface, affording highways open to the nations. To conduct international commerce by water the ships of one country must enter the ports of another. Thus both on the open sea and in the harbors there is an interest, common to seamen of all nationalities, in the advance of marine surveys and in the publication of charts. ”

“ Notices to Mariners are published at frequent intervals, giving all important corrections, which should be at once applied by hand to the charts, such as rocks or shoals discovered and lights and buoys established or moved. New charts, new editions, and canceled charts are also announced.
These notices should be carefully examined and the necessary corrections made on all charts of the sets in use on the vessel. A chart should be considered as a growing rather than a finished instrument, and constant watchfulness is required to see that it is kept up to date. ”

“ A coast is not to be considered as clear unless it is shown to be; buoys may get adrift and be in a different position or be gone altogether; fog signals vary in distinctness owing to atmospheric conditions; extreme or unusual tides may fall below the plane of reference; owing to strong winds the actual tide may differ from the predicted tide. Errors sometimes creep in from various sources, [Pg 148] such as those due to different reference longitudes or the use of a corrected longitude for a portion of the chart without changing other positions to which the same correction is applicable ”

“ In thick weather sound affords a valuable aid to the navigator. In narrow passages noting the echo of the whistle from a cliff is a method resorted to, as for instance in Puget Sound and along the Alaska coast. Fog whistles and bell buoys are maintained at many places. Submarine bells have recently been introduced at a number of points along the Atlantic coast, and vessels may be equipped to receive these submarine signals transmitted through the water, which indicate also the general direction from which the sound comes. ”

“ Even if objects cannot be seen, due to distance or thick weather, the chart furnishes a valuable aid when a vessel has approached within the limits where it is practicable to obtain soundings. Modern navigational sounding machines permit of obtaining soundings to depths of nearly one hundred fathoms without stopping the vessel. ”

“ Nearly all charts are now oriented with the meridian, that is, north is the top of the chart, and on a mercator chart the east and west border lines are parallel with the meridians and the north and south border lines with the parallels. ”

“ The main topographic features are represented for a moderate distance from the coast, with such detail as is useful, depending on the scale of the chart. Elevations are given in figures for prominent summits, islands, and rocks; the general configuration of hills and mountains is represented by contours on large scale charts or by hachures or shading on small scale charts. Rivers, streams, lakes, marshes, towns, roads, prominent buildings, and other important topographic features are shown by appropriate symbols. ”

“ The unit used for elevations is also stated on the face of the chart, as also the plane to which elevations are referred. On the United States charts this is generally mean high water and on British charts the high water of ordinary spring tides. Rocks and islets usually have figures shown beside them, either in brackets or underscored, which indicate the height above high water. Rocks which are bare at low water sometimes have a note "dries" or "bares" so many feet, indicating their height above low tide, although they are covered at high tide. ”