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December 1939

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December 1939 U. S. COAST AND GEODETIC SURVEY FIELD ENGINEERS BULLETIN NUMBER 13 DECEMBER 1939 Opinions, recommendations or statements made in any article, extract or review in this Bulletin do not necessarily represent official policy. U. S. COAST AND GEODETIC SURVEY FIELD ENGINEERS BULLETIN NUMBER 13 DECEMBER 1939 CONTENTS Page The New EXPLORER Frontispiece Precise Alignment 1 Carl I. Aslakson Hydrographic Survey Aids 11 H. C. Warwick Through the Straits of Magellan on the PATTERSON 16 R. R. Lukens Triangulation--Ship Intersection Method 20 Charles Pierce Lighted Survey Buoys 24 G. C. Mattison Comment: Herbert Grove Dorsey 26 Submarine Scarp off Cape Mendocino, California 27 Harold W. Murray Comment: N. H. Heck 32 New Shoals for Old 34 Earle A. Deily Relation of the Tide to Property Boundaries 44 R. S. Patton Strange Property Descriptions - Eastern Long Island 58 James E. Connaughton Some Reflections on Echo Sounding 59 Paul A. Smith Plane Table Traverse by Wire 71 Ross A. Gilmore Where the Decimal Point Goes 72 (From Engineering News-Record) The Bering Sea Survey 73 E. W. Eickelberg A Lamentable Ditty 80 Pacific Coast Wire Drag Survey - 1939 81 I. E. Rittenburg Least Depth on Shoals Effectively Determined 83 George F. Jordan Page Theodolite Micrometer Tests 84 Carl I. Aslakson Heeling Error in Radio Direction Finder Bearings 93 Crawford F. Faily Survey Pests 95 (From The Canadian Surveyor) Basic Bench Marks in New Orleans 98 Karl B. Jeffers Portable Depth Recorder 100 Herbert Grove Dorsey Tracking Down Survey Buoys 106 R. P. Eyman Serial Temperatures by Bathythermograph 108 John C. Mathisson Control Survey Criteria 114 (Are our Survey Specifications too Exacting?) Hugh C. Mitchell Comment: 118 C. L. Garner Trucks and Triangulation 121 P. L, Bernstein Proposal to Honor Isaac Ingalls Stevens 125 Charts on Board Ship 127 (From The Nautical Magazine) Sono-Radio Buoy Improvements 128 Millard F. Timm Comment: Herbert Grove Dorsey 130 R. P. Eyman 133 Simplicity in Maps 134 Charles H. Deetz Hydrographic Control - 1939 Ships OCEANOGRAPHER and LYDONIA (Sketch) 135 Ship HYDROGRAPHER (Sketch) 136 Toward a More Perfect Chart George F. Jordan 137 The Care of Surveying Instruments 140 (From Civil Engineering) Comment: Robert H. Griffin (From Civil Engineering) 144 H. W. Hemple 145 Page Sono-Radio Buoy "Gulf" in 400 Fathoms 148 L. P. Raynor It's a Small World After All 150 Special Survey, Sitka, Alaska 151 L. C. Wilder Plotting Three Point Fixes Without the Use of a Protractor 155 Charles Pierce It Happened in a Field Station 160 Marie H. Smithson Honors to Dr. Bowie 161 Motor Vessel E. LESTER JONES 162 L. C. Wilder Geographic Names of Interest 164 Sea Manners 174 What Not to Paint 174 (Extracts from Bulletins of Bureau of Marine Inspection and Navigation) Magnetic Observations in Alaska 175 (Extract from 1939 Season's Report of E. H. Bramhall) Outsiders' Impressions 186 From Maritime Commission Cadets The Launching of the New Ship EXPLORER 187 Proposed Assignment of Officers for Field Season, 1940 189 Statistics and Distribution of Field Parties 191 Field Memoranda, 1939 196 Circular Letters, 1939 198 1 PRECISE ALIGNMENT Carl I. Aslakson, Hydrographic and Geodetic Engineer U. S. Coast and Geodetic Survey Within recent years numerous scientific studies of minute move- ments of large structures have been made. In general, these studies require accurate determination of both the horizontal and vertical changes of various points on the structures relative to certain points outside the structures, which latter, owing to their mass and location, are assumed to remain fixed. In particular, these studies are important in connection with large concrete dams. The largest dams place a tremendous additional load on the earth's crust, due, not only to the mass of material in the dam itself, but to the water load accumulated behind the dam. Frequently temperature extremes add to the static stresses caused by the load. Movements of various orders must take place in the structures and as a consequence precise alignment, a little explored field of surveying, is becoming a neces- sity. At the outset it may be well to define what is meant by precise alignment. Since limited lengths are usually involved in structures requiring studies, we may first limit the length of line over which we desire precise alignment. This paper will be restricted to a discussion of the alignment of intermediate points between two fixed points which are 1300 feet apart. Applicability of the methods de- scribed herein to alignment problems over greater distances will be considered later. By precise