Page 527 echo sounding 5572 



holes are bored in the bar to permit the free flow of water, it can be kept in position 

 more easily. At each end of the bar there should be a wire or rope yoke in the form of 

 an inverted V to which the supporting lines are attached. The supporting lines should 

 be wire or have wire cores, like the material used for leadlines. The lines should be 

 marked and then lengths should be verified in accordance with the instructions for the 

 leadline (4621 and 4622). Measurements are made from the upper surface of the bar. 



If the bar is made of wood, a type suitable for submersion, such as cypress, should 

 be used. The bar should be kept well painted, for waterlogged wood is inefficient as a 

 reflector. Narrow metal strips should be fastened to the underside of the bar to prevent 

 warping. 



Two weights should be attached, one at each end, to the underside of the board 

 to keep the supporting lines taut. 



Under favorable conditions satisfactory reflections should be received from a bar 



9 inches wide to a depth of 15 fathoms; but under unfavorable conditions and at greater 

 depths, more distinct echoes will be received from a wider bar. 



Air is an almost perfect reflector of sound in water. If any difficulty is experienced 

 in receiving echoes from a bar in calm waters, construct a bar which incorporates a thin 

 layer of air near its upper surface. A cellular rubber sheeting will serve for this purpose. 



5572. Frequency of Bar Checks 



For echo sounding with an 808 Fathometer, or similar instrument, used in a launch, 

 bar checks shall be made under the following conditions, and at the following frequencies: 



(a) In protected waters where there is every reason to believe the results of the bar checks are- 

 dependable, and the range of depths sounded can be covered by the bar-check range, the index shall 

 be adjusted by bar check, and bar checks shall be made on the descent and ascent of the bar at each 



10 feet throughout the range of depths sounded — three times daily, at the beginning and end of the 

 day's work and once near the middle of the day. 



(5) Where most of the depths sounded are beyond the range of the bar check, other conditions 

 being as in (a), the index shall be adjusted by bar check, and one check shall be made at the deepest 

 depth at which a dependable result can be obtained, and one check shall be made at a depth in the 

 overlap between phases — three times a day as in (a). 



(c) In exposed waters, where dependable results cannot be obtained, the index shall be adjusted 

 by bar check (preferably in a protected place) — at least twice, and preferably three times daily — but 

 no other bar checks shaU be made. 



(d) In adjusting the index (see 5545), before the start of the day's hydrography the index shall 

 be adjusted to make the bar depth record correctly, but after any sounding the bar depth reading must 

 be recorded on the fathogram before the index is adjusted as well as after it has been adjusted. 



(e) Where the results of bar checks are to be used for correcting or compensating for the velocity 

 of sound (see 5617), not less than three complete bar checks per day shall be made, the bar check must 

 cover at least 75 percent of the range of depths sounded, and the bar checks must be most accurate- 

 and dependable (See also 5616a and 5617.) 



56. CORRECTIONS TO ECHO SOUNDINGS 



In addition to the corrections that are so nearly constant that they can be made by 

 some instrumental adjustment, there are other corrections that vaiy with time, place, 

 and depth to such an extent that instrumental compensation is usually not attempted. 

 These latter are the correction for tide, the correction for a velocity of sound differing 

 from the calibration velocity of the instrument, and the correction for slope. 



The correction for tide is usually applied algebraically — it is always so made in 

 the Coast and Geodetic Survey to soundings recorded in the Sounding Record, regard- 

 sles of the method of obtaining the soundings, and in accordance with 822. In river 



