82 HYDROGRAPHIC MANUAL PaGE 770 



changed during the course of the survey. This is not necessary for launches and small boats using 

 portable compasses. (See 144.) 



d. Index of cuts. — An index of all objects such as signals, survey buoys, aids to navigation, land- 

 marks for charts, rocks, and breakers, the positions of which have been determined by sextant angles, 

 shall be entered as follows: If a separate volume contains all such data, the index shall be entered on 

 page 2 of that volume; if the data are interspersed through the Sounding Records, the index shall be 

 entered on page 2 of volume No. 1, giving the volume and page numbers on which are recorded all 

 data for the location of any given station or object (see 248). 



References to positions taken to determine the azimuths of ranges shall be included in this index 

 (see 7845). 



e. Index of hydrographic information. — A separate index of special hydrographic information such 

 as currents, tide rips, overfalls, and swirls shall be entered on page 2 of volume No. 1. Any detached 

 determinations of the character of the bottom not recorded in the Sounding Records shall also be 

 indexed. 



/. List of stations. — An alphabetical list of the stations used on each hydrographic sheet shall be 

 prepared, in accordance with 2154. If typed, this should be pasted inside the title page of sounding 

 volume No. 1 for that sheet or, if desired, it may be lettered on the first record pages, from which 

 hydrography has been omitted for this purpose. 



Where a station has been relocated by a more accurate method, after having been used on the boat 

 sheet, this fact shall be noted in the list (see 241). 



g. Buoy positions. — Where buoys have been located by sextant cuts recorded here and there 

 throughout the Sounding Records, the cuts are to be indexed in accordance with d "Index of cuts," 

 Where final buoy positions based on these cuts have been determined by the field party by computa- 

 tion, by plotting on an aluminum sheet, or otherwise, that fact must be made clear in the index. Those 

 cuts that are to be plotted on the smooth sheet to determine buoy positions shall be properly identified. 



All buoy stations whose final positions have been determined by the field party, by any method 

 other than by plotting on the smooth sheet, must be included in the list of buoy stations required 

 in the Descriptive Report (see 8435). 



82. REDUCTION OF SOUNDINGS 



Recorded soundings must be corrected for any departure from true depth due to the 

 method of sounding or to a fault in the measuring apparatus and for the height of the 

 tide above or below the plane of reference at the time of sounding. Vertical columns 

 are provided in Form 275, Soundings, in which to enter the corrections and the soundings 

 after reduction. 



821. Correction Units 



Corrections (sometimes called reducers) shall be entered in the same unit in which 

 the soundings have been recorded (see 3112 and 8143). Parts of units shall be entered 

 in decimals — fractions shall not be used. Where the soundings have been recorded in 

 feet, the corrections shall be entered in feet and tenths; where the soimdings have been 

 recorded in fathoms, the corrections shall be entered in fathoms and decimals. Lines 

 shall be drawn thi'ough the inapplicable unit at the top of each column on each page. 



822. Corrections to Recorded Depths 



Each correction, preceded by the proper arithmetic sign, shall be entered on the 

 horizontal line opposite the first sounding to which it is applicable and need not be 

 repeated, except opposite the first sounding on each page and at the beginning of each 

 sounding line. A correction or sign once entered shall be considered applicable to 

 all following soundings until a different correction or sign is entered. 



