G. R. Putnam

Chart working. In crossing the open and deep portions of the ocean, where the only data given may be the projection lines and soundings far deeper than can be reached with navigational sounding machines, the chart is used to lay out in advance the general course to be followed and to plot the positions of the vessel at intervals either as determined by observations or, lacking these, by dead reckoning. When necessary the courses of the vessel are modified as the plotted positions are found to fall one side or the other of the proposed general track.

The principal operations on a chart are plotting or taking off positions by latitude and longitude, laying down or taking off bearings, directions, and courses, plotting or measuring distances, and laying down or taking off angles.

To plot a position by its latitude and longitude on a mercator chart, set a parallel ruler on the adjacent parallel and then move it to the required latitude as shown by the border scale at either side; then with a pair of dividers at the upper or lower longitude border scale take the distance from the nearest meridian and lay this distance off along the edge of the parallel ruler. The latitude and longitude of a point are taken from the chart by reversing this process, or with the dividers only. A direction is laid down on the chart or read from the chart preferably by using some form of protractor and measuring the angle from the projection lines. 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.

The use of protractors on charts in plotting by angles in the three-point problem will be referred to later.

The course to be steered to allow for a set due to current or wind may be obtained by a graphical solution on the chart, though it will be preferable to do this on other paper, using a larger scale. (Fig. 38.) The direction and velocity of the set and the course and speed of the ship may be considered as two sides of a parallelogram of forces, of which the diagonal is the distance and course made good. To obtain the course to steer to reach a given point with a given current and speed of vessel, lay down the direction of the destination; from the starting point lay off the direction of set and the amount in one hour; from the extremity of this describe an arc with radius equal to the speed of the vessel in one hour. A line drawn from the extremity of the direction of set to the point of intersection of the arc and the course to be made good will give the direction of the course to be steered, and the point of intersection will also be the estimated position of the vessel at the end of the hour's run.

Methods of locating a vessel. 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. The full discussion of these methods pertains to navigation and pilotage, and they will be only briefly referred to here as to their graphical application to charts.

Astronomical methods. There are a number of methods of obtaining the position of a vessel by astronomical observations. When the position is computed the chart enters into these only in the plotting of the final result, so that with one exception these methods will not be referred to further here.

The elegant method discovered by an American seaman, Captain Sumner, in 1843, is in part graphical, to be worked out upon the chart. This method is based on the obvious fact that at any instant there is a point on the earth having the sun in its zenith and which is the center of circles on the earth's surface along the circumference of any one of which the sun's altitude is the same at all points. A short portion of such a circle may be considered as a straight line and can be determined by locating one point and its direction, or two points in it. This is known as a Sumner line. (Fig. 39.)

From an observation of the sun's altitude and azimuth and an assumed latitude a position is computed and plotted and a line drawn on the chart through this position at right angles to the azimuth of the sun as taken from the azimuth tables and laid off from a[Pg 127-129] meridian. Another method is to compute positions with two assumed latitudes and plot the two resulting positions and draw a line through them. The vessel must be somewhere on the resulting Sumner line. A good determination may be obtained by the intersection of two Sumner lines obtained from two observations of the sun with sufficient interval so that there will be a change of azimuth of as much as 30 degrees to give a fair intersection. Allowance must be made for the movement of the vessel between the two observations by drawing a line parallel to the first and at a distance equal to the distance made good. An excellent intersection may be obtained by observation of the sun, and before or after it of a star in the twilight at a different azimuth.

COURSE TO ALLOW FOR SET, GRAPHICAL SOLUTION
FIG. 38.

POSITION BY INTERSECTION OF SUMNER LINES
FIG. 39.

POSITION BY COMPASS BEARINGS
FIG. 40.

Even a single Sumner line, however, furnishes valuable information, as it may be combined with other sources of information to obtain an approximation to the position. The vessel must be somewhere on this line, and this gives a good check on the position by dead reckoning, or an intersection may be obtained with a line or bearing of a distant land object, or a line of soundings may be compared on the chart with the Sumner line.

If an observation is taken when the observed heavenly body is bearing abeam, it is evident that the resulting Sumner line will be the direction of the course of the vessel, and this fact may be useful in shaping the course when nearing the land or a danger.

Dead reckoning. When impossible to obtain the position by any other means, it is computed or plotted from the last determined position, using the courses and distances run as shown by compass and log and allowing for effect of current and wind. Because of uncertainties in all these elements, positions so obtained may be from five to twenty miles in error in a two-hundred-mile run, depending of course to some extent on the speed of the vessel.

