Page 89 control and signal building 2311 



There are two general methods of locating signals for hydrographic control. One 

 method is by planetable, alidade, and stadia rods. This is the older method but is 

 being supplanted to a considerable extent by a new method, namely, air photography. 

 By both methods stations along or just inshore from the beach are located for use as 

 control points for the hydrography. The positions of these stations are plotted on the 

 boat sheet from which the hydrographer determines his position on the water by means 

 of sextant angles observed between three such stations. 



When survey parties are engaged in combined operations, the topographic surveys 

 naturally precede the inshore hydrography. The signals, shoreline, and other detail 

 are generally located on aluminum-mounted sheets by conventional topographic 

 methods, and are transferred to the boat sheets in sections as they are completed. 

 In the average area one planetable party can keep well ahead of one launch party 

 sounding withhandlead or wire. The recent use of portable graphic-recording echo- 

 sounding instruments has doubled and, in some areas, more than doubled the progress 

 of launch hydrography, and consequently its progress is now dependent to a great 

 extent on the speed at which the topographic parties operate. 



2311. Reference Plane 



The plane of reference used by the Coast and Geodetic Survey for the elevations 

 and contours for topographic surveys along coastal areas is mean high water (MHW), 

 regardless of locality. It is never mean higher high water. The plane of reference for 

 elevations must not be confused with the plane of reference for depths, which is depend- 

 ent on the locality (see 2172), or the plane of reference for the control of precise leveling, 

 which is mean sea level. 



2312. Offshore Details 



The topographer shall locate all rocks, pinnacles, reefs, etc., along the shoreline, if 

 possible. Breakers or sunken rocks, when noted, shall be located approximately, if 

 their positions cannot be determined definitely, so that the hydrographic party may 

 know in advance of the existence of such dangers in order to locate them definitely 

 when working in that area. (See also 363, 364, and 365.) 



2313. Method of Location 



Except for air photographic surveys, it is assumed that conventional planetable 

 methods will be used to locate the topographic stations, and since a survey by planetable 

 is a graphic survey there are no notes nor record of any of the measurements by which 

 the positions are determined. 



When special or unconventional methods are used to locate a station or group of 

 stations in an area, special mention of this shall be made in the Descriptive Report 

 accompanying the topographic survey. If all stations are located by conventional 

 planetable methods, a statement to this effect shall be included in the Descriptive 

 Report. 



When a new method is used successfully, a special report shall be made to the 

 Washington Office describing the method in detail with particular emphasis on the 

 probable accuracy obtained by its use. 



232. Accuracy of Topography 



One of the principal objects of the topographic survey is to locate the minor control 

 stations for the sucsequent use of the hydrographic party. Such methods and instru- 



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