Having devoted his entire professional career to the U. S. Coast and Geodetic Survey, Admiral Colbert (Sc. D., Tufts) knows whereof he writes. He has been navigator and executive and commanding officer of Survey ships in the coastal waters of the United States, Alaska, and the Philippines. During World War I he served as Lieutenant Commander on the U. S. S. Northern Pacific, a troop transport. In 1931 he was engaged in a comprehensive hydrographic survey of Georges Bank, utilizing advanced methods of offshore surveying. Since 1938 Admiral Colbert has been Director of the Coast and Geodetic Survey.a
Where in the world are we? How deep is the ocean? How high is high water? Finding the answers to these questions has been the job of the Coast and Geodetic Survey during more than a century of surveying and charting our coastal waters, and of establishing geodetic control in the interior of our country. The close relationship that exists between surveys and charts and the stars and planets is perhaps not generally known; yet astronomy is at the very foundation of all surveying operations that deal with extensive areas and that require an accurate orientation on the surface of the earth. Astronomy is the medium by which we hitch our planet to the stars and to the other planets. The engineer, the surveyor, the navigator who deal with practical astronomy — all owe a debt to the theoretical astronomer, whose determinations of the motions of the heavenly bodies and whose formulation of tables expressing their projected positions on the celestial sphere furnish data for use in establishing position on the surface of the earth — be it of an engineering structure on land, a lighthouse on the coast, or a vessel at sea.
It is an interesting fact that at the time of the inception of the Coast Survey in 1807,b there was not a single working observatory in the country for the training of astronomers, and there was not a college in the United States that included a course in geodetic surveying in its curriculum. This is but one example of the magnitude of the task that faced Professor Ferdinand R. Hassler when he accepted commission from President Jefferson to direct the survey of our coast.
One of Hassler's first hurdles involved astronomy. Being a man of outstanding scientific attainments, Hassler planned a survey along lines of lasting worth and proved value. There were many in those days who saw no need for elaborate base measurements and triangulation systems and who advocated that the surveys of p373 our coast be controlled by astronomic determinations alone. Hassler understood clearly that accurate surveys of extensive areas could only be accomplished by the geodetic process, which is, properly, a combination of the astronomic and the geodetic. Fortunately, Hassler's scientific approach prevailed, and the plan he proposed became the model for our work.
Ferdinand R. Hassler
Some of Hassler's contemporary workers and a number of those who followed him were among the foremost astronomers in the country, and their contributions, in both methods and equipment, to the development of practical astronomy are indelibly engraved in the records of Coast Survey achievements.
In 1834 Captain Andrew Talcott, of the Corps of Engineers, discovered the differential method for the determination of astronomic latitude with the zenith telescope. Through its adoption and refinement by the Coast Survey, the Talcott method received great impetus toward world-wide use. The accuracy of the results obtained by this method is superior to that of every other field method and compares favorably with the results obtained with the largest observatory instruments.
The application of the Talcott method for determining latitude indicated some radical errors in the published star places. The various observatories of the country immediately undertook the necessary additional observations to furnish more accurate star places; so that one of the by‑products of the method was an improvement of the star catalogue.
In the early history of the Coast Survey, the determination of astronomic longitude was one of the important problems that required solution. In those early days longitude was determined by observing certain celestial phenomena, such as the eclipses of Jupiter's satellites and the occultations of stars by the moon, which occurred everywhere at the same instant of time. The most practical method, and the one which gave the best results, was the use of highly accurate timepieces known as chronometers. A number of these were used to compare the difference in time between the two places where the observations were being made for difference of longitude. To minimize any error in rate, the chronometers were transported back and forth between the stations.
Within a few months after Morse flashed his first telegraphic message over the wires between Baltimore and Washington, Professor Bache, who succeeded Hassler as head of the Coast Survey, began experiments for an application of the telegraph to longitude determinations. On October 10, 1846, the telegraphic method was put into successful operation, and time signals were exchanged between Washington and Philadelphia.
The determination of longitudes by this method was extended to other parts of the country as rapidly as the electric telegraph came into practical use. Although this established a coordinated network of these accurately located points, it still p374 left the longitude relationship of America with the prime meridian through Greenwich, England, dependent upon chronometric determinations.
Between 1849 and 1855 the Coast Survey instituted expeditions to exchange chronometers between Cambridge, Mass., and Liverpool, England. In the last year six voyages were made, three in each direction, and fifty chronometers were transported.
