COLLECTION OF INFORMATION FOR CHARTS

FIG. 5. STATE OF ADVANCEMENT OF HYDROGRAPHIC SURVEYS OF THE COASTS OF THE WORLD, 1904.By G. W. Littlehales.

Shoals and dangers are shown either by the least depth or by rock or reef symbols. The characteristic soundings are shown on the chart, with abbreviations indicating the nature of the bottom. Depth curves are drawn, joining together points of like depth, and inclosing areas of less depth, on the same principle that contours are used on land maps; usually also the shoaler spots are made more prominent by sanding or tinting the area within them. Lighthouses, buoys, and other artificial aids to navigation are represented, with descriptive abbreviations. The coast is shown by a bold solid line for high water and a dotted linefor low water. 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. 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. On the larger scale charts only, vegetation features, particularly areas covered by trees, are represented by symbols. The land area is usually clearly distinguished from the water area by a tint or stipple. Latitude and longitude are given by the projection lines and the subdivided border, or sometimes on harbor plans by a note giving the position of some one point. Brief information as to the time and range of the tides is stated in a note. Data regarding currents, whether due to tidal or other causes, are given by current arrows placed on the chart, or by explanatory notes. Compasses are for convenience printed on the charts, and data given as to the magnetic variation and its rate of change. On large scale charts scales are provided for use in measuring distances. Ranges and channel lines are given when required. The ports are indicated where storm warning signals are displayed. The areas of forbidden anchorages are shown, and when important, the positions of submarine cables. The lines dividing the high seas from inland waters are sometimes stated on United States charts. Life saving stations are given, and time balls are usually noted. Views of important features are shown on some charts.

FIG. 6. SYMBOLS USED ON CHARTS OF THE UNITED STATES COAST AND GEODETIC SURVEY.

The layman who looks at the printed chart probably does not appreciate the amount or the variety of information that must be gathered and sifted and put in proper shape for a single chart.

Need of thorough surveys.As has been stated, a good chart requires that a thorough and correct survey be first made of the region to be charted. It is said that men are very apt to accept as true anything they see on a map. As to the nautical chart the mariner is likely to be somewhat more critical, however, and it is well that he is. The difficulty of charting an invisible surface such as the bottom of the sea is great, and the proportion of the navigable waters surveyed in sufficient detail to be at all certain of the absence of uncharted dangers is small.

The planning of surveys in a new region, such, for instance, as the Philippine Islands, presents many interesting problems, on the solution of which the effectiveness in chart results and the cost of the work materially depend. Many local conditions must be taken into account. The surveys are made on opposite coasts according to the seasonal winds and rainfall. In some parts fair-sized steamers are necessary; in others launches and small boats can do the work more economically. Shore parties with land transportation are used for portions of the work where the country permits. Natives are employed as far as practicable for the classes of work they can do; the Filipinos, for instance, make good sailors on the vessels and excellent penmen in the office.

The following is a brief outline of the steps of a complete survey for charting purposes, according to the present practice of the United States Coast and Geodetic Survey. These are given in their logical order, though in actual work this order must often be departed from. In this Survey the methods of control have been of a high standard; that is, the main stations have been accurately determined and permanently marked and described, and this has proven an advantage in the joining together of the original surveys and resurveys.

Astronomical observations.To locate on the surface of the earth the area to be charted, astronomical observations are required for the latitude and longitude of one or more points. In the best practice the longitude of a point is obtained by observing the transits of stars to get the local time, and sending time signals by telegraph to obtain the difference from the local time of some other place whose longitude is known. The latitude is observed by measuring the difference of zenith distance of pairs of stars crossing the meridian north and south of the zenith. The azimuth or true direction of some line is also obtained from star observations, usually by observations with a theodolite on a circumpolar star. Much existing chart work depends on positions determined by less accurate methods, as, for instance, longitudes obtained by transporting chronometers between the known station and that to be determined, or by observations of moon culminations, and latitudes obtained by direct observations of the altitudes of stars with theodolite or sextant.

FIG. 7. SYMBOLS USED ON CHARTS OF THE BRITISH HYDROGRAPHIC OFFICE.

FIG. 8. TRIANGULATION OF A BAY, SHOWING LOCATION OF SURVEY SIGNALS AND LANDMARKS.

Triangulation.The main framework of the survey consists of a series of triangles connecting prominently located points which are permanently marked in theground and the location described so that they can be found at a future time. At long intervals in the survey base lines are laid out and carefully measured with steel tape. Signals are erected over the points, including those at the ends of the base line, and angles are then measured at the various stations. 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 distance between two points as measured by triangulation thus differs from the distance computed from the astronomically determined positions. If this irregularity were not taken care of by adopting mean positions, the discrepancy in joining up different surveys would in extreme cases amount to about half a mile.

FIG. 9. TRIANGULATION STATION AND SIGNAL, ON ALASKA COAST.

FIG. 10. MEASURING ANGLES WITH THEODOLITE AT TRIANGULATION STATION ON ALASKA COAST.

