Page 773 records and reports 8224 



Echo-sounding instruments are intended to be adjusted and operated so that tlie 

 velocity and tide corrections are the only corrections that need be applied to echo 

 soundings (see 55 and 56). If for any reason, an additional correction is found to be 

 necessary, it should be entered separately in the ''Correction" column headed "Slope" 

 and appropriately headed. 



The effect of an erroneous motor speed on the calibration velocity should be noted 

 (see 5553). This should be taken into account by basing the velocity corrections on 

 the actual instrumental velocity instead of on the standard calibration velocity (see 

 table 20). 



8224. Tide Reducers 



The recorded soundings must be corrected for the height of the tide above or 

 below the plane of reference at the times of the soundings. These corrections are 

 entered in the ^'Correction" column headed "Red. for Tide." 



The planes of reference adopted for the reduction of soundings and the publica- 

 tion of charts of the Coast and Geodetic Survey are as follows (see also 2172): 



(o) For the Atlantic Ocean and Gulf of Mexico — the mean of the low waters (MLW). 

 {b) For the Pacific Ocean — the mean of the lower low waters (MLLW) — except for the Pacific 

 entrance to the Panama Canal where it is the mean of the low water springs (MLWS). 



(c) For certain of the larger navigable rivers and lakes special planes have been adopted. 



The location of the tide gage to be used in connection with each day's work shall 

 be entered in the appropriate space in Stamp No. 38, Processing (fig. 183), at the end 

 of the day (see also 4672). 



The tide reducers are derived from marigrams from automatic tide gages, generally 

 established especially for this purpose, or occasionally from observed tides (heights 

 read from a staff). 



After the height of the plane of reference for the particular gage or staff has been 

 received from the Washington Office (see 1215) the reducers can be scaled directly from 

 the marigrams, if available, and entered in the Records. 



In most cases the marigrams or records of staff readings must be sent to the 

 Washington Office before the plane of reference is determined. In such cases copies 

 must be made of those parts of the marigrams needed, or the hourly heights must be 

 scaled, from which curves can be reconstructed. The reducers can then be scaled 

 from the copies or the reconstructed curves when needed. 



Full instructions for deriving the reducers are contained in Special Publication No. 

 196, Manual of Tide Observations. 



823. Units of Final Reduced Soundings 



The algebraic sum of all corrections applicable is added to or subtracted from each 

 sounding, the result being entered in the double column headed "Field" under "Re- 

 duced Soundings." These reduced soundings will be in the same depth units as the 

 recorded soundings. The unit shall be indicated by drawing lines through the inappli- 

 cable subheadings at the top of the double column on each page. (See fig. 172.) 



This may result in some reduced soundings being in fathoms and tenths and some 

 in feet and tenths on the same hydrographic survey, or even on the same page of the 

 Record. In the latter case the change of unit must be boldly indicated, at the place 

 in the column where it occurs. 



Only one depth unit is used on one hydrographic sheet (see 771). For convenience 

 in plotting, all soundings shall finally be converted to the one depth unit to be used 



