Chapter II: Preface (2)

It is surprising to note with what lack of care and of sufficient evidence reports of dangers at sea have sometimes been made, and how incomplete are many of the reports even when the existence of the danger is beyond question. It is unfortunately true that some of these reports are the result of effort to escape blame for accident by throwing the fault on the chart. Many such reports also result from various illusory appearances. A large tree covered with weeds, an overturned iceberg strewn with earth and stones, a floating ice-pan covered with earth, the swollen carcass of a dead whale, a whale with clinging barnacles and seaweed, reflections from the clouds, marine animalculæ, vegetable growth, scum, floating volcanic matter, and partially submerged wrecks covered with barnacles, have been mistaken for islands, shoals, or reefs. A school of jumping fish has given the appearance of breakers or caused a sound like surf, and tide rips have been mistaken for breakers. Raper very properly calls attention to the obligation upon every seaman of carefully investigating doubtful cases and making reliable reports. "Of the dangers to which navigation is exposed none is more formidable than a reef or a shoal in the open sea; not only from the almost certain fate of the ship and her crew that have the misfortune to strike upon it, but also from the anxiety with which the navigation of all vessels, within even a long distance, must be conducted, on account of the uncertainty to which their own reckonings are ever open. No commander of a vessel, therefore, who might meet unexpectedly with any such danger, could be excused, except by urgent circumstances, from taking the necessary steps both for ascertaining its true position, and for giving a description as complete as a prudent regard to his own safety allowed."

As to the older doubtful dangers now shown on the oceanic charts, it is estimated that the positions may be considered as uncertain by 10 miles in latitude and 30 miles in longitude, and areas of this extent must be searched to determine definitely the question of their existence.

The following are interesting or typical cases of reported dangers:

The master of an Italian bark in September, 1874, reported sighting a large rock in latitude 40° N. and longitude 62° 18´ W. Fortunately for the charts there were two independent reports from other vessels in the same month of sighting a partially submerged wreck in this vicinity.

The Spanish steamer _Carmen_ was wrecked in 1891 by running on a rock off the southwest coast of Leyte; the rock was reported to lie one mile off shore, a dangerous position for vessels using Canigao Channel. A survey made in 1903 showed 58 feet of water in this location, and that Carmen Rock on which the vessel struck was really within one-fourth mile of the beach. The rock had, however, for twelve years been shown on the charts in a position which made it an obstruction to navigation.

The ship _Minerva_ in 1834 was reported to have struck a rock near the middle of the broad entrance to Balayan Bay; the fact that this occurred at 2 A.M. indicated a very doubtful position, but it was stated that an American ship had previously been wrecked on the same rock. It consequently appeared as a danger on the charts for seventy-one years, when a survey showed no depth of less than 190 fathoms in this vicinity, and it was removed from the charts.

A British steamer was wrecked in San Bernardino Strait in 1905; the master reported that he was in a position where the chart showed 51 fathoms, and that he was 1-1/2 miles distant from Calantas Rock, and on these grounds the finding of the official inquiry was that "no blame can be attached to the master, officers, or any of the crew for the casualty." Very shortly after the disaster, the surveying steamer _Pathfinder_ definitely located the wreck and made a survey of the vicinity. The previous chart of Calantas Reef was found to be fairly correct, and the stranding was determined to have occurred well within this reef in a position where the chart showed soundings of 3-3/4 to 4-3/4 fathoms, and 1/2 mile from Calantas Rock, which rises 5 feet above high water.

A transport entering San Bernardino Strait a few years ago ran on a rock and was damaged; the position was reported as about two miles southeast of San Bernardino Island and near the middle of the passage. The rock was not put on the charts, as prompt investigation showed 50 fathoms of water in this vicinity, and that in all probability the transport actually touched a small reef making out from the island.

The master of the brig _Helen_ reported that his vessel was wrecked on a reef lying six miles from Rockall. When surveyed Helen Reef was found to be about one-third this distance from Rockall.

An island has been reported in eight different positions, ranging in latitude from 30° 29´ to 30° 42´ N. and in longitude from 139° 37´ to 140° 38´ E.

There have been a number of reports of islands in the area from latitude 40° 00´ to 40° 30´ N. and longitude 150° 30´ to 151° 00´ W. The master of the bark _Washington_ reported in 1867: "On my passage from the Sandwich Islands to the northwest coast of the United States, when in latitude 40° 00´ N., in a dense fog, I perceived the sea to be discolored. Soundings at first gave great depths, but diminished gradually to 9 fathoms, when through the mist an island was seen, along which I sailed 40 miles. It was covered with birds, and the sea swarmed with seal and sea elephants." A United States vessel searched in this vicinity without seeing any indication of land, and obtained soundings of 2600 fathoms. A British ship in 1858 searched for fourteen days over this area without finding anything. Searches were also made in 1860 and 1867 without success, and the present charts show no islands in this part of the Pacific.