Upon the completion of the first successful Atlantic cable in 1866, the Coast Survey, under the direction of Dr. Benjamin Gould, the noted astronomer, organized a project making use of the cable to measure the difference of longitude between Greenwich and the Naval Observatory at Washington. The results proved that the chronometric determination of 1855 was in error by over a second in time, or •about .25 mile.
The perfection of the methods used in the United States for determining latitude and longitude placed this country well in the forefront of astronomic achievement. It was freely stated at the time that geographical values of the positions of the principal astronomic stations of the Coast Survey were determined with greater accuracy than the values known for any European observatory.
The introduction of the radio method of determining longitude in 1922 did not increase the accuracy of the results, but merely made the determination more convenient and more economical.
In recent years, important contributions to practical astronomy have been made by international cooperation in which the Coast and Geodetic Survey has participated. One of these has been the establishment of a world longitude net for the coordination of longitude work throughout the world; another, the establishment of variation of latitude observatories at five places around the world on approximately the same parallel of latitude. In this country these observatories are at Gaithersburg, Md., and Ukiah, Calif. The results furnish information for the study of the wanderings of the geographical pole.
When the geodetic surveyor has located a survey point on the surface of the earth from astronomic observations, he has established a starting point for his land operations. The next step is to determine the direction to a second station by an observation on Polaris or other celestial body for azimuth. The third is to make an accurate measurement of the distance between the two stations. These provide the foundation for a method of locating other points on the surface of the earth without making direct measurements. This method is known as triangulation. It is the only feasible method of measuring great distances with a high degree of accuracy, independent of the character of the intervening terrain.
p375 The earliest triangulation work in the United States was executed in 1817 in the vicinity of New York. It was here that Professor Hassler made his first astronomic observations. In his report Hassler says: "A special station was made upon Weasel Mountain for latitude and azimuth, and a solar eclipse occurring just at the same time gave occasion for an observation of longitude at the same place."
First Coast Survey
From an accurately measured base line (shown by heavy lines) and the measured angles of a series of connected triangles, the lengths of the other lines are determined.
Since this early triangulation, an elaborate network has been extended over the entire country, and tens of thousands of control points have been established. These are permanently monumented with markers for use as starting points in surveys for topographic, geologic, and other forms of mapping, and for every engineering project requiring an accurate knowledge of the horizontal relationship between points on the earth's surface.
This instrument used by Professor Hassler in the early triangulation required a special horse-drawn carriage to transport it from one station to another. The streamlined theodolites of today can be carried by one man.
Top of the Monument
Triangulation party making observations from the top of the Washington Monument, Washington, D. C. •(555 feet high), in order to fix its position in latitude and longitude. Note the compactness of the modern theodolite as contrasted with the one used by Hassler.
The shape and size of the earth must be taken into account in the survey of a country as large as the United States; otherwise, serious errors will be introduced in the results. To determine curvature, astronomy again enters the picture.
From many precise measurements involving astronomic observations and triangulation, the earth has been found to approximate closely the mathematical figuring known as the oblate spheroid, with a polar diameter •about 27 statute miles less than its equatorial diameter; hence, a spheroid of reference has been adopted for correctly placing the network of triangulation. The adopted reference spheroid deviates from the true figure, and this deviation is greatest in the vicinity of the high mountain ranges and the great ocean deeps. It is because of these deviations that position determination by astronomic observations alone is unsatisfactory. Many cases are on p377 record of such deviations, one of the most pronounced being on the island of Puerto Rico, where a high mountain range runs east and west the length of the island, with the deep waters of the Atlantic to the north and the Caribbean to the south. Astronomic stations had originally been established on the north and south coasts of the island. When these were connected by triangulation, the true distance between them was found to be •about 1 mile less than that given by the astronomic determinations.
In any engineering or scientific undertaking involving a large area, it is important that a full coordination and correlation be secured of the surveys, maps, and charts affected. A hydrographic or topographic feature on the earth can have but one latitude and longitude, which must be the same on every map or chart on which it appears. This can only be accomplished through the adoption of a single horizontal datum for the triangulation points in the country.
Triangulation Net of North America
The Dominion of Canada and the Republic of Mexico are connected by triangulation to the United States and Alaska to form one of the largest continuous arcs in the world.
In the United States the origin for all Federal mapping is station Meades Ranch in central Kansas. Its latitude and longitude on the reference spheroid were fixed by a mathematical adjustment based on a study of numerous astronomic stations throughout the country that had been connected by a continuous system of triangulation.
Station Meades Ranch was selected because it was near the geographical center of the United States and because it was at the junction of two great arcs of triangulation, one along the thirty-ninth parallel and the other along the ninety-eighth meridian.
