4. CONTROL AND SIGNAL BUILDING 



conspicuous points, rock outcrops, and large 

 boulders. 



All topographic stations specially marked 

 for future recovery shall be marked with a 

 standard bronze topographic station disk 

 stamped with the name and year of estab- 

 lishment. Reference marks are not set at 

 topographic stations. The stations may be 

 located by planetable or photogrammetric 

 methods (see 4-7). 



4-25 Description of recoverable topo- 

 graphic stations. — New recoverable topo- 

 graphic stations shall be described on Form 

 524. If the station is located by photogram- 

 metric methods a C.S.I, card, Form 152, or 

 153, shall be submitted also. Form 524 

 should contain a full description of the sta- 

 tion just as for a triangulation station (see 

 4-5). If there are suitable reference points 

 in the vicinity, the distances and directions 

 to them should be shown in a sketch. 



The same form shall be used to report the 

 recovery of a topographic station. A con- 

 spicuous capital R should be printed in the 

 upper right-hand comer of the card. If the 

 original description is correct and adequate 

 a statement to this effect is sufficient. If 

 it is inadequate a complete new description 

 should be written. If the station is not re- 

 covered, state whether it should be con- 

 sidered lost, giving particulars, and the time 

 spent in searching for the station. 



4-26 Sextant angle location of signals. 



Occasionally it is necessary to locate a sig- 

 nal by sextant angles to supplement the 

 established control. The station may be lo- 

 cated by observing a strong three-point fix 

 at the station (see 1-18). A check angle to 

 a fourth station should always be measured. 

 A navigating sextant should be used and 

 wherever practicable the angles should be 

 observed to triangulation stations. The posi- 

 tion on the boat and smooth sheets may be 

 plotted with a three-armed metal protractor 

 or the position may be computed. 



The station may be located by fixing the 

 position of the survey vessel by strong three- 

 point sextant fixes and simultaneously ob- 

 serving a cut to the station. At least three 



119 



well-intersected cuts are required. The ves- 

 sel should be stationary when the cuts are 

 observed. Accurate results cannot be ob- 

 tained from a vessel underway. 



A third method which may be used is to 

 occupy three or more control stations with a 

 sextant, observing at each an angle from an- 

 other control point to the new station. The 

 stations should be selected to provide strong 

 intersections of the cuts. 



Stations located by sextant angles shall not 

 be used for the purpose of locating other 

 stations except as a last resort. 



4-27 Unconventional methods of signal 



location.— Waterways with sufficient depth 

 of water, or important enough to be useful 

 in navigation, require accurate, detailed sur- 

 veys, and the control in them must be estab- 

 lished by conventional methods. In sloughs 

 through swamp or mangrove or in minor 

 tributaries, where the depth of water is 

 trivial, less accurate methods can be tol- 

 erated. The control may be established by 

 sextant triangulation. A traverse may be 

 run using sextant angles to carry the azimuth 

 and distances may be measured by stadia or 

 by sextant angle using a sextometer rod. The 

 hydrographer, by exercise of some ingenuity, 

 can devise methods appropriate to the in- 

 struments available, such as subtense bars, 

 range finders, or floating calibrated wire. 

 Sextant angles should always be measured 

 with a navigating sextant. The traverse or 

 graphic triangulation should be plotted on 

 an aluminum-mounted sheet, preferably at a 

 scale twice as large as that of the hydro- 

 graphic sheet. Such traverses should seldom 

 be more than a mile in length and the end 

 of the traverse should be firmly positioned 

 by connection to established control of a 

 higher order if possible without undue 

 expense. 



4-28 Hydrographic signal building. — 



Signals appropriately located and erected are 

 a very important prelude to successful launch 

 hydrography. The ease and smoothness with 

 which a launch unit operates depend to a 

 large extent on the competency with which 

 the signal building party does its work. 