In a number of cases erroneous positions have been due simply to blunders. Thus Lots Wife, first seen by Captain Meares in 1788, was shown on his chart in latitude 29° 50´ N., longitude 156° 00´ E., and stated in his book to be in latitude 29° 50´ N. and longitude 142° 23´ E. Massachusetts Island by one report was in longitude 177° 05´ E. and by another in 167° 05´ E. The apparent blunder of 10° is now immaterial, as the island has disappeared from the charts altogether. The Knox Islands were placed by the Wilkes Exploring Expedition in latitude 5° 59´ 15´´ N., longitude 172° 02´ 33´´ E. The old British charts showed islands of this name also in latitude 5° 59´ N., longitude 172° 03´ W., the longitude being doubtless transposed. In the case of Starbuck Island, discovered south of the equator, the latitude was apparently transposed, as on old charts it was also shown in the position, latitude 5° 40´ N., longitude 156° 55´ W.

A pinnacle rock can sometimes be located only with great difficulty even when known to exist. Rodger Rock, on which the bark _Ellen_ struck and was damaged, lies in latitude 0° 41´ 15´´ N. and longitude 107° 31´ E. It has but three feet over it at low tide. The British surveying ship _Rifleman_ searched four days before finding it, although the plotted tracks showed that she and her boats had passed very close to it. This indicates that great caution must be used in removing a reported danger from the charts.

The old charts of the Atlantic indicated a danger 30 to 45 miles to the southwest of Cape St. Vincent. This danger was omitted from the charts about 1786 owing to lack of confirmation. Later, in 1813 and 1821, it was reported that vessels were lost or damaged by striking this rock. Soundings of over a thousand fathoms are now shown on the chart in this vicinity and the rock no longer appears.

A comparison of a Pacific Ocean chart of about forty years ago with one of the present time (Fig. 19) illustrates in a striking manner how many doubtful dangers, or vigias, have gotten on the charts and how after laborious search many of them have now been removed. This condition was especially true of the Pacific, owing to the numerous reports of an indefinite nature from whaling ships, among whose captains there was a saying "that they do not care where their ship is, so long as there are plenty of whales in sight."

PREPARATION OF INFORMATION FOR CHARTS.

#Chart schemes.# Before commencing the preparation of a chart it is necessary to arrange a definite scheme for it, and the usefulness of the chart will depend materially on this preliminary plan, in which must be outlined its scale, size, limits, and features to be represented. New charts have sometimes been prepared simply to fit the surveys as they progressed or to fill immediate or local requirements. It is, however, desirable that general plans for series or groups of charts be made, and with changing needs, information, and conditions it is sometimes necessary that existing schemes be modified.

#Compilation of information.# Considerable work must usually be done to get the field records in shape for the published chart. The soundings must be plotted and the characteristic depths selected. Only a part of the soundings that are made can be shown on the original sheet and only a small part of these are used on the final chart. A selection is made showing the least soundings on shoals and bars, the channel depths, and the characteristic soundings in anchorages and other areas. The original surveys are generally made on a considerably larger scale than that on which the chart is published, in order that the soundings may be more thoroughly plotted. The sheets must then be reduced to the scale of publication, and this can conveniently be done by means of photography or with a pantograph.

The best judgment is required in selecting the important features to be shown on the chart and omitting the less important and not essential features which might tend to obscure the others. In charts of new regions where complete surveys are lacking, care must be exercised in weighing, combining, and adjusting information from various sources and which is, perhaps, more or less conflicting.

#Projections.# 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.# This is a rectangular projection in which the meridians are straight lines spaced at equal intervals and the parallels are straight lines so spaced as to satisfy the condition that a rhumb line, or line on the earth cutting successive meridians at the same angle, shall appear on the developed projection as a straight line preserving the same angle with respect to the meridians.

This projection may be considered as the unrolling upon a plane of the surface of a cylinder tangent to the earth along the equator, and upon which the various features of the earth's surface have been projected in such manner as to satisfy the above requirement.

On this projection there is a constant distance between the meridians, whereas on the earth they actually converge toward the poles. The distance between the parallels increases in passing toward the poles, approximately in the proportion of the secant of the latitude. For each small portion of the map the relative proportions are maintained as on the earth.

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; great circles and consequently lines of sight are curved lines excepting the meridians and the equator; rhumb lines or lines having a constant angle with the meridians are straight, and for the same angle are parallel in all parts of the chart. These qualities are all rigid and the projection can therefore be used for all areas, small or large, up to the extent of the earth's surface, except that it cannot be extended to the poles, as there the length of the minute of latitude would become infinite.