Origin for U. S. Mapping
Station Meades Ranch in central Kansas is the origin of the North American datum of 1927, to which all Federal mapping is tied. Many thousands of control points, permanently marked with bronze discs set in concrete, furnish starting points for local surveys and engineering construction.
Today the continent of North America, from the lower end of Mexico through the United States, Canada, and Alaska to Bering Strait, is coordinated on this single datum. In this network is contained the longest arc of continuous triangulation in the world.
The control points that the geodetic engineer has established along the coasts and in the interior of the country are used by the topographic engineer as the framework for his map. The details he fills in with the plane table or by the more modern method of aerial topography. Photographs taken from the air are reduced to map form by means of specially designed stereoscopic plotting instruments.
Designed in the Coast and Geodetic Survey for stereoscopic plotting of topographic maps from 9‑lens aerial photographs.
Just as without astronomy our land surveys could not be properly positioned on the surface of the earth, so it is with surveys of the water areas. The same fundamental problem is involved, although there is a difference in the methods and instruments used. When the navigator leaves the shores of one country and sets out for the ports of another, he has nothing to guide him over the trackless sea except the stars and his chart. Whether he determines p378 his position at sea by observations on terrestrial objects or celestial bodies, or by electronic methods, he must have a chart on which he can properly relate that position to the land area and to the surrounding submarine features. The foundation for the nautical chart must therefore be hydrographic surveys that are tied into the same system of spherical coordinates as is used in establishing control points on land and in delineating the topographic features.
Hydrographic surveying involves a measurement of depth and a determination of position. The revolutionary advances made in this field during the past two decades have had a profound effect on the usefulness of the nautical chart. The measurement of depths by hand lead and wire has given to measurement by sound — better known as echo sounding. In this method depths are registered automatically on a graduated dial and can be read with equal facility whether measuring a depth of 2 or 2,000 fathoms of water. Graphic recorders are rapid and sensitive enough to detect and record wrecked ships lying on the ocean floor.
Echo Sounding and Electronic Position Determination
While the ship's fathometer measures the depths under the vessel by sound, the position of the vessel is determined by electronic methods.
Determination of a depth has little value unless the latitude and longitude are known, so that the depth can be shown on the field survey sheet or on the published chart. When in sight of land the position of the survey vessel is determined by means of the well-known three-point fix method from sextant angles taken on board ship to objects on shore previously correlated with the coastal control net. Beyond the limit of visibility of shore objects, underwater sound ranging has been used. By exploding a bomb in the water near the survey vessel, measuring the time of arrival of the sound impulse at two or more previously located hydrophone stations, and employing the knowledge of the velocity of sound in sea water, the distance to the p379 vessel can be determined. Used in conjunction with this method is the taut-wire sun‑azimuth traverse. The distances between buoys are measured by means of a taut-wire apparatus, and the azimuths of bearings are determined from observations on the sun.
Taut-wire Sun-azimuth Traverse
In surveying offshore, distances between control buoys are sometimes measured by wire, and bearings are determined from observations on the sun.
Occasionally it is desirable to begin a hydrographic survey at the offshore end and work in toward shore. Such was the case with the Georges Bank Survey begun in 1930. Georges Bank is an important area to both the fishing and the shipping industries. In order to modernize the charts of the Bank as early as possible, it was desirable to begin the survey near the offshore end of the Bank, •about 200 miles from Cape Cod, where the westbound shipping lanes cross the continental shelf. The basic position for the project — which comprised an area of •about 28,000 square miles — had therefore to be an astronomic one.
A buoy was anchored near the outer edge of the Bank and its position determined by a long series of star observations from the surveying vessel anchored nearby. Eighteen sets of sights were made by three observers on eight days, each set consisting of four or more stars. The position obtained was held fixed for the entire survey, which covered a period of three working seasons. From it, additional control buoys were established by acoustic methods and by sun azimuths. When the connection was made with the coastal triangulation, the error in the system was found to be •less than .25 mile — a result far better than was expected when the survey was begun.
Buoy Control Scheme for Survey of Georges Bank
A long series of star observations fixed the position of buoy "W" from which the control was expanded to a connection with shore stations.
Astronomic sights are used in hydrographic surveying in the same manner as in navigation, except that they are taken with greater precision and care. An observed altitude on a heavenly body determines a circle of position on which the observer is located. Observations made on two or more stars define circles of position, the intersection of which determines the position of the observer.
