ASTRONOMIC DETERMINATION OF DIRECTION* By Captain Edward M. Burd, Coast Artillery. Whenever artillery is fired at invisible targets the determination of the direction of the gun-target line is perhaps the most vital element of the system. Present indications point to the firm establishment of indirect methods cf fire for heavy artillery used in land warfare. With increasing range it is not improbable that seacoast armament may be fired more by Case III in the future than in the past. While a positionfinding system need not necessarily be oriented, it requires but little more work and is a great deal more convenient to have all directions indicated by their true azimuths, and is almost essential when it comes to extending the system or connecting it to other systems. Consequently, the Artillery Officer is confronted with the necessity for determining true north-geographic north-whenever firing data is to be prepared, or any surveying or map work is to be done. During the past emergency great efforts have been made to extend and improve the art and science involved in the use of artillery, and Meridian Determination has received its full share of attention. The two or three methods more or less standard among Army and civilian engineers have been increased to about a dozen. Many of the supposedly new methods are put forth without any apparent hesitation, to say the least. The practical astronomer or the experienced field engineer, knowing that the subject has been exhaustively investigated for hundreds of years, would suspect that in time the more impractical methods will naturally gravitate into the discard-a survival of the fittest. But most of us are too busy with other duties to become accomplished scientists, and when we need to make a determination of direction it is desirable that some ready reference indicate the procedure for two or *A comparison of the many methods, with a general statement of the principles involved in each, and of its advantages and disadvantages for actual field use. SUN three of the more suitable methods, saving time and thought which might otherwise be wasted on impractical impractical academic solutions of the problem. RESULTS OF COMPARATIVE OBSERVATIONS FOR DETERMINING Y-AZIMUTH OF A LINE. ALTITUDE METHOD (15) AM obs-338 44 15 55 30° 139 148 39 338° 44′ 52" +336-45 11 28 338 45 390-49 78 +2' 13° - 48 10338 46 55 108 1° 30° 113 +23° - 1 53 (mean deviati yon: A.M-28°; PM-30 d41 ין 17 33845 17 +10° 69° Results too erratic and approximate to be considered. +03769 + 38 +1' 30° -19 Burd Jam 13, 1919. This article is the outgrowth of a series of experiments undertaken as a matter of purely personal interest, with the idea of making an impartial and unprejudiced comparison of the many methods from the standpoint of actual field use. Α convenient line was arbitrarily chosen and permanently marked upon the ground, and some sixty separate and distinct. observations were made to determine the azimuth of the line, using as many methods as possible. The observations and computations being all made by the same person, using the same transit, and keeping all other external conditions the same, the results should be fairly comparable. Also, no attempt was made to do the work with greater care or precision than would be expected of ordinary field work under rather unfavorable conditions, as will be evidenced by the results, which have been tabulated on Plate I. In fact, all the observations were made during rather disagreeable winter weather, and the results as here shown are not exhibited as a measure of the precision which should attend the planning and execution of the azimuth determination for any permanent or extensive work. Each and every observation made was computed and the results are here shown, regardless of the lack of agreement of those results, since the discarding of any observations would have vitiated the impartiality of the comparison. The results of all this work are shown on Plate I. No further comment or comparison of these figures appears to be necessary, as the reader will probably perfer to draw his own conclusion. In the course of this investigation careful search on the subject was made through most of the standard texts on astronomy and surveying, the publications of the Coast Artillery School and of the Heavy Artillery School, A.E.F. Each authority usually limits its discussion to a few methods, explaining them in detail and possibly commenting upon their relative merits; but usually no general comparison is made, and the books do not necessarily select the same few methods from the number available. Plate II gives a summary of the apparent preferences of each authority, and indicates the detail and space allotted to each of the methods considered. An attempt has been made to estimate the order in which each authority would rank the methods, as indicated by the figures. An average or composite rank, as shown at the bottom of Plate II, has been made by averaging the individual figures. In a way this average may be assumed to approximate a fair ranking of the methods, judged largely by theoretical considerations. by the best authorities on the subject. This comparison has also been attacked from another angle, from the standpoint of engineers who have had actual field experience in this kind of work, and appreciate the shortcomings of each method from that point of view, which is only obtained by experience. This is the viewpoint most RELATIVE VALUE OF NINE METHODS OF MERIDIAN DETERMINATION as taught at HEAVY ARTILLERY SCHOOL, A.E.F. and and as deduced from STANDARD ENGINEERING TEXTBOOKS. Notation and numbers show apparent relative value as deduced from the treatment of the methods by each authority. often lacking in books, and it was rather singular that in only one of the books just compared was any consideration given to the many points which always arise in the course of field work of observation, questions which often more vitally affect the results than minor defects in the theory. As a basis of comparison a rating scale was devised. It was used independently, without comparison with the others, by each RELATIVE VALUE OF TEN METHODS OF MERIDIAN DETERMINATION who are also CIVIL ENGINEERS, WITH CONSIDERABLE FIELD EXPERIENCE. Upper figure is rated value; rated by accompanying scale Lower, heavy figure shows relative position among the methods rated. of the officers who rated these methods. The results of these ratings are shown on the upper table of Plate III, being the upper, light-faced figures under each method. On the basis of each of these ratings the methods were ranked as indicated by the lower, heavy-faced figures. At the bottom |