An interesting fact regarding a rhumb line oblique to the meridians is that it is a spiral continually approaching but never reaching the pole; this spiral makes an infinite number of revolutions around the pole, and yet it has a finite length for the reason that the length of each revolution diminishes as the number of revolutions increases.

The mercator projection has been extensively used for nautical charts, for which it presents important mechanical advantages, in that adjacent charts can be joined on all their edges while still oriented with the meridian; all charts are similar; the border may be conveniently subdivided, giving a longitude scale applicable to any part of the chart, but a latitude scale that may be used in the same latitude only; courses are laid down as straight lines and can be transferred with parallel rulers from one part of the chart to another without error. 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. Although distance scales vary with the latitude, distances can be taken from this chart with fair correctness by the use of the latitude border scale for the middle latitude, subdividing the total distance if there is much range of latitude. The inability to take off the great circle or shortest course directly from the mercator chart is from a navigational point of view a defect, but the most convenient solution for this appears to be the supplementary use of a gnomonic chart as will be described. The fact that lines of sight are not straight lines on this projection is another defect, as by the plotting of bearings and angles on approaching the land the positions of vessels are located on the chart; fortunately, however, the error due to this cause usually falls within the other uncertainties involved in locating a ship; if need be it would be practicable to allow for this curvature. In the polar regions, however, the faults of the mercator projection become so much exaggerated that it is not used for navigational purposes, but because of the absence of commercial navigation there this is a minor matter in the general question of chart projection. For the plotting of original surveys the mercator projection is not suited and is not used, for the reasons above mentioned.

Tables of "meridional parts" are published which give the distance in terms of minutes of longitude from the equator to the various parallels; with these tables a mercator projection may readily be constructed.

Airy proposed a graphical method of sweeping the arc of a great circle on to a mercator chart, and tables are published for this purpose. The method is only approximate and is limited in application, and the supplementary use of a gnomonic chart would appear to be preferable.

#Polyconic projection.# In plotting the original surveys it is essential that a projection be used which will for the area included on a survey sheet show the points in their correct relation both as to direction and distance. These conditions are substantially fulfilled by several projections, of which the polyconic is used in the United States. If a hollow cone were placed so that it would either be tangent to the earth's surface along one of the parallels of latitude or cut it along two parallels, and the points projected on to this cone, and the cone then unrolled and laid out flat, the result would be a conical projection, of which there are several variations. If successive tangent cones be used and each parallel of latitude be developed as the circumference of the base of a right cone tangent to the spheroid along that parallel, the result is the polyconic projection, which has been used for field sheets and for the large scale charts, as well as for the topographic maps of the United States. This projection has valuable qualities for moderate areas of the earth's surface, within which the scale is approximately uniform, areas retain nearly their true proportions, and great circles and consequently all bearings and directions are approximately straight lines. The parallels of latitude are arcs of circles with radiuses increasing as we recede from the pole; therefore they are not truly parallel and the length of the degree of latitude increases either side from the central meridian. The meridians converge toward the poles and become slightly curved as we recede from the central one; the longitude scale is everywhere correct, but the latitude scale is strictly correct only on the central meridian. The angles of intersection of parallels and meridians are right angles or nearly so. The polyconic projection is not used for very extensive areas of the earth's surface, as for instance a hemisphere.

#Gnomonic projection.# In this projection the eye is assumed to be at the center of the earth and the features are projected upon a plane tangent to some point on the earth's surface. It is practicable to use this projection for oceanic areas, and it has the very important quality that every straight line on it represents a great circle of the earth. To obtain the great circle or shortest course between two points it is therefore only necessary to draw a straight line between the points on a gnomonic chart. Because of the great distortion near the edges this projection is not otherwise adapted to navigational use, and it is employed only to mark out the general course, and sufficient points are then transferred to a mercator chart. 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. A modified or composite course can readily be laid out on a gnomonic chart.

#Arbitrary projection.# The few charts published of the polar regions are sometimes on an arbitrary projection, in which the meridians are straight lines radiating from the pole and the parallels are equidistant circles with the pole as center. The latitude scale is uniform. At some distance from the pole the longitude scale becomes very much distorted, but the projection is a practicable and convenient one for the immediate polar regions. Gnomonic and conical projections are also used for the polar charts, differing little from the foregoing for moderate areas.

#Scales.# Charts are published on a variety of scales to suit different needs of navigation, and the usual classification depends on scale. In addition to the ocean charts covering a single ocean in either one or several sheets and intended for navigation on the high seas, there are for our Atlantic coast the following series:

Sailing charts, scale about 1/1200000, for general coastwise navigation.