alignment of the intermediate points is meant that the probable error of alignment shall be of an order many times smaller than that ordinarily attained by the engineer using ordinary transit or wire methods. The writer recently had the privilege of assisting in a prob- lem in precision alignment at the David W. Taylor Model Basin of the U. S. Navy at Carderock, Md. This project was to establish inter- mediate points at approximately 40-foot intervals between two points 1300 feet distant from each other. The line lay along a wall inside the towing basin structure and was to be used to control the align- ment of the steel rail on which the driving wheel of the towing car- riage was to run. Since elimination of all sources of friction was of paramount importance, the ultimate in accuracy of alignment of the towing carriage rail was desired. Two methods of alignment were employed. One of them, desig- nated as the "Taut-Wire Method," was used by the physicists of the U. S, Navy. The other, which will be called the "Optical Method," was employed by the writer. A discussion of the two methods with their agreement and their relative merits and applications follows. Taut-Wire Method At first thought, it might seem that the establishment of a straight line from a taut wire should prove to be an easy problem. Unfortunately, a number of practical difficulties arise. In the first place, there is a large catenary in a wire 1300 feet in length. At the Model Basin a piano wire with an area of .000806 square inch was employed, and although this was stretched until it was under a tension of 186,000 pounds per square inch, there was a 4-1/2-foot 2 3 catenary sag in this wire. Obviously there was also the problem of accurate plumbing from this wire. Certain other practical diffi- culties were encountered. In a line of that length, absolute free- dom from air currents was necessary. The question also arose as to whether or not a magnetic bow might not exist in a steel wire caten- ary of that length. It was because of this latter uncertainty that a check on the alignment by the optical method was desired. Briefly, the "Taut-Wire Method", as employed, is described in the following paragraphs: The piano wire was fastened to the wall near one end of the line. At the other end it passed over a pulley and to it was hung a 150-pound lead weight. Since the area of the cross section of the wire was .000806 square inch, this gave a tension of 186,000 pounds per square inch in the wire. Near the end points of the line the wire was about seven feet above the wall, while at the mid-point it was about 2-1/2 feet above the wall. Steel posts 1-1/2 inches by 6 inches in cross section, protrud- ing from the concrete wall about 6 inches, were spaced along the wall at approximately 8-foot intervals. One of these posts may be seen in Plate I. Every fifth post contained a l/2-inch brass bolt with a polished head set into it flush with the top of the post. (This bolt is not visible in Plate I; the one which appears near the objective end of the telescope was used in the vertical control work) . These bolts were set on line by transit so that it was known that the precise line would fall somewhere on each bolt. The method of plumbing used was the unusual feature of the Taut- Wire Method. This was accomplished as follows. A small brass plumb bob was suspended by a fine silk thread from the wire. The silk thread was looped over the wire to prevent eccentricity. The plumb bob hung down past one end of the steel post and was immersed in water to damp its movements. A 12-power telescope (See Plate I) was placed on line on an adjoining post eight feet away. The line upon which the telescope was placed was also established by careful plumb- ing from the piano wire. The telescope contained a glass diaphragm 4 with an etched scale upon which the position of the plumb bob thread could be read to .01 millimeter. A long series of readings of the thread was made and then a sharp stylus was lined in on the brass bolt at the mean reading of the thread on the telescope scale. This process was repeated on each post upon which it was desired to set an alignment point. The manner in which the piano wire, posts and telescope were arranged while readings were being made is illustrated in Figure 1. It is obvious that the approximate method of establishing the tele- scope on line introduces no serious error into the results. Since the ratio of the distances of the plumb thread and the telescope to the brass bolt is 0'-2" to 8'-0" or 1 in 48, it is apparent that only l/48th of the error of alignment of the telescope will enter into the results of line as established on the brass bolt.
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