Page 383 equipment and instruments 46 



46. DEPTH FINDING EQUIPMENT AND INSTRUMENTS 



The instruments and accessories used by the Coast and Geodetic Survey for hand- 

 lead and wire soundings are described in this section. Echo-sounding instruments are 

 described in chapter 5. The wire drag and sweep are described in Special Publication 

 No. 118, Construction and Operation of the Wire Drag and Sweep. 



461. Sounding Pole 



Shallow depths over an extensive tidal flat in protected waters can be measured 

 more easily and accurately with a sounding pole than with a leadline. The soundings 

 are read to the nearest half-foot, but are limited to depths not exceeding 12 feet. 

 Between soundings the pole is not raised out of the water, but is turned end for end, 

 alternately. This permits soundings to be taken faster, provides a close spacing of 

 soundings, and is easy for the leadsm.an. 



The sounding pole is a 15-foot length of l}^-incli round lumber, capped with a 

 metal shoe at each end which may be weighted to hasten sinking. Any convenient 

 system of marking which is symmetrical toward both ends and minimizes errors may 

 be used. The following system has proved satisfactory: Mark each foot and half- 

 foot permanently by a small notch cut in the pole. Paint the entire pole white, and 

 the spaces between the 2- and 3-, the 7- and 8-, and the 12- and 13-foot marks black. 

 Other foot marks are indicated by }2-inch colored bands, red at the 5- and 10-foo,t 

 marks, and black at the 1-, 4-, 6-, 9-, 11-, and 14-foot marks. Half -foot marks are 

 }4-inch colored bands, white where the pole is black and vice versa. 



462. Leadline 



A leadline is a length of sash cord or tiller rope to which a sounding lead is attached, 

 and is used for measuring depths of water. It is graduated by fathom or foot marks and 

 seizings. The lead is lowered until it touches bottom and at the instant the line and 

 lead are vertical and taut and the lead is on the bottom, the depth is read from the 

 markings on the leadline. 



To avoid large corrections to the observed soundings, it is important to use a lead- 

 line material that will not stretch nor shrink with prolonged use. The Coast and 

 Geodetic Survey has adopted, as standard leadline material, mahogany-colored tiller 

 rope with a phosphor-bronze wire center, of six strands of seven wires, each No. 33 B. 

 and S. gage. The wire core should not break with prolonged flexing. The rope is size 

 No. 8, about one-quarter inch in diameter, made of solid-braided long staple cotton, and 

 waterproofed. The braid should be tight so that wire strands that break are less 

 likely to protrude through the rope covering to injure the leadsman's hands. 



In 1941, Samson mahogany tUlcr rope, manufactured by Samson Cordage Works, 

 Boston, Massachusetts, was found to be most suitable. The Washington Office will 

 furnish this material upon requisition. 



A lead weighing not less than 8 pounds should be used for handlead sounding in 

 depths to 7 fathoms. For greater depths, a 12- to 14-pound lead should be used. (See 

 4661.) For sounding where there are subsurface currents a still heavier lead may be re- 

 quired to ensure vertical casts. A leadsman should not be required to sound with an 

 excessively heavy lead for long periods of time without relief. To avoid subjecting a 

 leadline to different tensions, which may introduce errors in the soundings, the same- 

 sized lead should always be used on the same line. 