General coast charts, scale 1/400000, for local coastwise navigation.

Coast charts, scale 1/80000, for approaching the coast at any point and for inside passages.

Harbor and channel charts, of various large scales from 1/5000 to 1/60000, for entering harbors and rivers and passing through channels.

The expression of scales by miles to the inch or inches to the mile is the more familiar. The expression of scale in the manner used by the Coast Survey and by most of the European countries, by standard fractions as 1/80000, meaning that any distance on the chart is 1/80000 of the actual distance on the earth, has some advantages. For instance, the relation of these fractions gives at a glance the relation of the scales of the charts. Thus a 1/80000 chart is on a scale five times as large as a 1/400000 chart.

For the more important harbors charts have been published on several different scales to meet various needs. Thus New York Harbor is shown on charts of scales of 1/10000, 1/40000, 1/80000, 1/200000, 1/400000 and 1/1200000, each of course including a different area.

The selection of suitable publication scales is of prime importance; a large scale permits of greater clearness and of showing more detail, but on the other hand restricts the area and the points that can be shown on a single sheet, or else makes a chart of excessive dimensions. In general in chart preparation the scale should be restricted to the minimum that can be used to fulfill the particular object and clearly represent what is desired. A chart of very large scale is not convenient for plotting, and a moving vessel may pass quickly beyond it or into range of objects beyond the limits of the chart.

PUBLICATION OF CHARTS.

#Methods of publication.# An ideal process of publication for nautical charts would include the following features; rapidity in getting out new charts, facility in reprinting and correcting existing charts, clearness and sharpness of print, durability of paper and print, and correctness of scale. It is difficult to fulfill all these requirements by any method as yet developed. In the Coast and Geodetic Survey several different processes are in use at present; charts are engraved on copper and printed directly from the copper plate, or they are transferred from the copper plate to stone and printed from the stone, or a finished drawing is made and transferred to stone by photolithography and printed from the stone, or an etching is made on copper from a finished drawing and printed from a transfer to stone. Charts in other countries are in large part printed from engraved plates, excepting some preliminary charts by lithography.

#Copper plate engraving and printing# have long been used in chart preparation. A drawing is prepared as a guide for the engraver; this must be correct as to all information to be shown but need not be a finished drawing. A true projection is ruled upon a copper plate. By photography a matrix is made from the drawing and a wax impression taken from this matrix. This is then laid down on the copper to fit the projection, and the impression is chemically fixed on to the copper. The work thus marked out is engraved by hand or by machine. A high degree of skill is required in the accuracy and finish necessary for chart engraving. Machines have been invented in recent years which can be used for portions of the work on copper plates, as for instance for cutting the sounding figures, the bottom characteristics, the border and projection lines, border divisions, compasses, line ruling, and stipple ruling. Stamps and dies have been successfully used for some symbols and notes, and roulettes for shading. By means of these various machines, many of which are American inventions, the process of chart publication from plates has been materially facilitated.

When the plate is completed an alto, or raised copy, is made by depositing copper on to it in an electrotype vat, and from this alto another basso or sunken copy is made by the same process. This latter basso is used in printing. A copper plate may be used for about 3000 impressions, after which it may become too much worn for satisfactory chart printing. By printing from a duplicate basso the original plate is preserved and additional copies can be made when needed. The use of the alto also greatly facilitates matters when a considerable correction to the chart is required. All the portions of the chart to be changed can be scraped off the alto, and when a new basso is electrotyped from this scraped alto all such areas will of course appear as smooth copper, on which the new work can be engraved. Numerous small corrections are called for on charts, and on copper plates where these are to replace old work the latter is removed either by hammering up the back of the plate or by scraping its face.

Printing directly from plates is a laborious process. After the press bed has been carefully padded to take up inequalities in the plate, the surface of the latter is covered with ink and then carefully wiped off by hand, leaving the ink only in the engraved lines. The paper, first dampened, is laid on the plate, and passes with it beneath the cylinder of the press under considerable pressure. The prints are calendered by being placed in a hydraulic press under 600 tons pressure. The charts are beautifully clear and sharp, not equalled by other methods of printing. Owing to the wetting and drying of the paper, the finished print is, however, quite appreciably smaller in scale than the plate, and the shrinkage is greater in one direction than in the other. The average day's work for one press and two men is 75 prints. This is small compared with the output practicable with lithographic presses. On the other hand a plate can be prepared for printing more readily than a lithographic stone. For small editions the plate printing compares well in economy with lithographic printing, and the plate can also be printed on short notice. Because of changes in aids to navigation and other corrections, it is usually desirable to print at one time only a sufficient number of copies of a chart to meet current demands, and not to carry a large stock on hand.