Compass bearings. 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. This will be very weak if the angle of intersection is acute, and will become stronger as it approaches a right angle. A bearing of a third object should be taken when practicable, as it affords a valuable check in that the three lines should intersect in the same point; if they do not do so when plotted the error is either in the observations, or the compass, or the plotting, or the chart. (Fig. 40). All compass bearings are of course dependent upon the accuracy of the compass and the knowledge of its errors due to the local magnetic effect of the ship, and also upon the correctness with which the magnetic variation from true north is known. Bearings of near objects should therefore always be preferred, and those of distant objects considered as giving only approximate positions. An error of one degree in the bearing of an object 30 miles away will deflect the plotted line about one-half mile. 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. (Fig. 41.)

Two successive bearings of a single object, as, for instance, a lighthouse, noting the distance run in the interval, afford a convenient and much used means of locating the position with respect to that object. Such bearings are drawn on the chart in reversed direction from the object. The distance run between the bearings, as read by the log and corrected for current if practicable, is scaled off with dividers and the course of the vessel is set off with parallel ruler; the latter is then moved across the two plotted directions until the distance intercepted between them equals that scaled with the dividers, and the edge of the ruler then represents the track of the vessel. (Fig. 42.) If the angle from the bow, or from the course of the vessel, for the second bearing is double that for the first bearing, the distance from the object at the second bearing is equal to that run by the vessel in the interval, and the use of this simple relation is designated as "doubling the angle on the bow." If the angles between the course and the object are respectively 45° and 90° when the two bearings are taken on an object on the shore, the distance that the ship passes offshore when the object is abeam is equal to the distance run between the two bearings; this is a much used navigational device, known as the "bow and beam bearing" or the "four-point bearing." There is an advantage, however, in using bearings at two and four points (or 22.5° and 45°), as these give the probable distance that the object will be passed before it is abeam.

Ranges. A valuable line of position is obtained by noting when two well-situated objects are in range, that is, one back of the other in the line of sight from the vessel, as, for instance, a church spire appearing behind a lighthouse or a rock in line with a prominent point. Such ranges are of course entirely free from compass errors, and should be noted whenever there is favorable opportunity. The value of the range in plotting will increase with the distance between the objects, and if the two are close in proportion to the distance to the vessel the direction will be weak owing to the uncertainty in drawing a direction through close points. Artificial ranges are often erected as aids to navigation, usually to indicate the course to be followed in passing through a channel. Ranges afford a valuable guide in avoiding dangers, as for example an inspection of the chart may show that if a certain lighthouse is kept in line with or open from an islet a dangerous shoal will be given a good berth; on coasts not well buoyed such danger ranges are sometimes marked on the charts. (Fig. 43.)

DANGER BEARING
FIG. 41.

Horizontal sextant angles. The location of a position by the three-point problem, using sextant angles, is much more exact than by bearings, but is less used because not so well known and also because additional[Pg 133-135] instruments are required and the conditions are not always favorable. It is so valuable a method, however, that it should be used, when necessary, on every well-equipped vessel. A single horizontal angle taken with a sextant between objects, as two lighthouses, defines the position of the vessel as somewhere on the circumference of a circle passing through the two objects and the vessel. A protractor laid on the chart with two of its arms set at the observed angle and passing through the two objects, will permit of locating two or more points of this circle on the chart. This furnishes a line of position which may be combined with other information to locate the vessel. With a compass bearing of one of the objects the position may be plotted directly from the single angle. Two sextant angles measured at the same instant between three objects furnish one of the most accurate means of locating the position of a vessel, this being the same method that is ordinarily used in hydrographic surveying, known as the three-point problem. (Fig. 44.) The two angles are conveniently set off on a three-arm protractor, which is shifted on the chart until the three arms touch the three points, when the position of the center is plotted. A third angle to a fourth point furnishes a valuable check in case of doubt. Two angles may also be taken to four objects without any common point, and in this case portions of the two circles of position are plotted and their intersection will be the ship's position.

The value of this method depends largely on the selection of favorably located objects, and it is quite important that the principles of the three-point problem be understood. If the ship is on or near the circumference of a circle which passes through the three objects the position will be very weak, and the same is true if the distance between any two of the objects is small as compared with the distance from them to the vessel. A useful general rule is that the position will be strong if the middle one of the three objects is the nearest to the vessel, provided that no two of the objects are close together in comparison with the distance to the vessel.

