Introduction.—The nautical chart plays an important r61e in the economic development of maritime nations. It raises the blockade laid down before their harbors by hidden rocks and reefs and reveals the channels to safety; d opens their ports to merchant fleets; and it guides their commerce in safe lanes in the intercourse between nations. The chart thus fulfills a mission vital to a nation’s economic freedom, and directly or indirectly influences the life and freedom of each and every individual of that nation.
Nautical charts may possibly now be considered as having passed through three epochs and as entering upon the fourth: the exploratory, the reconnaissance, the survey eras, and, for lack of a better term, the fourth may be classed as the modern era. During the exploratory period, charts consisted in general of crude depictions of the explorer’s conception of the general appearance of a region, with little attempt to attain accuracy of topographic depiction and with no delineation of the ocean’s bottom. The advent of the projection and of crude instruments ushered in the reconnaissance era in which surveys of a sort were made, coast lines and headlands were sketched in fair position relative to other features and some sounding was done. During the survey epoch, accurate projections have been laid down, the shore lines have been surveyed in detail, and the bottom of inland and coastal waters has been faithfully delineated. The efforts of hydrographers during this period have, however, been confined principally to areas within sight of land and any work beyond landmarks and signals has been more of a reconnaissance nature, due to the lack of a means for accurate location of the survey vessel out of sight of land. Modern science, through radio acoustic ranging, now offers such a means, and surveys of an accuracy comparable with inshore work are now being extended well offshore.
The first nautical charts of which we have a record were constructed by Marinus of Tyre during the second century, followed soon after by the charts of the Egyptian Ptolemy. Though naturally crude and inaccurate in the state of development of that period, they did show a knowledge of the British Isles, of East Africa, of India, and of the Malay coast. The charts of Marinus and Ptolemy served early mariners until the middle of the tenth century; then the construction of the Portolano, a new type of chart, was brought about through the development of commerce by the cities of Italy, by the Crusades, and by the adoption of the magnetic compass. A portolano constructed by Petrus Vesconte in 1311 is the oldest chart now extant of which the date can be definitely stated. A chart of Juan de la Cosa in 1500 is the earliest chart now extant showing the American coast, Juan de la Cosa sailed with Columbus on two voyages, and his chart shows the discoveries of those expeditions.
All these early charts, even to the early years of the nineteenth century, were crude depictions of new territory, principally contributions by individual pioneers, inspired by the spirit of the true explorer—the urge to raise the horizon. The modern chart, however, is an accurate engineering product, not dependent upon the individual skill of the explorer, but is the result of a general development of precise cartographic methods and is based on systematic surveys, the procedure of which has gradually evolved from the less precise practice of the art of navigation to the present precision based on the developments and principles of modern science.
Throughout history the exigencies brought about by national stress have left their imprint on the customs and the attainments of the immediate post-war periods. The last Great War has been no exception; the world has moved faster during the past ten years than during several decades prior to the war. During the period of stress, the immediate necessity to create, conceived by the inborn instinct of self-preservation and urged by the human trait of national pride, brings about intensive research work of the physical scientist and of the engineer in the devising of weapons of destruction and of defense; after the first lull following the war, these are made to serve purposes quite foreign to the original intent. So it has been with hydrographic surveying for the construction of the nautical chart. The past decade has seen a revolutionary advance in the technique employed in hydrographic surveying, with a corresponding increase in the volume and accuracy of this work. The improvements since the World War, both in methods and instruments, have undoubtedly surpassed those of any other period in the history of the art. Hydrographic surveying during this post-war period has assumed a scientific aspect of no small degree, in which the general principles involved have been based to a large extent on intensive research work during the war for the detection of hostile submarines.
The hydrographer of today has searched the laboratory of the physical scientist for his methods and has invaded the shop of the radio engineer for his instruments; on offshore hydrography he employed in the past a material means, piano wire, for obtaining measurement of ocean depths; today with his echo-sounding instruments for obtaining soundings indirectly, and with his radio-acoustic ranging apparatus for accurately locating these soundings when surveying out of sight of landmarks, he is applying the resources of modern science to the extension of the knowledge of the contours of the ocean bottom, both to meet the requirements of navigation and incidentally to further the studies of the geomorphologist. A higher order of accuracy than for navigation alone is now required; the scientist is voicing a strong and insistent demand for accurate submarine configuration in connection with his physiographic studies.