The copper plates, bassos, and altos make a very convenient and enduring means of preserving the chart ready for printing or for further correction. A large number of plates can be placed in a small space, and if properly cared for they may be stored indefinitely without deterioration.

With plate printing it is not practicable to print more than one impression on the chart or to use more than one color, and plate-printed charts are therefore in black only.

#Engraving on stone.# On the United States Lake Survey the charts are first engraved on stone, and by a special process the work is then transferred to small copper plates, which are preserved. The final publication is by lithography, transferring again from the plates to stone.

#Photolithography# is a quick method of publishing a chart. It would be practicable by this means to reproduce the original survey sheets, but ordinarily these are not suitable as to scale and legibility, and it is necessary to make a new drawing, usually on tracing vellum. This is photographed on to glass plates, on the scale of the proposed chart. From these glass negatives positive prints are made on sensitized lithographic paper. These prints are fitted together and then inked, taking the ink only where the lines appear. This transfer print is then laid face down on the lithographic stone and run through a press under pressure, the stone absorbing the ink from the paper. The stone is then treated so that the inked portion remains slightly raised, and from this stone an indefinite number of charts can be printed in a lithographic press at the rate of 1000 an hour. The paper is not moistened, and consequently there is little distortion or change of scale in prints from stone. If desired to shade the land or use another color for any other purpose, additional impressions can be made on the same charts from other stones. Because of the bulk of the stones, work cannot ordinarily be retained on them, but the chart is cleaned off and the stones repeatedly used until worn thin. The original drawing as well as the negatives is preserved, from which the chart can again be published. For republication, the process is, however, not entirely satisfactory; the negatives are not always permanent, the work must again be assembled and transferred to the stone, changes or corrections are not very conveniently made on either drawing or negative, and after repeated changes the drawing becomes difficult to use in photolithography. Whether the charts are actually printed from copper or stone, there are decided advantages therefore in the matter of correction work and future editions in having the charts engraved on copper. On the other hand, the advantages of the photolithographic process are the ability to publish new drawings promptly, to use more than one shade on a chart, to obtain prints with little change of scale or distortion, and to print large editions rapidly.

#Lithographic printing by transfer from engraved plates.# An impression on transfer paper may be taken from an engraved plate and this laid down on the stone in a manner similar to that used in laying down the prints from the glass negatives in photolithography. Prints are then made from the stone the same as in photolithography, but with superior results as to clearness. This general process is extensively used in both map and chart publishing in this country, as it combines the advantages of the plate in preservation of the chart record and facility of correction, and the advantages of the lithographic printing in less distortion of the printed chart, ability to print more than one shade, and facility for large editions. As the transfer from the plate can be readily made it is also better applicable to small editions than is photolithography. It is, however, not as convenient in the latter respect as plate printing, and it does not give a resulting impression equal in clearness or durability to the impression directly from the plate.

#Etching on copper# for chart publication has been recently developed in the Coast and Geodetic Survey. A finished tracing is made, the surface of a smooth copper plate is sensitized, and by exposure to the sun a print is made on the sensitized surface. It is essential to use an air-exhausted printing frame so as to get good contact between the vellum and the plate. The work is then etched into the copper and the plate cleaned and touched up, after which it may be used the same as a hand-engraved plate, either for transfer to stone or direct plate printing. The expense and time required in the etching process are much less than for hand engraving. The process has been successfully used for a number of harbor charts. The etching of course will be of the same scale as the vellum at the time of the print, and vellum varies somewhat in scale with weather conditions and age. Unless overcome by the substitution of some more invariable material in place of vellum, this might be an obstacle to the use of the process for general charts where a true scale on the copper plate is desirable because of future work to be done on the plate. It must also be taken into account that the etching requires a finished tracing in ink, which is not essential for the hand engraver; if, however, the chart is first published by photolithography, as is the usual practice in the Coast and Geodetic Survey, the same tracing is used for both processes.

#Distribution of charts.# Charts published by the government are sold to the public at a small price, estimated to cover the cost of paper and printing. The charts may be obtained direct from the publishing office or from the chart agents who are to be found in all the principal seaports. Catalogues are published from time to time giving complete lists of the current charts and the main facts regarding them. Index maps show graphically the area covered by each chart. The notices to mariners contain announcement of new charts or new editions published and of charts or editions cancelled, as well as of all corrections.

CORRECTION OF CHARTS.

#Need for revision.# The making of the survey and the printing of the chart do not complete the problem of the chart maker. Both nature and man are constantly changing the facts the representation of which has been attempted on the charts, and also the needs of man are always varying. The original surveys are made to meet the reasonable requirements of the time, but breakwaters and jetties are built, and channels and harbors dredged and otherwise improved, and cities built, and new paths of commerce are opened which bring vessels into waters previously thought of minor importance.