Acoustic methods of surveying have steadily pushed seaward the frontiers of accurate hydrographic surveys and have been used to explore the intricate patterns of our deep coastal slopes. These methods are responsible for the discovery of many p380 new and amazing facts about the continental shelf, the seaward extension of the land mass. The continental shelf is not of uniform width. Off certain portions of the Pacific Coast and Alaska it is quite narrow, whereas along the Atlantic Coast it varies from •10 miles in the vicinity of Palm Beach, Fla., to •about 200 miles off Cape Cod, Mass. In general the •600‑foot depth line is considered to be the edge of the shelf. From here the submerged land drops abruptly to ocean depths.
On September 28, 1945, a Presidential proclamation extended the jurisdiction and control of the United States over the natural resources of the subsoil and sea bed of the continental shelf. This added •approximately 300,000 square miles to our jurisdiction in continental United States and •about 550,000 square statute miles in Alaska.
Modern hydrographic surveys covering the area of the shelf have modified radically our former belief that the ocean floor is a flat, monotonous plain, devoid of rugged relief. How erroneous this concept was has been amply demonstrated by the many basins and ridges, troughs and peaks, that have been discovered since the advent of the new hydrographic methods. Submarine topography has been found in many areas comparable in extent and magnitude p381 to some of the major topographic formations that exist on land. The continental shelf — particularly along the northeast Atlantic Coast — is indented by many submarine canyons, penetrating many miles into the shelf. One of the most pronounced of these, and perhaps one of the greatest in the world, is the one that, lying •120 miles southeast of New York Harbor, marks the submerged gorge of the ancient Hudson River. If this canyon could be stripped of its blanket of ocean, we could in some places see across it; the floor of the canyon would lie •some 3,600 feet below. The mouth of this canyon has been traced to depths of •6,000 feet below sea level. The Hudson Canyon had been known to geologists for some years, but its true form and size were not known until modern methods of hydrography surveying were used in 1936.
In the Gulf of Mexico, in the vicinity of the delta of the Mississippi River, a large depression, or submarine trough, has been found that penetrates the otherwise flat and smooth continental shelf for a distance of •about 20 miles. To the northeast of this trough is a small dome-shaped hill which, geologists believe, is a salt-dome uplift. It stands •about 160 feet above the sea floor and rises to within •200 feet of the water surface. In shape this dome has been found to resemble closely some of the buried salt domes along the coastal plains of Louisiana and Texas where many of our oil reserves are located. Numerous other, similar formations have been found along the edge of the shelf here, •100 and more miles from shore.
Off the coast of California, •about 50 miles southwest of San Francisco, a huge submerged mountain has been found, rising 4,000‑8,000 feet above the ocean bed and to within 2,500 feet of the sea surface. Other, similar seamounts, some more and some less pronounced, abound along the Pacific Coast and in the Gulf of Alaska.
U. S. Coast and Geodetic Survey Ship Explorer
One of the latest additions to the fleet of survey vessels used to explore the hidden mysteries of the deep, the ship contains the latest in surveying equipment.
p382 An unusual and interesting submarine feature along the northern California coast is the Mendocino Escarpment, a clifflike formation, 6,000 feet high on the northern face, that juts into the Pacific for •66 miles, almost at right angles to the general trend of the coast line. This remarkable feature is on the seaward extension of the famous San Andreas Fault, which extends throughout the length of California. The San Francisco earthquake of 1906 was due to earth movement in the vicinity of this fault.
In the Gulf of Alaska, extending from Yakutat Bay and paralleling the Kenai and Alaska peninsulas and the Aleutian Archipelago, there is the remarkable Aleutian Trench. This elongated depression is traceable for a distance of 2,200 statute miles, the last 1,400 of which conform to the p383 arc of a circle of a radius of 760 statute miles. The floor of the Trench stands at its maximum about 25,000 feet below the surface of the sea and about 9,600 feet below the adjacent floor of the Pacific Ocean south of Attu Island. It was along the steep slopes of this Trench that seismograph records fixed the epicenter of the seaquake of April 1, 1946, which gave rise to such pronounced sea waves in the Hawaiian Islands and along the California coast.
In the Philippine Islands, just east of Mindanao, is the impressive Mindanao Deep, where the greatest known ocean depth — 35,400 feet, or slightly over 6.5 miles — was obtained with an echo sounder. If we could put Mount Everest into this great ocean deep, the world's highest peak would still be covered by a mile of sea water.
Recently, electronic methods have been developed by the Coast and Geodetic Survey for use in hydrographic surveying. These will permit accurate surveys to be extended greater distances offshore than heretofore. Shoran equipment — a form of radar used during the war for precision bombing — has been adapted to, and successfully used for, position determination in the western Aleutians and off the coast of Maine. By this method a survey ship at distances of •50‑100 miles offshore can be located with an accuracy comparable to that of a three-point visual vix on shore objects.
