5612 hydrogrAphic manual Page 530 



ayer and these are summed numerically or graphically to find the corrections for various depths, in 

 accordance with 5613 or 5615. 



(4) The velocity corrections for the various depths are then plotted and a graph is drawn through 

 them, from which the correction for any echo sounding may be scaled, as described in 5614. 



5612. Mean Regional Temperature and Salinity Curves 



It is desirable to correct registered soundings for the error due to velocity with as 

 great an accuracy as practicable, but it is not considered practicable to utilize each 

 serial temperature and each separate temperature and salinity observation indi- 

 vidually for this purpose. For practical reasons it is necessary to derive curves repre- 

 senting the average conditions throughout a given area and through a given time 

 period which will be close enough to reality so that no appreciable errors will be 

 introduced from the use of the averages. At all events the over-all requirements of 

 accuracy of depth measurements, as prescribed in 3111, must be met, and the velocity 

 corrections should be determined with an accuracy so that no sounding will be in error 

 from this correction alone by more than one-half percent. (See 6313.) 



To fulfill these requirements a sufficient number of temperature and salinity 

 measurements must be made in the project area. The frequency with which the 

 observations should be made is considered in 6322 and 6332. Previously acquired 

 knowledge of the temperature and salinity conditions in nearb^^ areas can sometimes 

 be used as a guide as to the probable number of observations required. Lacking any 

 previous knowledge of conditions in the area to be survej^ed, the observations should 

 be made wuth a greater frequency at the beginning of the project, and these will disclose 

 the necessary frequency and distribution of observations needed for the remainder of 

 the project. 



After all of the serial temperatures and separate temperature and salinity observa- 

 tions for the season have been plotted on Form 5-1528-5, a study must be made of the 

 results to see how these may be grouped by areas and by time periods, for the purpose of 

 preparing mean curves. The area and the time period for which a mean curve may be 

 adopted will vary from an entire season's work to the area surveyed in one trip on the 

 working ground. Past experience has shown that all of the serial temperatures of an 

 entire season's hydrography on the Pacific Coast, in Alaska, or in the Haw^aiian Islands 

 may often be combined for the average curves for an entire season. It is a matter that 

 must be left to the hydrographer's judgment, but it must be borne in mind that, to com- 

 ply with the requirements, the average temperature from surface to bottom used to 

 correct any sounding must be within 2° of the actual mean temperature (see 6313). 



A study of the serial temperatures may disclose that all of them can be combined 

 into one mean curve, except those taken in one locality or during one period of time. 

 When this is so, these latter serial temperatures should be segregated from the rest 

 and used for the derivation of supplemental mean curves for that particular locality 

 or time period. 