A single sextant angle furnishes a means of avoiding a known danger by using what is known as the horizontal "danger angle." (Fig. 45.) Note two well-defined objects on the coast either side of the danger to be avoided and describe a circle through them and passing sufficiently outside of the reef to give it a safe berth. With a protractor on the chart note the angle between the objects at any point on the outer part of this circle. If in passing, the angle at the ship between the two objects is not allowed to become greater than this "danger angle" the danger will be given a sufficient berth. This method as well as any use of sextant angles or bearings depends of course on the accuracy of the chart, and caution must be used where it is not certain that the chart depends upon an accurate survey.

Soundings. 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. A rough check is at once[Pg 137-139] obtained by comparing such soundings with those given on the chart for the position carried forward by dead reckoning. If a number of soundings are taken and plotted on a piece of tracing paper, spaced by the log readings to the scale of the chart, and this tracing paper is laid over the chart and shifted in the vicinity of the probable position until the soundings best agree with those on the chart, a valuable verification of position may be obtained. This is particularly the case if the area has been well surveyed, and the soundings taken on the vessel are accurate, and the configuration of the bottom has marked characteristics. For instance, in approaching New York the crossing of the 30, 20, and 10 fathom curves will give a fair warning of the distance off the Long Island and New Jersey coasts, and soundings across such a feature as the submerged Hudson gorge extending to the southeastward of Sandy Hook will give a valuable indication of position. The taking of soundings should be resorted to even in favorable conditions, in approaching shoal water, as a check on other means of locating the vessel. Many marine disasters are attributed to failure to make sufficient use of the lead, the simplest of navigational aids.

Vertical angles. The vertical angle of elevation of an object whose height is known will give the distance, and combined with a bearing or other information this permits of locating a vessel where better means cannot be used. Distance tables are published for this method. (Fig. 46.) The vertical angle is measured with a sextant and must be the angle at the ship between the top of the object and the sea level vertically beneath it; for a hill or mountain, therefore, the eye of the observer should be near the water. The object should not be so distant that curvature becomes appreciable. The "vertical danger angle" is a means of avoiding a known danger, on a principle similar to that of the horizontal danger angle; that is, the angle of elevation of a known object is not permitted to become greater than a fixed amount depending on the distance from the object to the danger to be avoided.

Positions by sound. 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.

Need of vigilance. Too great importance cannot be attached to frequent verification of positions by the best available means, particularly when approaching the land. Neglect of this or overconfidence has caused many disasters. A notable instance was the loss of one of the largest Pacific steamers on the coast of Japan in March, 1907. In the afternoon of a clear day this vessel ran on to a well-known reef about a mile from a lighthouse, resulting in the total loss of vessel and cargo valued at three and a half million dollars. The captain was so confident of his position and that he was giving the reef a sufficient berth that he laid down no bearings on the chart and took no soundings.

Instruments. The principal instruments needed for use with charts are; dividers for taking off distances and latitudes and longitudes, parallel ruler for transferring directions to or from a compass rose and for taking off or plotting the latitude on a mercator chart, protractor of 180 degrees for reading the angle with the meridian of any direction or for laying off on the chart any given angle with the meridian, and three-arm or other full-circle protractor for plotting a position by the three-point problem.

Parallel rulers on the principle of Field's are strongly recommended for chart work, as they combine in a single instrument the advantages of a parallel ruler and a 180-degree protractor. Any direction can be read or laid off by simply moving the parallel ruler to the nearest projection line, which is a process not only more convenient than referring to the compass rose printed on the chart but also more accurate because of the longer radius. These instruments can also be used the same as a plain parallel ruler. Field's parallel rulers are made in two forms, one rolling and the other sliding. The former is a single ruler with edge graduated 90 degrees either way, and mounted on rollers; it is the most rapid instrument for reading or laying off a direction, but it requires a smooth surface. The latter is an ordinary two-bar parallel ruler with edge when closed graduated 90 degrees either way; it is a very serviceable instrument and probably more to be depended upon for ordinary use than the rolling form. Some form of combined protractor and parallel ruler should be in every navigational equipment, and it is unfortunate that these instruments are not better known in this country. There are other forms of half-circle protractors which are used on the same principle, that is, of bringing the center on to a projection line and reading where the line cuts the border graduation of the protractor. Thus a semicircular protractor is used with a separate straight edge, along which it is slid to the nearest meridian; another form is the simple circular protractor with a thread fastened at the center. All these forms of protractors, it will be noted, are intended to work from the true meridian, and they are usually graduated in degrees only; the use of degrees instead of points is becoming much more general in navigational work, and reference to the true meridian is also more common than formerly.

