3. EQUIPMENT AND INSTRUMENTS 



69 



the current. If calibration cannot be ef- 

 fected alongside the buoy, one of the follow- 

 ing methods may be used : 



If the distance circles curve very slowly, 

 calibration may be effected by circling at 

 slow speed close to the buoy. Scale the ap- 

 proximate azimuth from the ship to the buoy 

 along the distance arcs through the corrected 

 buoy position. Read the phasemeter dials as 

 the ship crosses the distance arcs as shown 

 by pelorus bearings. Correct the observed 

 values as above and compare with the buoy 

 distance arcs. The ship will cross each arc 

 twice in one complete circle. If the two 

 values do not check within a few tenths of a 

 lane, repeat the observations until a check 

 is obtained on opposite headings. 



While stopped and drifting near the buoy 

 observe Raydist distances, true bearing to 

 the buoy, and a range finder distance or 

 depression angle to the buoy simultaneously. 

 Apply ship's head corrections and correct 

 i?2 distance if three-antenna system is used 

 at the Red station. Scale off the distances 

 corresponding to the position determined by 

 the distance and bearing to the buoy and 

 compare them with coirected observed dis- 

 tances. The calibration sheet should be on a 

 scale larger than that of the survey sheet. 

 At least three comi)arisons should be made 

 at various positions with respect to the 

 buoy, and the mean used for subsequent 

 corrections. 



3-61 Calibration records and report. — All 



calibrations and checks on calibrations shall 

 be recorded in the sounding volumes or in a 

 separate volume used exclusively for this 

 purpose. The corrections should be shown 

 on the plotting abstracts. All gains or losses 

 in lanes shall be noted on the abstract and 

 in the sounding volume as they occur, and 

 the time noted. 



A table of corrections shall be compiled 

 and submitted with a special report on Ray- 

 dist corrections at the end of each season or 

 project (see 7-27). A copy of the correction 

 tables for each survey sheet shall be included 

 in the Descriptive Report to accompany it, 

 and reference shall be made to the special 

 report (see 7-9). 



3-62 I.H.B. Special Publication No. 39.— 



The electronic equipment used by the Coast 

 and Geodetic Survey has been briefly de- 

 scribed in the preceding paragraphs. For a 

 more complete discussion of these and other 

 radio aids used in hydrographic surveying, 

 see Special Publication No. 39, Radio Aids to 

 Marine Navigation and Hydrography, pub- 

 lished by the International Hydrographic 

 Bureau in July 1955. 



Depth-finding Equipment and Instruments 



3-63 Standard depth-finding equipment. 



— All soundings for hydrographic surveys 

 shall be obtained by one of the following: 

 sounding pole, leadline, wire sounding ma- 

 chine, 808 type Fathometer, EDO 255 echo 

 sounder, or the EDO 185 (often referred to 

 by military designation UQN) echo sounder 

 with or without a Precision Depth Recorder 

 attached (see 1-35). Other types of echo 

 sounders or sounding equipment shall not 

 be used for hydrography except by specific 

 authority of the Director. Clearances over 

 obstructions, when required, shall be deter- 

 mined by Wire Drag methods as described 

 in Publication 20-1. 



3-64 Sounding pole. — Shallow depths 

 over an extensive flat area in protected wa- 

 ters can be measured more easily and ac- 

 curately with a sounding pole than with a 

 leadline and a pole shall be used in areas 

 too shoal for echo sounders. The soundings 

 are read to the nearest half-foot, but are 

 limited to depths not exceeding 12 feet. The 

 sounding pole is seldom used in general 

 depths greater than 6 feet except to provide 

 supplemental soundings to assist in inter- 

 pretation of fathograms when sounding in 

 areas where grass or other vegetation may 

 obscure the bottom trace. 



The sounding pole is a 15-foot length of 

 11/2 inch 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 is satisfactory : 



Mark each foot and half -foot permanently 



