40 



-10 



-6 



+ 1 



+ 2 



HYDROGRAPHIC MANUAL 



11 12 13 14 15 16 17 18 



19 20 



Figure 4. — Construction of predicted tide curve. 



dieted tide curve for a reference station is 

 furnished by the Washin^on Office, a time 

 correction and range factor will usually be 

 necessary to make the curve applicable to 

 the project area. These corrections will gen- 

 erally be indicated on the curve by the Office. 

 The predicted tide curve furnished by the 

 Washington Office is on a long roll of paper 

 6 inches in width. If it is to be used exten- 

 sively it will be convenient to construct a 

 small wooden box containing two spools, 

 each of which can be turned by a crank. By 

 these cranks the roll of paper can be wound 

 from one spool to the other, passing over the 

 flat top surface of the box where it is ex- 

 posed to view between the two spools. 



a. Time correction. — If the tides in the 

 area sounded occur earlier or later than at 

 the reference station, the original number- 

 ing of the hour marks must be decreased or 

 increased, respectively, by a corresponding 

 amount. The hour marks on the tide curve 

 are usually not numbered, but the beginning 

 of each day can be identified by a double 

 downward jog in the datum line. With a 

 time allowance of +1 hour this double jog 

 will be taken as 1 :00 to adapt the curve to 

 the working ground, but if the time allow- 

 ance is —1 hour the double jog will be taken 

 as 23 :00 or the preceding day. 



b. Range factor. — The allowance for the 

 diff"erence in range can be most conveniently 