With the increase of commerce and speed of vessels more direct routes are demanded for reasons of economy. Inside routes not originally used are sometimes developed for defensive reasons. The average draft of the larger vessels has also increased remarkably since the modern hydrographic surveys were commenced, and surveys once made to insure safety for the deepest vessels of that time are now not adequate. The average loaded draft of the 20 largest steamships of the world has increased as follows: 1848, 19 feet; 1873, 24 feet; 1898, 29 feet; 1903, 32 feet. The average length of these vessels was 230 feet in 1848, 390 feet in 1873, 541 feet in 1898, and 640 feet in 1903. The number of vessels drawing as much as 26-1/4 feet rose from 36 in 1902 to 185 in 1904. In 1906 there were 17 vessels afloat, drawing 32 feet and upwards. There are now two steamers on the Atlantic 790 feet long, 88 feet beam, and 37-1/2 feet draft when fully loaded, and larger vessels are already planned.

Great natural agencies are also constantly at work effecting changes in features shown on the charts. The action of currents and waves is continually cutting away or building the shore, particularly on sandy coasts exposed to storms. When surveyed in 1849 Fishing Point on the east coast of Maryland was but a bend in the shore line. By 1887 it had built out about two miles in a southerly direction, and in 1902 about two-thirds of a mile further, curving to the westward. Altogether in about half a century this tongue of land has grown out nearly three miles.

Rivers are bearing vast quantities of sediment and depositing these near their mouths, pushing out the coast line and filling in the bottom. The main mouths of the Mississippi are advancing into the Gulf, but at a comparatively slow rate. A break from the main river at Cubit's Gap just above the head of the passes, however, has done an enormous amount of land making, filling in an area of about 50 square miles between 1852 and 1905.

The mouth of the Columbia River in Oregon shows an interesting example of the movement of an island. The chart of 1851 shows the center of Sand Island 3-1/4 miles southeast of Cape Disappointment, the chart of 1870 shows it 2-3/4 miles southeast, and the chart of 1905 shows it 1-1/4 miles easterly. This island has thus moved 2 miles northwesterly directly across the middle of the river entrance, closing up the former north channel. The southern point of the entrance, Clatsop Spit, has built out about the same distance.

_Photo by U. S. R. C. Service._

FIG. 37. BOGOSLOF VOLCANO, BERING SEA.]

Volcanic action in well authenticated cases has caused islands to rise or disappear. In the present location of Bogoslof Island in Bering Sea the early voyagers described a "sail rock." In this position in 1796 there arose a high island. In 1883 another island appeared near it. In 1906 a high cone arose between the two, and a continuous island was formed over 1-1/2 miles long and 500 feet high. The latest report (September, 1907) was that this central peak had suddenly collapsed and disappeared. Bogoslof is an active volcano, and the main changes have been the result of violent volcanic action. The history of this island for over a century past forms a remarkable record of violent transformations in the sea.

Earthquakes sometimes cause sudden displacements, horizontal or vertical, of sufficient amount to affect the information shown on the charts. A careful investigation of the effects of the earthquake in Yakutat Bay, Alaska, in September, 1899, showed that the shore was raised in some parts with a maximum uplift of 47 feet and depressed in other parts, and that at least two reefs and four islets were raised in the water area where none appeared before. Undoubtedly there were changes in the water depths, but definite information is lacking because there had been no previous hydrographic survey. The San Francisco earthquake of 1906 caused little vertical displacement, but there were horizontal changes of relative position as much as 16 feet; so far as known this earthquake did not affect the practical accuracy of the charts. Related to earthquake phenomena are the gradual coast movements of elevation or subsidence which are taking place but at so slow a rate as not to sensibly affect the charts in ordinary intervals of time.

Another agency at work is the coral polyp on the coral reefs; although the rate of growth appears to be very slow, the resulting reefs and keys are an important feature in tropical seas.

Practically all of the land features shown on charts are likewise subject to changes, the more rapid of which are mainly due to the works of man.

The changes of channels and of commercial needs cause many alterations to be made from time to time in the lights and buoys which are shown on the charts.

#Methods of correction.# The problem of keeping a chart sufficiently up to date is one of much practical importance and one which must be taken into account in planning what should be shown on the chart in the first place so as to bring it within the range of practicable revision.

Certain features are corrected at once on the charts as soon as the information is received, such as dangers reported, and changes in lights and buoys. Where harbor works are in progress the periodic surveys made in this country by the Corps of Engineers furnish data which are applied promptly to the charts. Reported dangers in channels and bars are investigated by special surveys and the information is put on the charts. Examinations are made from time to time for the revision of the features along the coast line. Complete resurveys have been made, at long intervals, of some important portions of the coast where there has been evidence of change, and these, when they become available, are applied to the charts. All parts of the coast where the exposed portions are not of very permanent material will require resurveys at intervals, depending on their importance and the rate of change.

