2341 HYDROGRAPHIC MANUAL PaGE 92 



view of a hydrographer surveying the adjacent waters from a launch. He should place 

 the temporary signals, whitewashes, banners, tripods, etc., at points best situated to 

 give strong fixes throughout the entire area. If the topographer succeeds in this, 

 excellent control will be available for the hydrographic survey, and the hydrographer 

 will not be delayed by having to build. and cut in supplemental signals nor by the dif- 

 ficulty of identifying one signal from another as where they are unnecessarily numerous 

 and similar in appearance. 



2341. Hydrographic Signal Building 



Stations along a sandy beach or a low flat area should be placed well back from 

 the beach, where practicable, to provide stronger sextant fixes with fewer stations to 

 control the hydrography close to the shore. In any case they should be at least far 

 enough from the beach so they will not be destroyed by wave action due to high water 

 or storms. 



Where signals are built along an irregular coast, one should always be located at 

 the head of each inlet or small cove. Stations on cliffs, high hills, or bluffs are unsatis- 

 factory for sextant fixes close inshore, because the sextant may have to be tilted so far 

 from the horizontal plane to measure the angles that corrections have to be applied 

 (see 3338). 



The signals need not be erected with more permanency than is required to remain 

 intact until the hydrography in the area has been completed. Whitewashes, driftwood 

 tripods, flags, banners on stumps, wreckage, and numerous other objects make splendid 

 signals for launch hydrography when properly dressed. 



Care must be taken to vary the shapes, sizes, and spacing of the signals to prevent 

 confusion. Too much importance cannot be placed on the spacing of stations,-^ size of 

 the signal, and range of visibility. When viewed from a mile or two offshore, signals 

 spaced too close together, especially if they are small, dim, and of similar appearance, 

 may be difficult to identify when reflected in sextant mirrors. 



In the spacing of stations, one every half-mile may be sufficient if they can be 

 located a quarter of a mile inshore from the beach. Where the beach is irregular and 

 vision inland is obstructed by trees, bluffs, cliffs, etc., stations spaced 300 to 400 meters 

 apart will usually be sufficient. Signals widely spaced, unless they are well back 

 from the beach, furnish weak fixes for positions close inshore. 



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. Whitewashing or signal building in the fog is unsatisfactory and should not 

 be resorted to; vision is hmited and distances cannot be judged correctly, with the 

 result that the stations are frequently too close together and often are all of similar 

 appearance. 



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 and shape so as to be quickly and unmis- 

 takably identified, then the control will be adequate for hydrographic surveying. 



235. Recoverable Topographic Stations 



Changes in topography and culture occur as the country develops, or as a particular 

 area increases in importance, and revision surveys become necessary. Such surveys 

 should not be undertaken until a thorough search has been made for stations estabhshed 