The standard three-arm protractor, or station pointer, as it is known to the English, should be a part of every navigational outfit because of its value in locating a position by the three-point problem. A recent American invention, Court's three-arm protractor, is an instrument made of celluloid for the same purpose. It should not be considered as a substitute for the standard metal instrument, but it is a simple, cheap, and handy supplement to it, as it may be readily used for small angles and short distances where there are mechanical difficulties in working with the metal three-arm protractor. Other protractors can be used for the three-point problem, as, for instance, Cust's protractor on celluloid, on which the angles are drawn in pencil and erased, and the tracing-paper protractor.

Degree of reliance on charts. The value of a chart must not be judged alone from its general appearance,[Pg 145-147] as skill in preparation and publication may give a handsome appearance to an incomplete survey. On the other hand a thorough survey might through poor preparation result in a chart defective either in information or in utility.

The degree of completeness of the soundings, the character of the region, and the date of the survey should be taken into account in deciding as to the amount of reliance to be placed on the chart. Areas where the soundings are not distributed with fair uniformity may be assumed not to have been completely surveyed. Caution should be used in navigating on charts where the survey is not complete, and even where careful surveys exist care must be taken if the bottom is of very irregular nature with lumps near the navigable depth, as for instance on some of the coral reef coasts. Isolated soundings shoaler than the surrounding depths should be avoided, as there may be less water than shown. In such a region, unless the whole area is dragged, it is impossible to make it entirely certain that all obstructions are charted.

While an immense amount of faithful work has been put into the preparation of many charts, the user must constantly exercise his own judgment as to the reliance to be placed on them. 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, 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; clerical and printing errors may occur; there are sometimes omissions in surveys; a feature may get plotted in two different positions; tide rips are reported as breakers and floating objects as rocks or islands, and thus many dangers have gotten on the charts which cannot be found again, and false reports are sometimes made to shield some one from blame. Most of these classes of errors and uncertainties, however, disappear in the use of charts of a thoroughly surveyed coast.

Use the latest editions of charts. The latest edition of a chart should always be used and should be corrected for all notices since its issue. Carelessness or false economy in not providing the largest scale or the latest chart has been the cause of more than one marine disaster.

The British Board of Trade issue the following official notice to shipowners and agents: "The attention of the Board of Trade has frequently been called to cases in which British vessels have been endangered or wrecked through the masters' attempting to navigate them by means of antiquated or otherwise defective charts. The Board of Trade desires, therefore, to direct the especial attention of shipowners and their agents to the necessity of seeing that the charts taken or sent on board their ships are corrected to the time of sailing. Neglect to supply a ship with proper charts will be brought prominently before the Court of Inquiry in the event of a wreck occurring from that cause."

The following is a translation of a notice in the preface to the catalogue of charts published by the German government: "Owners and masters of vessels are apprised that cases of marine accidents in which the casualty was due to antiquated or erroneous charts, have frequently been before the admiralty courts. In consequence of this, the 'Instructions for the prevention of accidents to steamers and sailing vessels,' issued by the Seeberufsgenossenschaft have been amended by the following additional paragraph: 'It is obligatory upon every master, except when engaged in local coastwise navigation, to keep the Notices to Mariners regularly, and with the aid of them to carefully keep his charts up to date.'"

The British shipping laws provide that a ship may not be sent to sea in such an unseaworthy state that the life of any person is thereby endangered, and the House of Lords has defined the term "seaworthy" to mean "in a fit state as to repairs, equipment, and crew, and in all other respects, to encounter the ordinary perils of the voyage." Proper charts and sailing directions are a necessary part of the equipment of a vessel, and the courts have frequently inquired into this.

The records of the British courts, however, show that even in recent years many ships have been damaged or lost owing directly or indirectly to failure to have the latest information on board. The following are instances from these records.

In 1890 the steamer Dunluce was lost owing to the use of an old edition of the Admiralty chart which showed a depth of 412 fathoms on the Wikesgrund, whereas the later chart showed much less water. In this case the master had requested his ship chandler to send him the latest chart.