Survey sheetsare next prepared, of suitable size and scale. On each sheet a projection is laid down, that is, the meridians and parallels are drawn, and all the points determined in the triangulation are plotted in their true relation. Usually separate sheets are prepared for the topography or shore survey and for the hydrography or survey of the water area.

Topography.The topographic survey of the shore and as much of the adjacent area as is required isusually made with a plane table, on which the map is actually drawn in the field as the work progresses. Points are located on the plane table sheet either by direct reading of the distance on a stadia rod or by intersections from two or more stations. On the plane table sheet it is customary to locate the shore or high-water line, the low-water line, off-lying rocks, streams, rivers, roads, towns, lighthouses, and all prominent features near the coast. Elevations are measured with the plane table or obtained from the triangulation, and are represented on the sheet both by figures and by contours, which are lines joining together points of the same elevation. For instance, a 100-foot contour represents the line where a plane 100 feet above sea level would cut the surface of the ground. It is particularly important in this topographic work to locate accurately objects which are good landmarks and likely to be of use to the mariner. In some regions auxiliary methods are used in filling in the topography, as, for instance, along a difficult coast each feature of importance may be located by sextant angles, or a traverse line may be run along the shore by the transit and stadia method.

FIG. 11. TOPOGRAPHIC SURVEY PARTY AT WORK WITH PLANE TABLE ON THE PRIBILOF ISLANDS.

FIG. 12. SURVEY SIGNAL OF IRON PIPE ON THE BAR OFF THE MOUTH OF THE YUKON RIVER.

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 sextantangles on survey signals or by angles from the shore. 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. The deepest sounding thus far made, 5269 fathoms, or nearly six miles, was obtained by this method in the Pacific Ocean near Guam.

FIG. 13. HYDROGRAPHIC PARTY SOUNDING WITH LAUNCH IN BALTIMORE HARBOR.

FIG. 14. THE LUCAS AUTOMATIC SOUNDING MACHINE FOR DEPTHS TO 5000 FATHOMS, WITH ENGINE.

FIG. 15. THE SIGSBEE SOUNDING MACHINE ON A SURVEYING VESSEL.

FIG. 16. LONGITUDINAL SECTION OF SURVEYING STEAMERFATHOMER, SHOWING GENERAL ARRANGEMENTS.Fig. 16 enlarged(58 kB)

FIG. 16. LONGITUDINAL SECTION OF SURVEYING STEAMERFATHOMER, SHOWING GENERAL ARRANGEMENTS.

Fig. 16 enlarged(58 kB)

The offshore soundings are made from a surveying steamer; the inshore work is usually done by a launch or small boat.

So far as the navigational use of charts is concerned it is important that the hydrography shall show the limiting depths and the freedom from dangers, of channels, entrances, harbors, and anchorages. It is also desirable that the soundings shall be carried off shore at least as far as the one-hundred-fathom curve, as with the modern forms of navigational sounding machines it is possible for vessels under way to obtain soundings to this depth, and such soundings may be of value in identifying the location of the vessel. For depths greater than one hundred fathoms the soundings have less direct value to navigation except as proving the absence of shoaler areas, but soundings throughout the oceanic regions are of great geographical interest as well as of direct practical value in the laying of cables.

It is obvious that the plan of mapping the sea bottom by dropping a lead at intervals over its hidden surface is far from an ideal one. The lead gives the depth only at the point at which it touches the bottom, and no information as to the space between the casts except such as may be inferred from the relation of successive soundings. In numerous cases, after what was considered a very thorough survey of a region had been made, at some later day a pinnacle rock or other danger has been discovered. For instance, a very detailed hydrographic survey of Buzzards Bay was made in 1895; the sounding lines were run at intervals of 50 to 100 yards, and 91,000 soundings were made for a single sheet. Within this area the cruiserBrooklynin 1902 touched a rock which was found to have 18 feet over it. (Fig. 17.) The least depth in the vicinity developed in the original survey was 31 feet.

For the satisfactory development of hydrographic work some invention is much needed which as it passes along the bottom will give a continuous depth curve. Several devices have successfully accomplished this in shoal water, but great credit awaits the inventor who designs something of more general application.

FIG. 17. PORTION OF ORIGINAL HYDROGRAPHIC SHEET, BUZZARDS BAY, ON SCALE 1-10000, SHOWING AREA CLOSELY SOUNDED IN 1895, WHERE THEBROOKLYNSTRUCK IN 1902.

Tides and currents.Information must be obtained as to the movement of the water, both vertical and horizontal. The rise and fall of the tide are obtained by tide gauges, either automatic, which draw a continuous tidal curve on a roll of paper, or simple tide staffs, which must be read at intervals. The currents, whether due to the tides or other movements, are measured by noting the movement of partially submerged floats. Less accurate but useful information as to currents is obtained from the logs of vessels.

FIG. 18. DRAGGING FOR DANGERS WITH A LONG WIRE.