A brief comparison of the results of the old and of the new methods will indicate in general the degree of the advance of the art of hydrographic surveying for chart construction. A sounding in 20,000 feet of water with piano wire required about an hour; today this same sounding is made with the vessel at full speed by means of echo-sounding apparatus in a little over eight seconds. By the old methods of offshore hydrographic surveying, the position of the vessel was approximated by dead reckoning; today it is accurately determined by the explosion of a depth bomb composed of a small quantity of T.N.T. timed to sink about 100 feet before explosion. The chronograph on the ship receives electrically from its hydrophone, and graphically records, the impulse from the explosion. The subaqueous sound wave also travels at known velocity to hydrophones at two or more suitably located stations on the seashore where the vibrations thus set up cause an electrical impulse to travel through amplifiers to a thyratron in a temporary radio station on shore. The actuation of the thyratron causes the radio transmitter to send out automatically a signal at the exact instant of the arrival of the sound wave at the shore station; the signal from each shore station is received by the survey ship’s radio receiver and transmitted to the chronograph, where its receipt is graphically recorded on the same tape which recorded the impulse of the explosion. Since radio transmission may be considered instantaneous for these distances the elapsed times indicated on the tape are those required for the sound wave to travel via the water from the ship to each shore station. These time intervals can be measured from the chronograph tape within a hundredth of a second, the distances from the shore stations then computed, and the position of the ship thus readily determined. On one occasion the sound wave from the explosion carried through a distance of 206 miles; distances of 75-100 miles are not uncommon.
Progress of hydrography.—Hydrography though a most important factor, can be regarded as only one of the several field operations essential to chart construction. Though the most essential part of a chart is the water area, yet without adequate control of a high order hydrographic surveys over large areas cannot produce charts of the accuracy necessary to meet the requirements of present-day navigation, since an inaccurate chart is actually a menace rather than an aid to the speedy, deep-draft modern vessel; precise triangulation furnishes this control, or fundamental structure, for charting operations. It may be likened to the steel framework which binds together the other material entering into the construction of a large building. Other operations also enter into the field activities carried on for constructing a chart: tidal observations for the reduction of soundings to a common datum plane; magnetic observations for the determination of the magnetic elements affecting the mariner’s compass; and topographic surveying for delineating the shore line which is shown on the chart in true relation to the water areas.
The marine surveyor, therefore, is actually a combination of navigator and engineer, with a clear understanding of the requirements of the mariner and with engineering ability to carry on not only hydrographic work of a high order, but also all the related field operations necessary to the construction of the chart.
In hydrographic surveying two principal factors are involved: the determination of the depth of water, and the accurate location of the spot at which the depth is determined. Under the well- known methods formerly employed, the depths were in general obtained on inshore Work by means of a lead line, and their positions were determined by a graphic solution of the three-point problem; on offshore work the depth was determined in the earlier work by a hemp line, and later by piano wire and sinker; the position determination was based almost entirely on more or less refined methods of navigation, on celo-navigation, on dead reckoning, or on a combination of both. During the period since the World War radical departures from these methods have taken place, both in technique and in the instruments and means employed; the general principles involved are based to a large extent on intensive research work incident to the war, for purposes quite foreign to their present application.
By echo-sounding methods depths are determined indirectly by the measurement of the time interval required for a sound wave to travel from the survey vessel to the bottom of the ocean, and to return as an echo to the ship. The measurement of this time interval must, of course, be of a comparatively high order of precision ; it is made by means of a specially designed, time-measuring instrument, the construction of which is based on a precise knowledge of the velocity of the transmission of sound in sea water. This velocity depends upon the temperature and the salinity of the water, and varies over an extreme range of about 4,740-5,220 feet per second.
The velocity of the transmission of sound in sea water is also employed for the determination of the position of the sounding relative to the coast line; that is, its geographic position. This method of position determination, known as radio acoustic ranging, has been developed by the United States Coast and Geodetic Survey, with the co-operation of the Bureau of Standards, for use when out of sight of land where positions cannot be determined by angles on shore objects.
It is obvious that the hydrographer of today with these modern methods is not dependent upon clear weather to see signals on shore in order to carry on his surveying operations, he can now survey, with accurately determined positions, during foggy weather or at night. Modern science, therefore, has furnished two primary factors, among others, toward the expeditious prosecution of hydrographic surveying—the obtaining of soundings of ocean depths in seconds as compared with hours by the older methods, and the freeing of the hydrographer from the limitations imposed by darkness and by fog.
Need for detailed surveys.—While science has assisted the hydrographer in carrying on more expeditiously his mapping of the ocean floor, it has likewise added to the requirements imposed upon him by a world gone mad for speed and more speed, for faster and more exacting schedules for its transportation, and for its pleasure. The navigator of the past was compelled at times to grope his way with only the knowledge that he had a sufficient depth of water under his vessel. The nautical chart of the future, however, must supply him a wealth of detail of submarine valley, Plateau, or mountain range, for with his echo-sounding equipment he too has the means of obtaining at full speed a continuous record of these submerged features under his ship. He is thus furnished a useful aid in the determination of his position in thick weather when celestial bodies are not visible, provided, of course, the hydrographer has surveyed and charted for him this characteristic submarine topography.
