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334. Large-Scale Surveys 



More precise methods of control on larger scales are necessary for unusually intense 

 surveys of important localities, or detailed surveys to verify dredging operations in the 

 vicinity of docks or slips or for use in planning subsurface construction of any kind. 

 The methods to be used in such cases may be grouped in three classes: 

 (a) Surveys controlled entirely on board the vessel. 

 (6) Surveys controlled entirely from shore. 

 (c) Surveys controlled partly on board and partly ashore. 



3341. Surveys Controlled Entirely on Board 



Surveys on particularly large scales (usually larger than 1 : 10,000) may be controlled 

 entirely on board the vessel with the usual sextant three-point fix method, but special 

 precautions must be taken to guard against errors that would not be appreciable on 

 smaller scales. The control must be located more accurately. The survey vessel 

 must proceed at slow speed and the strongest fixes must be chosen. The observers 

 should stand close together and close to the sounding apparatus when marking the 

 angles. The angles must be marked simultaneously. The soundings must be vertical. 

 The exact times of soundings and positions must be noted with great care. 



It is impracticable to make such a survey satisfactorily without ranges. Parallel 

 ranges equally spaced may be established in advance for the special use of the hydro- 

 graphic survey party, or if a prominent object in the distant background is where it 

 can be seen throughout the area to be surveyed, it may serve as the rear range mark 

 to a series of accurately located front range marks along the shore. In the latter case 

 the sounding lines radiate from the rear object, but if it is sufficiently distant, the diver- 

 gence in the spacing between lines may not be detrimental; in fact it may be advanta- 

 geous as it results in a spacing increasing proportionately to the distance offshore, which 

 may be desirable if the depths of water increase as one goes offshore. Radiating lines 

 are of particular advantage for surveying around points of land. 



For most precise operations, ranges on shore, either parallel or radiating, should 

 be established by a topographic party, the distances between them being measured 

 with a tape. 



3342. Shore-Controlled Surveys 



For surveys of the greatest accuracy in river and harbor work the positions of the 

 soundings are determined by means of two or more theodolites or transits set up on 

 shore at suitably situated triangulation stations. Two stations will usually be suffi- 

 cient to determine positions, but three are advisable to provide for a check at each 

 position and to ensure that a position is obtained in the event that one shore observer 

 misses his observation because of an intervening object or for any other reason. 



In the most precise surveys, the depth of water permitting, the depths are measured 

 in feet and tenths on a graduated pole, simultaneous cuts being taken from the instru- 

 ment stations on shore when the pole is in a vertical position. When a position is 

 required the shore observers are notified by a flag being displayed on the vessel; this is 

 dropped at the instant the observation is required. Rarely is every sounding so located ; 

 it is usually sufficient to locate only every third or fourth sounding, according to the 

 accuracy demanded. 



The soundings on the vessel must be coordinated with the shore observations and 

 it is essential that the timepieces used be in agreement, and if practicable be compared 

 several times a day, and if necessary reset. The times of the positions and of the 



