178 



HYDROGRAPHIC MANUAL 



an average load and be in average trim. This 

 value may be assumed to be a constant for 

 the season's work. Where the result of the 

 test shows that the combined effect of set- 

 tlement and squat is less than 0.2 foot, it 

 may be neglected, but if it is more than this, 

 an instrumental adjustment should be made 

 to all echo-sounding instruments used in 

 shoal water to compensate for it, or an arith- 

 metical correction must be made to the 

 soundings. 



Where the index has been adjusted to com- 

 pensate for the amount of settlement and 

 squat at normal sounding speed, it is neces- 

 sary to make an arithmetical correction to 

 any soundings taken from the vessel while 

 stationary or running at slow speed. 



Either of two methods may be employed 

 to determine the combined effect of settle- 

 ment and squat, but the first method de- 

 scribed is preferred. In both, the tests 

 should be made at either high or low water, 

 when the tide level is varying slowly. Pro- 

 vision must be made to measure any tidal 

 change which does occur during the tests. 



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 approxi- 

 mately 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 condi- 

 tions 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 made. 



With the vessel stopped at the marker buoy, 

 a level rod. is held on board the vessel ver- 

 tically over the transmitting and receiving 

 units, or over the mid-point 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 



on the same spot, and the rod should be read 

 again with the same instrument on shore. 

 The difference between the two readings, cor- 

 rected 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 satis- 

 fies the requirements as to depth and bot- 

 tom 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 repeated several times and 

 the average value determined. 



5-109 Adjustment of index. — All echo- 

 sounding instruments used by the Coast and 

 Geodetic Survey are provided with facilities 

 for adjusting the index, or initial setting. On 

 portable sounders the adjustment is accom- 

 plished by means of a knob on the cover of 

 the instrument. Adjustment of the index on 

 the EDO-185 is less easily accomplished and 

 shall be made only by a qualified technician. 



The index may be set to compensate for 

 the algebraic sum of the draft of the trans- 

 ducer, settlement and squat, and instrumen- 

 tal error, but it is more easily maintained 

 on one of the printed lines on the fathogram. 

 The index should be maintained at the 

 adopted position and arithmetical corrections 

 to soundings are required for variations from 

 the correct setting. 



When the speed of operation of an echo 

 sounder is changed the value of the initial 

 setting is changed. For example, an 808 

 recorder can be operated to sound in units of 

 feet or fathoms. If the initial is set at 2 feet 

 and the mode is changed from feet to fath- 

 oms, the value of the initial setting is 