Dragging for dangershas long been resorted to for the investigation of isolated spots. A valuable and successful means has been employed recently of making sure that an area is free from shoals or rocks having less than a certain depth. This is done by dragging through the water a wire from 500 to 1400 feet long, and suspended at the required depth, with suitable buoys and weights, and kept taut by the angle of pull. If, for instance, the wire is set at a depth of 30 feet it will indicate the presence of any obstruction of less depth by catching on it and upsetting the buoys, and such spots are at once marked and investigated. Considerable work has been done with such drags in the last few years on the Atlantic and Gulf coasts and on the Great Lakes. This is of course a somewhat tedious process and gives no information as to depths greater than that for which the wire is set, but the experience already had indicates its great value. It will probably be found desirable in time to thus drag all water areas important to navigation where the depth is near the draft of vessels and the irregular nature of the bottom gives indication of dangers. In extensive dragging operations near Key West and in Jericho Bay, Maine, a number of shoals have been picked up which were not found in the original surveys.

A remarkable instance of the value of the drag was the recent discovery of a rock in Blue Hill Bay on the coast of Maine. This rock has but 7 feet of water over it, and is only 6 feet in diameter at the top. It is surrounded by depths of 78 feet, from which it risesnearly perpendicularly. The original survey gave no indication of a danger here, and its existence was not suspected until it was discovered with the wire drag.

Another method of dragging that has been employed is by means of a pipe suspended beneath a ship's bottom.

Magnetic variation.As the compass is a universal navigational instrument, information as to the magnetic variation is needed for the charts. The angle between the direction of the magnetic needle and the true north is measured at various points on both land and sea, and at some stations these observations are repeated after a number of years. From these results magnetic maps are made, from which both the variation and its annual change may be taken.

Reports of dangers.Aside from the more systematic surveys as outlined above, much information has been placed on the charts from other sources. On the earlier charts and on those of more remote regions at the present day much of the work has been sketched rather than surveyed. Even in the better surveyed portions reports come in as to dangers or other matters not shown, and if of importance and the report appears to be reliable these are sometimes at once put on the chart pending further investigation, or in other cases an examination is first made.

Shoals, rocks, and even islands have in numerous instances been shown on the charts which no one has been able to find again, and many of them after repeated searches have been removed. The same island or danger has sometimes been charted in two or more different positions as reported at varioustimes. The treatment of such cases is one of the serious and interesting problems of the chart maker. It is generally less harmful to show a danger which does not exist than to omit one which does exist. On the other hand a non-existing danger shown on a chart may be the cause of actual expense and loss of time in compelling a vessel needlessly to go out of its course.

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 themisfortune 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 steamerCarmenwas 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 onthe charts in a position which made it an obstruction to navigation.

The shipMinervain 1834 was reported to have struck a rock near the middle of the broad entrance to Balayan Bay; the fact that this occurred at 2A.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 112miles 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 steamerPathfinderdefinitely 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 334to 434fathoms, and12mile 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, andthat in all probability the transport actually touched a small reef making out from the island.

The master of the brigHelenreported 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 barkWashingtonreported 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 longitude142° 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 barkEllenstruck 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 shipRiflemansearched 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 aboutforty 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."

FIG. 19. PORTION OF CHARTS OF 1869 AND 1903, OF THE PACIFIC OCEAN WEST OF THE HAWAIIAN ISLANDS, TO ILLUSTRATE THE REMOVAL OF DOUBTFUL DANGERS.

FIG. 20. PORTION OF CHART OF PONCE HARBOR, SCALE 1-20000, TO SHOW SELECTION OF SOUNDINGS FROM ORIGINAL SURVEY GIVEN BELOW.

FIG. 21. HYDROGRAPHIC SURVEY OF SAME PORTION OF PONCE HARBOR, REDUCED TO ONE-HALF SCALE OF ORIGINAL SHEET.

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 thatthe length of each revolution diminishes as the number of revolutions increases.

FIG. 22. MERCATOR PROJECTION OF NORTH PACIFIC OCEAN, SHOWING GREAT CIRCLE ROUTES YOKOHAMA TO PUGET SOUND, AND YOKOHAMA TO HONOLULU AND THENCE TO SAN FRANCISCO.

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 notstraight 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.

FIG. 23. POLYCONIC PROJECTION OF PORTION OF NORTH PACIFIC OCEAN.

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 alongone 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.

FIG. 24. GNOMONIC CHART OF NORTH PACIFIC OCEAN, SHOWING GREAT CIRCLE ROUTES YOKOHAMA TO PUGET SOUND, AND YOKOHAMA TO HONOLULU AND THENCE TO SAN FRANCISCO.

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 importantquality 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.

FIG. 25. NORTH POLAR CHART ON ARBITRARY PROJECTION.

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 usualclassification 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 about11200000, for general coastwise navigation.

General coast charts, scale1400000, for local coastwise navigation.

Coast charts, scale180000, for approaching the coast at any point and for inside passages.

Harbor and channel charts, of various large scales from15000to160000, 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 as180000, meaning that any distance on the chart is180000of 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 a180000chart is on a scale five times as large as a1400000chart.

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 of110000,140000,180000,1200000,1400000and11200000, each of course including a different area.

FIG. 26. NEW YORK HARBOR, PORTIONS OF CHARTS ON FOUR DIFFERENT SCALES.

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 shownon 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.

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.

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