554 HYDROGRAPHIC MANUAL PaGE 514 



The tests should be made at a place where the bottom is known to be smooth 

 and level and in a depth of water which is approximately seven times the draft of the 

 survey vessel. If the survey vessel is habitually used to survey in depths considerably 

 less than this, an additional test should be made at the lesser depth. 



First method: A leveling instrument may be mounted on shore, preferably on the end of a pier 

 ■off which are the required conditions as to depth and bottom, and past which the vessel can run at 

 normal sounding speed. A marker buoy should be anchored with a short scope at the point where 

 the test is to be oaade. 



With the vessel stopped at the marker buoy, a level rod is held on board the vessel vertically 

 over the transmitting and receiving units, or over midway between them if one is forward of the 

 •other, and the level rod is read with the instrument on shore. The height of the tide should be noted. 

 Then the vessel should run past the marker buoy at normal sounding speed, with the rod held over 

 the same spot, and the rod should be read again with the same instrument on shore. The difference 

 between the two readings, corrected for tidal changes, will be a measure of the combined effect of 

 settlement and squat at the location of the acoustic units. Several such tests should be made and 

 a mean of the results used. 



Second method: Select an area which satisfies the requirements as to depth and bottom and 

 anchor a marker buoy with a short scope. With the vessel stopped alongside the marker buoy the 

 •depth of water should be measured accurately with an echo-sounding instrument. Then the vessel 

 should run past the marker buoy at normal sounding speed, taking another accurate echo sounding 

 when in the same position relative to the buoy. Provision must be made for a record of the tidal 

 change during the test. The difference between the echo soundings underway and stopped, corrected 

 for change in tide, will be the combined amount of settlement and squat. The test should be re- 

 peated several times and the average value determined. 



554. Adjustment of Index 



All echo-sounding mstruments used by the Coast and Geodetic Survey are pro- 

 vided with facilities for adjusting the index. Some instruments, such as the 312 

 Fathometer and the Veslekari, have one adjustment for the time of keying and another 

 adjustment for the position of the index on the dial, which two adjustments are made 

 independently. Other instruments are constructed so that the position of the index 

 registered on the dial is governed by the position of keying, and to these only one 

 adjustment is necessary. 



With the draft, instrumental error, and settlement and squat, determined, the 

 index of the echo-sounding instriunent should be adjusted for the algebraic sum of 

 these, by the methods described under the following headings for each type of instru- 

 ment. For transceivers and separate transmitting and receiving units that are elec- 

 trically coupled, the transmitted signal should register on the dial at a depth equal 

 to the algebraic sum of the corrections. For other instruments such as the 312 Fath- 

 ometer, the registration should be at a depth equal to the algebraic sum of the correc- 

 tions plus one-half the distance of separation (see 556), A notation shall be made in 

 the Sounding Record each time the index of an echo-sounding instrument is readjusted. 



5541. Dorsey Fathometer No. 1 



The index of the Dorsey Fathometer No. 1 is adjusted by means of a knurled 

 knob centrally located on top of the indicator cabinet. A lock nut at the base of the 

 knob must be loosened before an adjustment can be made, and after the adjustment 

 the lock nut must be tightened. If it is not tightened firmly, the index may shift 

 during operation. The fathometer attendant should be instructed to check the index 

 adjustment at frequent intervals by observing the registration of the transmitted signal 



