247 HYDROGRAPHIC MANUAL Page 108 



first. They usually consist merely of small banners tied on the branches of trees or 

 mangroves at points and other places where good views are obtained, and so arranged to 

 form the best figures. In executing the sextant triangulation all triangles should be 

 closed by the observation of all tlu-ee angles and adjusted for closure. Navigating 

 sextants should be used, but closing en-ors of several minutes may be expected. If at 

 all practicable the sextant triangulation at its farther end should be joined to control 

 points located by more accurate methods to determine the closing error and provide 

 for an adjustment of the scheme. A geodetic computation of such a scheme is usually 

 unwarranted, but to obtain sufficient accuracy it should be plotted on an aluminum 

 sheet and at a scale several times larger than that of the hydrogi'aphic survey of the 

 area. 



Sextant triangulation may be advantageously combined with the sextometer 

 method or with distance measurements with a floating calibrated wire, described in 

 2381. In any case, advantage should be taken of long vistas to observe directions to 

 stations as far ahead as possible. These will strengthen the azimuth of the scheme 

 appreciably. 



247. Natural Objects Located From the Vessel 



The offshore distance to which hydrography can be controlled visually depends 

 directly on the elevations of mountain peaks, hillocks, and other natural and artificial 

 objects visible from offshore, and the number of these which are located for use as control. 



The positions of some of these are determined by triangulation and by topographic 

 methods, but since the perspective is entirely different when viewed from seaward a 

 number of definite points, excellent for use as hydrographic stations, generally remain 

 unlocated. During combined operations the hydrographic party frequentl;^ detects 

 these in time to have them located by the topographer or the triangulation party, but 

 when these parties have completed their operations in the area the only recourse, if 

 they are to be available as hydrographic signals, is to locate them by hydrographic 

 methods. 



They are usually located from the ship by sextant cuts by the method described 

 in 2432. Vertical angles to any peaks or hillocks should be observed at each position 

 at which cuts to them are observed. The elevations computed from the vertical 

 angles serve to verify that the cuts have been taken to the same object. (See 382.) 



Since many of the objects useful in controlling offshore hydrography are only 

 visible at a considerable distance offshore, proper identification is one of the important 

 problems of location. Each object should be thoroughly described at the time of the 

 first cut, and a perspective sketch made of the object and its surroundings for use in 

 identification at the time of future cuts. When cuts are taken for the location of natural 

 objects the vessel should be brought to a full stop in the water and anchored if practi- 

 cable. After the vessel has swung to her anchor a number of cuts can be obtained to be 

 plotted or computed from the same fix if they are taken in rapid succession. Each 

 fix should be verified after the cuts have been taken to ensure that the angles have not 

 changed appreciably. ^Tien the vessel is stopped but not anchored, as many observers 

 as practicable should be used so that the angles of the fix and the cuts may be taken 

 simultaneously. At each stop several fixed positions may be required where a num- 

 ber of cuts is to be taken. A check angle should be taken at each position. It is 

 important that the cut be referenced to a control station close to the object, if available; 

 the fix itself need not necessarily include this control station. 