In 1891 the steamer St. Donats got ashore on a patch which was not shown on the chart in use, which was privately published in 1881; the danger was, however, shown on the Admiralty chart corrected to 1889.

Also in 1891 the steamer Trent was lost on the Missipezza Rock in the Adriatic. The ship was navigated by a private chart published in 1890 which did not show this rock, and by sailing directions published in 1866.

The steamer Aboraca, stranded in the Gulf of Bothnia in 1894, was being navigated by a chart corrected to 1881 which did not show that the Storkallagrund light-vessel had been moved eight miles.

The steamer Ravenspur was lost on Bilbao Breakwater owing to the use of a chart not up to date which did not show the breakwater. In 1898 the steamer Cromarty was lost in attempting to enter Ponta Delgada harbor, and in 1901 the steamer "Dinnington" was lost by steaming on to the new breakwater in Portland harbor; both of these disasters were likewise due to the use of old charts which did not show the breakwaters. In these three cases the masters of the vessels had authority to obtain the necessary charts at the owners' expense.

Not so, however, in the following case from the finding of a British marine court in 1877: "The primary cause of the ship's getting on shore was due to the master's being guided in his navigation by an obsolete Admiralty chart dated September 1, 1852, and corrected to April, 1862, and on which no lights are shown to exist either in ... or ... and to his not being supplied with the latest sailing directions. The Court, considering that the master was obliged to furnish himself with chronometer, barometer, sextant, charts, sailing directions, and everything necessary for the navigation of his vessel out of his private resources, which, under very favorable circumstances, might perhaps reach £150 a year, find themselves unable in this instance to pass a heavier censure upon him than that he be severely reprimanded."

The loss of the German steamer Baker on the coast of Cuba on January 31, 1908, was declared by the marine court at Hamburg to be due in part to the use of an unofficial chart which did not show the latest surveys on that coast.

Use the largest scale charts. The largest scale chart available should be employed when entering channels, bays, or harbors, as it gives information with more clearness and detail, positions may be more accurately plotted, and sometimes it is the first corrected for new information.

The records of the courts of inquiry also show cases where vessels have been wrecked owing to the use of charts of too small scale.

In 1890 the steamer Lady Ailsa was lost on the Plateau du Four. The only chart on board for this locality was a general chart of the Bay of Biscay, and the stranding was due to the master's mistaking one buoy for another. The court found that the chart, although a proper one for general use, was not sufficient for the navigation of a vessel in such narrow waters and on such a dangerous coast.

The Zenobia was stranded on the San ThomÉ Bank in 1891. On this vessel the owners were to furnish the chronometers and the master the charts and sailing directions. The master was, however, apparently satisfied with only a general chart of the South Atlantic for navigation on the coast of Brazil, and had no sailing directions at all.

The depth curves on charts furnish a valuable guide, and if the curves are lacking or broken in some parts it is usually a sign that the information is incomplete. The 100-fathom curve is a general warning of approach to the coast. The 10-fathom curve on rocky coasts should be considered as a danger curve, and caution used after crossing it. The 5-fathom curve is the most important for modern vessels of medium draft, as it indicates for them the practical limit of navigation. The 3, 2, and 1-fathom curves are a guide to smaller vessels, but have less significance than formerly because of the increase of draft of vessels.

The shrinkage of paper, especially in plate printing, has been referred to. This introduces two possible sources of error: first, the shrinkage being different in the two directions, any scale printed on the chart will be accurate only when used in a direction parallel to itself; second, for the same reason, angles and directions will be somewhat distorted. Fortunately these errors are not serious in the ordinary navigational use of a chart, but they should not be overlooked when accurate plotting or measuring of distances is attempted on a plate-printed chart.

The actual shrinkage measured on charts printed from plates varies from 13 inch to 1 inch in a length of chart of 36 inches. On British and American plate printed charts the shrinkage is usually from two to nearly three times as much in one direction as it is in the other.

Care of charts. In order that they may be properly used charts should be filed flat and not rolled. They should be systematically arranged so that the desired chart can be instantly found. They should be cared for and when in bad condition replaced by new copies. They can be most conveniently filed in shallow drawers, thus avoiding the placing of many charts in a single drawer. The latter is a common fault; it not only increases the labor of handling the charts but adds to the liability of their injury.