And, too, the marine surveyor's sphere of operations has necessarily been extended. In the old days the mariner made no attempt to obtain a depth for position determination until he "had come on soundings"; that is, on to the continental Shelf where the depths are in general less than 100 fathoms. In future by means of echo sounding he will be able to make use of many other characteristic features, such as submarine gorges, ridges, and mountains which rise about the surrounding ocean floor, as they are discovered and charted by the hydrographer. Later the 1,000-fathom contour, which in many places is as characteristic as the 100-fathom contour, will be equally valuable for position finding. It is obvious, therefore, that we must extend our surveying operations offshore, at least to the beginning of abysmal ocean depths.
Progress of hydrographic surveys.—Already a beginning is being made toward this end; within the past decade hydrographic surveys have been made along practically the entire Pacific coast line of the United States and from the shore out beyond the 1,000-fathom contour. The water areas along the outer coast of California in 1918 were only 27 per cent surveyed; today these surveys are 90 per cent completed. In 1919 the outer coasts of Oregon were only 14 per cent surveyed, and of Washington only 44 per cent; today these areas are 100 per cent surveyed.
Submarine features.—In the course of these surveys many outstanding characteristic features are being found and charted. In February, 1933, a submarine mountain, rising from a general depth of 10,000-11,000 feet to within 1,800 feet of the ocean surface, was discovered and surveyed by the Coast and Geodetic Survey ship Pioneer, 15 miles out beyond the general 2,000-fathom contour and 75 miles offshore from San Nicolas Island off the coast of Southern California. This mountain is well out beyond any previous work in this general locality, and was found in developing a ridge with slightly less water than the 2,000 fathoms over the surrounding areas, extending out beyond the general 2,000-fathom contour, and terminating in this peak. It is actually a circular plateau with an area of about 15 square miles. It rises to a ridge about 6 miles long and a few hundred feet wide. The Pioneer's positions while engaged on the survey of this mountain were determined by 3 hydrophone stations, one 75 miles distant on San Nicolas Island, another 70 miles distant on Santa Rosa Island, and the third 90 miles distant on Point Conception, the nearest point on the mainland.
Another submarine plateau, situated 20 miles west by south of Point Sur, California, was discovered by the survey ship Guide in 1932. (See Fig. 5.) This plateau rises rather abruptly to within 2,700 feet of the surface from a general surrounding depth of 3,600 feet. The Guide also discovered and surveyed a large mountain situated in lat. 37° 02' N. and long. 123° 20' W., 45 miles offshore from the Farollone Islands. Its summit rises abruptly to within about 5,400 feet of the surface from a depth on the seaward side of 10,800 feet. The development of the summit indicates that the mountain is of volcanic origin, with a well-defined crater.
Submarine valleys are of even more value to the navigator for fixing positions in thick weather, since they extend well into the continental shelf, generally normal to the lanes of coastwise traffic and, since their depths are much greater than those of the surrounding areas, they constitute unmistakable “landmarks.” Both edges of such gorges are well defined and furnish good lines of position for the mariner in one direction; the width of the gorge, as determined by dead reckoning by the navigator and compared with the charted width, furnishes a good line of position m the other direction.
Several such valleys, deeply indenting the continental shelf, have been discovered during this detailed modern survey of the Pacific coast. A narrow gorge was discovered in 1932 by the survey ship Guide; this valley extends into the continental shelf for a distance of about 3 miles. The prevailing surrounding depths on the shelf are from 65 to 80 fathoms; the gorge itself has maximum depths of over 200 fathoms throughout two-thirds its length, and in excess of 300 fathoms for half its length. The head of this submarine valley is only 6 miles southwest of Ana Nuevo Lighthouse. During the recent survey of Georges Bank off the coast of Massachusetts several submarine valleys with pronounced characteristic features were discovered and developed. Corsair Gorge, on the southeastern edge of the bank, is 2 miles wide, 8 miles long, and 1,800 feet deeper than the surrounding areas. This valley was discovered during the surveying season of 1930 by the survey ship Oceanographer. Another gorge, discovered during the 1931 survey season, is on the southern edge of the bank about 90 miles southwest from Corsair Gorge. This second valley is about 11 miles long, over 2 miles wide, and about 2,000 feet deep.