Another electronic device is being developed by which control of comparable accuracy with shoran will extend the offshore limit of accurate hydrographic surveys to as far as •300 miles. This may open up new fields for scientific research in an area that many believe to be a future reservoir of natural resources. Hitherto, one of the great drawbacks to such investigations has been the limitations in the methods of offshore surveying; even with the most nearly perfect geophysical methods there must still be a correlation with certain aspects of hydrographic surveying in order that a reliable topographic picture can be obtained of the area under investigation.
Our country is coupled to heavenly bodies in still another way. We are all familiar with the rhythmic rise and fall of the sea. This alternate elevation and depression — which at some places occurs daily and which we call the tide — can be expressed graphically in the form of a mathematical curve in which height and time are the two factors.
The Coast and Geodetic Survey is interested in tidal phenomena for two reasons. In its hydrographic work, the height of the tide must be known every instant during which a survey is in progress in order that depths obtained may be corrected and referred to a common datum plane for publication on the nautical chart. Tidal observations are also necessary in connection with the publication of the tide tables, which furnish the navigator predicted times and heights of the tide at the important ports of the world.
The connection between the tide and the sun and moon was recognized at a very early period, but it was not until Sir Isaac Newton formulated his theory of the law of gravitation, that a rational explanation of tidal phenomena was advanced. Newton proved that the tides were a necessary consequence of the law of gravitation; hence, the attractive forces of sun and moon on the earth and the waters overlying it are the principal tide-producing forces. The moon being nearer, its tide-producing force is more than twice that of the sun.
If we examine the tide curve for a period of a month, we find that the times of high p384 and low water of one day occur 50 minutes later than those of the preceding day. This period is characteristic of the tide practically everywhere and is irrespective of the range or type of tide found. The moon in its revolution around the earth shows the same retardation of 50 minutes.
The height of the tide during a month is affected by the relative positions of the sun, moon, and earth. At times of new and full moon — when sun, moon, and earth are in line — the tides have their greatest rise and fall; whereas during first and third quarters, the rise and fall are at a minimum.
Tides differ in the character of the rise and fall. At New York, for example, there is the semidaily type of tide (two tides of approximately equal range); at Pensacola, there is the daily type (one tide a day); and at San Francisco there is the mixed type of tide (two tides with unequal range).
The tide varies in range from place to place. At the Atlantic entrance to the Panama Canal it is less than a foot in six hours, whereas at the Pacific entrance, only •30 miles away, it averages 12.5 feet. At Anchorage, Alaska, the rise and fall is approximately 33 feet.
Since the tide-generating forces are of astronomic origin, it might be supposed that the tide would exhibit similar characteristics everywhere. This would be the case if there were no other influences. Actually, the tides as they occur in nature are the result of the varying responses of the different ocean basins to the tide-producing forces, as modified by local terrestrial features. This theory of the tides that makes them local phenomena, rather than a single world phenomenon, was developed in the latter part of nineteenth century by R. A. Harris, a tidal mathematician in the Coast and Geodetic Survey. In predicting tides for any port, it is necessary first to secure tidal observations for about a year. After that predictions can be made for any future time. Tide prediction is a complicated mathematical process, but the work has been greatly simplified by the use of the Tide Predicting Machine, designed in the Coast Survey. Astronomical equations involving as many as thirty-seven factors are solved mechanically, to produce the tide in nature.
Tide Predicting Machine
This robot predicts the times and heights of the tide for any time in the future.
Thus, we see that astronomy is at the foundation of all our survey operations: By means of triangulation we have tied together astronomic observations made throughout the country. This has made possible the determination of the figure of the earth and the best placement of our country on that figure. We have tied into the network of triangulation our mountains, rivers, coast lines, and other land features; and by extending our control network offshore we have tied in our waters, our continental shelves, and the ocean features beyond. In short, we have hitched our country to the stars.
a Admiral Colbert is now chiefly remembered as the director of the Coast and Geodetic Survey during World War II. Good biographical information can be found in Chapter 12 of Science on the Edge: The Story of the Coast and Geodetic Survey from 1897-1970 by John Cloud, NOAA Historian.
b In 1807, Prof. Hassler was teaching at the United States Military Academy at West Point. According to Sidney Forman ("The United States Military Philosophical Society, 1802‑1813", WMQ(3) 2:280 f. and notes), the Coast Survey was a direct outgrowth of a paper he read at a meeting of the Society in that year.
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