Notwithstanding the great progress made in hydrographic surveys, a considerable number of rocks and shoals dangerous to navigation and not previously shown on the charts are reported, averaging nearly 400 each year for the last six years, according to the British reports. Of the 367 reported in 1906, 11 were discovered by vessels striking them.

Immediate information in the form of _Notices to Mariners_ is published, of the more important corrections to charts which can be made by hand. These corrections show what charts are affected, and give sufficient data for plotting.

In the case of extensive corrections or new surveys a new edition of the chart is printed and all existing copies of the previous edition are canceled.

It is important that the user of the chart shall make certain that he has the latest edition and that all corrections from its date of issue have been applied from the _Notices to Mariners_.

It is unfortunately true that owing to failure to take proper account of the notices, or to economy, old editions or unconnected charts are sometimes used, and in a number of cases the loss of vessels has been directly due to this cause. Those responsible for the safe navigation of vessels should insist that the latest editions of charts are provided and that all charts to be used are inspected and corrected to date.

READING AND USING CHARTS.

#Reading charts.# A chart is a representation on paper of hydrographic and topographic information by means of various conventional methods and symbols. It is evidently important for those making use of charts to understand the system and conventions used, and to be able to interpret readily the various parts of the chart. The ability to read a chart must include an understanding of all its features, such as scale, projection, geographic position, directions, depths, plane of reference, aids to navigation, tides, currents, elevations, topography, and date of survey and publication.

#Scale.# For American and British charts the scale is usually expressed by the inches or fractions of an inch to the minute or degree of latitude, or by the fractional proportion of a distance on the map to the corresponding distance on the earth. These fractions are sometimes stated on the British charts, and nearly always on those of the United States Coast Survey. The chart catalogues give the scale in one or the other form. A familiarity with the meaning of scales is of value in selecting the most suitable chart, in judging of the relative uses of charts, and in estimating distances. Where the fractional scales are stated they furnish a simple means of comparing charts, as, for instance, a chart on 1/50000 scale will show all distances just twice as long as a chart on 1/100000 scale.

The following are scale equivalents:

Scale 1/10000 is equivalent to 7.30 inches to one nautical mile.
Scale 1/20000 is equivalent to 3.65 inches to one nautical mile.
Scale 1/40000 is equivalent to 1.82 inches to one nautical mile.
Scale 1/50000 is equivalent to 1.46 inches to one nautical mile.
Scale 1/80000 is equivalent to 0.91 inch to one nautical mile.
Scale 1/100000 is equivalent to 0.73 inch to one nautical mile.
Scale 1/200000 is equivalent to 0.36 inch to one nautical mile.
Scale 1/400000 is equivalent to 0.18 inch to one nautical mile.
Scale 1/1000000 is equivalent to 0.07 inch to one nautical mile.
Scale 1/1200000 is equivalent to 0.06 inch to one nautical mile.

For use in measuring distances on large scale charts the length of one or more nautical miles is usually drawn on the chart, and sometimes scales are also given in other units. 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; this definition makes the nautical mile equal 6080.27 feet. Lecky adopts 6080 feet as the nautical mile. The length of the actual minute of latitude on the earth's surface increases from 6046 feet at the equator to 6108 feet at the poles, an increase of about one per cent. It is, however, this somewhat variable unit of length which is ordinarily used in scaling distances on the sailing charts.

On small scale charts there is usually a border scale entirely around the chart, conveniently subdivided; this serves the double purpose of facilitating the plotting or reading of positions by latitude and longitude and of furnishing a scale of minutes of latitude for use in measuring distances. On a mercator chart this scale of course varies with the latitude and it must be referred to in the mean latitude of the distance to be measured. In general practice the minute of latitude is taken as equal to the nautical mile.

#Projection.# On only a few charts is there a statement of the projection used. Practically all general sailing charts are on the mercator projection, which can be readily recognized by the rectangular network of meridians and parallels and the increase with latitude of the distance between the parallels. On large scale local and harbor charts the kind of projection used is not of importance to navigation, as for such limited areas the difference between projections would not affect the use of the chart. On certain small scale charts of the United States Coast Survey which are on the polyconic projection this fact is stated on the chart, and can also be readily recognized by the convergence of the meridians and curvature of the parallels. Gnomonic charts intended for taking off great circle courses are always described in their titles and are also easily recognized by the increased scale and distortion toward all the borders. Charts of the polar regions are published on several different projections, which are distinguished from the mercator by their circular or curved parallels.

