120 



HYDROGRAPHIC MANUAL 



Where the stations are located and spaced so 

 that strong fixes will be available at any 

 point in the area and are varied in size, 

 shape, or color so as to be quickly and un- 

 mistakably identified, then the control will 

 be adequate for hydrographic surveying. 



Stations along a sandy beach or low flat 

 area should be placed well back from the 

 beach to provide strong sextant fixes with 

 fewer signals. In any case they should be at 

 least far enough from the beach so that they 

 will not be destroyed by wave action during 

 storms (see 5-40). 



Where signals are built along an irregular 

 coast, one should always be located at the 

 head of each inlet or small cove. Stations 

 high above the water on cliffs or bluffs are 

 unsatisfactory for fixes close inshore. Sig- 

 nals need not be established with more per- 

 manency than is required to remain intact un- 

 til the hydrographic survey is completed. 



Care must be taken to vary the size, shape, 

 elevation, and color of the signals to prevent 

 confusion. The largest and most conspicuous 

 signals should be placed at locations where 

 they can best be seen in the offshore areas 

 and at points which will provide strong 

 fixes. Smaller signals should be appropriately 

 spaced for inshore work. If the signals can 

 be built well back from the shoreline, a spac- 

 ing of approximately one every half-mile may 

 be sufliicient. Where the beach is irregular 

 and signals must be built close to the shore- 

 line, a spacing of 300 to 400 meters will be 

 adequate. 



4-29 Natural and artificial objects. — The 



term "signal" is used to designate any sort 

 of object, natural or artificial, for use in 

 measuring sextant angles to locate positions 

 of a survey unit while engaged in sounding. 

 It may be of any size or shape. It is obvious 

 that the measurement of angles will be fa- 

 cilitated when the objects at the control sta- 

 tions are conspicuous enough to be seen 

 readily by the observers. For this reason, 

 as well as for economy and durability, nat- 

 ural objects such as lone boulders, pinnacle 

 rocks, waterfalls, and lone trees, and artifi- 

 cial objects such as beacons, lighthouses, 



tanks, spires, and building gables, should be 

 used as signals wherever available. 



4—30 Signal building materials. — Where 

 natural objects are not available, the most 

 satisfactory and economic types of signals 

 are those made of whitewash or white cloth. 

 Whitewash properly brushed on solid rock 

 will make the most durable signal. Such sig- 

 nals can usually be made in sizes appropriate 

 to the distance at which they will be used. 

 The nature of the project and availability 

 of electronic equipment for offshore hydrog- 

 raphy are the determining factors which 

 influence distribution of large signals. There 

 should be enough large signals to provide 

 control for calibration of Shoran at maximum 

 distances. For the inshore launch work small 

 whitewashes or signals constructed of white, 

 red, or orange cloth in various shapes such 

 as cross banners, flags on stumps, or wrapped 

 on trees or poles are adequate. Black cloth 

 should be used if the signal shows against 

 the skyline. Any signal made of cloth, ex- 

 cept black, will show best when it reflects 

 sunlight and will provide the best reflection 

 if it is set at an angle of about 60 degrees. 

 Red or orange cloth can be seen for rela- 

 tively short distances, but provide a break in 

 a series of white signals which reduces pos- 

 sibilities of confusion. 



When cloth is used to construct a signal 

 it should be securely fastened with a stapling 

 gun or tacks and several 4 to 6-inch slits 

 cut to relieve wind pressure and discourage 

 vandalism. 



Whitewash is made of pulverized, un- 

 slaked lime. The lime deteriorates rapidly 

 and, when a stock must be kept on hand 

 over a considerable period of time, it should 

 be stored in water-tight metal containers. 

 The whitewash is prepared by filling a bucket 

 about one-quarter full of lime and adding 

 about one gallon of water. The mixture 

 should be stirred rapidly and a little water 

 added as necessary to keep it from boiling 

 over. A stick about 3 to 4 feet long should 

 be held at arms-length to stir the mixture 

 as it may spout up while boiling. The ex- 

 ploding bubbles may cause injury to the eyes 

 and the mixer should be careful to avoid such 