These gorges are probably gashes left by giant landslides of the terminal moraine constituting Georges Bank. Soundings directly off the mouths of the valleys indicate that these masses of material, having slipped from their insecure holds onto the steep side of the continent, now lie at a depth of 6,000 feet just off the edge of the bank.
The United States government has at the same time been extending the hydrographic surveys to its Pacific possessions. The first systematic hydrographic surveys of the Hawaiian Islands were begun after their annexation and as executed may be divided into two sections: one comprises the area in the immediate vicinity of the larger eastern group of islands and within the 1,000-fathom curve; the other extends to the westward, and includes the area of small islets and shoals scattered over the 1,200 miles between the Island of Niihau and Midway Islands. The survey of the first area is nearing completion, and the surveys of the second area has already been extended to Lisianski Island, about 900 miles to the westward.
A modern, systematic survey of the Philippine Islands was begun on January 1, 1901. This gigantic undertaking, except for several unimportant areas, is now practically completed. The first survey of the more important areas was completed several years ago. Sixty-four per cent of the total water areas of the islands was completed during the years 1901-18; the entire area is about 90 per cent completed at the present time. The hydrography yet to be accomplished in the islands consists of a small area along the northeast coast of Luzon and about 75 per cent of the west coast of Palawan Island. On offshore work about 10,000 square miles remain to be surveyed off the north coast of Borneo between the international boundary and completed work in the extreme southern part of the Sulu Sea. The completion of this vast project in 30 years, considering its comprehensive nature and the large area involved, may be considered an achievement in surveying operations.
The surveys of Alaska, on the other hand, have not progressed so rapidly. The comparatively short surveying season in those latitudes, coupled with lack of funds and with the great amount of weather unsuitable for surveying operations during a season, have militated against the efficient prosecution of that project. Due, too, to the enormous area and to the rugged character of the coast line, the greater part of Alaskan waters is yet inadequately surveyed, although the whole area has been covered by surveys of a reconnaissance nature, and the more important areas by comprehensive modern surveys.
On the Atlantic and Gulf coasts of the United States first surveys of all waters have been completed, but naturally, using the older methods, a large part of the offshore areas are somewhat sketchily covered. Modern surveys are now being laid down. The most interesting and unique of these surveys, that of Georges Bank, was begun in the spring of 1930 and completed in the fall of 1932. While the problems encountered on this survey taxed the ingenuity of the hydrographer, they have served to further the development of methods for the control of surveys of offshore areas fairly comparable with fixed position work controlled by observations on terrestrial objects.
The area comprising Georges Bank is one of the most difficult to survey of any along our coasts. The problems involved include remoteness from base and shore, prevalence of fogs, heavy traffic, numerous shoals, and strong and complicated currents. During the times of spring tides, which occur at the times of new and of full moon, these currents attain their greatest velocity, resulting in long rows of “over falls” and “tide rips,” as the masses of water, forced up by strong currents from greater depths, sweep over the shoal areas. The normal expectation of fog in this region is from 40 to 50 per cent of the time in June and July; 30 to 35 per cent in August, and 20 to 30 per cent in September. The traffic consists of scores of trawlers along the 40-fathom curve, sword fishermen in the deeper waters on the extreme eastern part of the bank, vessels plying between New York and Nova Scotian ports, and transatlantic liners between European ports and Boston and New York.
The control for this survey consisted of a chain of triangles with lengths of sides of from 10 to 15 miles, the vertices of which were marked with survey buoys, constructed and planted by the survey vessels. (See Fig. 7.) The lengths of all triangle sides were determined by radio acoustic ranging and the azimuths of many of the triangle sides were measured by sun azimuths. The geographic position of one of the buoys at a triangle vertex out near the tip of the bank was determined by numerous astronomical sights under favorable meterological conditions with a probable error of one- to two-tenths of a mile. This position determination was held fixed for the three field seasons of the survey until a connection was made, at the completion of the survey in the fall of 1932, with shore triangulation stations on Cape Cod and Nantucket Island. The relations of the remainder of the survey buoys to this basic buoy were computed by means of the distances between buoys, which were measured by the precise determination of the time of travel through the water of the sound produced by the explosion of a small quantity of T.N.T., exploded at a depth of about 100 feet alongside each survey buoy, the impulse of explosion being received and recorded on the survey vessel anchored near by, as well as its receipt in the hydrophone of another vessel anchored near the distant buoy, employing methods quite similar to the ordinary methods of radio acoustic sound ranging already explained. This marine triangulation was carried shoreward for 140 miles and the connection with shore triangulation stations was within 400 meters, requiring an adjustment of less than 3 meters per mile.