Page 193 hydrography 3115 



direct measurement by leadline or sounding pole (see 3421, 461, and 462), and (c) 

 direct measurement by sounding machine and wire (see 3422 and 463). The approved 

 types of equipment for use with each method are described under the appropriate head- 

 ings. Echo-sounding instruments and sounding machines are purchased by the Wash- 

 ington Office and only approved types are furnished. 



3115. Obsolete Methods of Depth Measurement 



The following methods and equipment are considered obsolete and shall no longer 

 be used in hydrographic surveys: 



a. Pressure tubes. — Echo sounding in recent years has rendered obsolete the use of pressure tubes 

 for depth measurement, and their use is not approved. Various types of pressure, or sounding, 

 tubes have been in use for many years in navigation and hydrographic surveying. The principle of 

 the method depends on the fact that when a tube of small diameter, closed at one end, is lowered to the 

 bottom, the water pressure forces water into the tube, compressing the air against the closed end — the 

 amount of compression depending on the water pressure, which is a function of depth. 



The type of tube formerly used in the Coast and Geodetic Survey was designed in the Bureau. 

 The tube was made of brass and designed so as to retain the water which entered it on descent. The 

 amount of water was measured by inserting a brass rod just far enough to bring the water level to the 

 top of the tube. The depth was then found by comparing this length of brass rod with a suitably 

 graduated scale. 



Chemical tubes are best known in navigation. The inside of a glass tube is coated with a chemical 

 that changes color when it comes in contact with sea water. Of course the water runs out of the tube 

 as it is raised to the surface, but a difference in color marks the extreme point to which the water rose 

 in the tube, so that the depth can be measured by reference to an appropriate scale. Tubes, frosted 

 on the inside, are also used, the frosted interior appearing more opaque where dry than where wet. 



b. Trolley soundings. — Echo sounding has also replaced trolley soundings entirely and the use of 

 this method is not approved. This was a method of measuring depths vertically by leadline or wire, 

 while underway, beyond the limit possible by hand leadline. The leadsman was stationed aft but the 

 lead was dropped from a point far enough forward along the ship's side so that when it reached bottom 

 it was vertically below the leadsman. The lead was carried forward to this point on a carriage rigged 

 on a wire. The line was rove through a system of sheaves to the sounding machine for heaving 

 in. 



c. Sonic Depth Finder. — Several types of echo-sounding instruments have been used by the 

 Coast and Geodetic Survey which are now considered obsolete for hydrographic surveying. Among 

 these are the Sonic Depth Finder, described briefly in 5131, and the hammer- or striker-type 

 fathometers (see 5161D), among which are the 412 Fathometer (see 5133) and the 432 and the 515 

 Fathometers. 



312. Project Limits 



The limits of the project will ordinarily be stated in the project instructions. 

 When no reference is made to the offshore limits the hydrography is to be extended 

 as far as the methods of control in use warrant, taking into consideration the desir- 

 ability of surveying offshore submarine features of value to navigators using echo- 

 sounding instruments (see 122) . 



3121. Inshore Limits in Protected Waters 



In protected waters the hydrographic survey shall extend as close to the high- 

 water line as practicable. The low- water line should be fully developed by the hydro- 

 graphic survey in all areas where the range of tide permits. The survey should be 

 planned so that sounding lines can be run close to the shore during periods of high tides 

 and calm weather. Tliis will result in a definite determination of the low-water line 

 and in much of the area between the low- and high-water lines being surveyed. 