#Geographic position.# For large scale and harbor charts the latitude and longitude of some point marked on the chart are sometimes stated on the face of the chart. For others of these, however, and for smaller scale and general charts, positions are obtained by reference to the border scale. There is a latitude scale down either side of the chart, and a longitude scale across the top and bottom. These scales are conveniently subdivided into degrees, minutes, or fractions of a minute. The minute is divided into tenths (6´´), sixths (10´´), quarters (15´´), or halves (30´´) on various charts.

#Directions# are indicated on charts both by the projection lines and by compass roses. 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. Formerly many charts were not so oriented. Some of these are still in use and can readily be recognized by the diagonal or inclined direction of the projection lines with respect to the border of the chart. Of course directions must not be referred to the border lines of these diagonal charts, and scales along such border lines must not be used. Directions with respect to true north may always be referred to the projection lines of the chart, but on a polyconic or polar chart a direction must not be carried so far from any projection line as to introduce error on account of convergence of the meridians. Compass roses are placed on charts to facilitate the taking off or laying down of directions, though in some respects their use is less accurate and convenient than the use of protractors, referring to the projection lines. The British charts and many of those of the United States Coast Survey have only magnetic compasses, with degrees outside and points inside, the former graduated to 90°. These are engraved on the chart with the magnetic variation for the date of publication, or for a few years in advance, and give the annual change in the variation. Because of expense of engraving they can be changed on the charts only at intervals of some years, and until this is done allowance for the change in variation is to be made if important. The German charts and those of the United States Hydrographic Office now have a threefold compass, the outer one degrees true, the middle degrees magnetic and the inner points magnetic; the degrees in both cases are graduated to 360°, reading from north through east, south, and west; thus northwest would be stated as 315° instead of N. 45° W. Small scale charts covering extensive areas have no magnetic compasses. They sometimes have true compasses, and usually have the isogonic lines, or lines of equal magnetic variation, marked on them, from which the variation at any intermediate point can be estimated.

#Depths.# The unit used for depths is always stated plainly on the chart, and it is important to note this carefully, as the British, American, and Japanese charts use fathoms for some charts and feet for others, and most other countries use meters. Some of the earlier charts of the United States coast have the depths inside of the 18-foot curve in feet and outside of that curve in fathoms.

Depth curves are shown on charts in order to bring clearly to the eye the different depth areas and the limits for navigation of vessels of various drafts. The shoaler areas are usually indicated by sanding the outer limit or the entire area within the depth curve. For the curves of greater depths various standard symbols are used which vary slightly in the different series but which may readily be recognized by the soundings on either side of them. On the British charts the 1 and 3 fathom curves are usually indicated by sanding the outer edge of the areas of these depths respectively; beyond these the standard curves shown on these charts are the 5, 10, 20, and 100 fathom curves. Similar curves are used on the United States charts. The German charts show the 2, 4, 6, 10, and 20 meter and various deeper curves, and the French the 2, 5, 10, and 20 meter and deeper curves. On the United States Lake Survey charts the areas included within the 6, 12, and 18 foot curves are shaded with a blue tint, heavy along the outer edge, which brings out strongly the shoal areas.

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. When a sounding is made without the lead reaching bottom, the depth obtained is sometimes 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. The rock awash symbol indicates a rock awash at some stage of the tide, unless more definitely stated. The position of a wreck is indicated by a special symbol. P. D. (position doubtful) and E. D. (existence doubtful) are placed after soundings or rocks or other features which depend on some doubtful report not yet verified.

The following are the relations between depth units found on various charts:

1 meter = 3.281 English feet = 0.547 English fathoms.
1 sajene (Russian) = 7 English feet = 1.167 English fathoms.
1 braza (old Spanish) = 5.484 English feet = 0.914 English fathom.
1.829 meters = 6 English feet = 1.000 English fathom.

#Aids to navigation.# Each series of charts has a definite system of representing the aids to navigation; these are similar in principle but differ as to detail. The characteristics of the lights, light-vessels, buoys, and beacons are usually explained by abbreviations placed by the side of each, and the entire system of representation is given on the explanatory sheet for the charts. Various methods of coloring lights and sectors and buoys are in use on different charts. It is evidently of importance that the user of the chart should readily understand the significance of the navigational aids as shown. For details regarding lights it is of course desirable to refer to the light lists; for the coasts of the United States detailed buoy lists are also published. Range and channel lines when shown are represented by distinctive symbols with bearings indicated. Danger ranges for the avoidance of shoals are sometimes shown. On the British charts bearings as stated on range and channel lines are magnetic; the custom varies on other charts and must be carefully noted in each case.