80 



U. S. COAST AND GEODETIC SURVEY 



255. Time allowance. — When there is much difference in the time 

 or height of the tide at the place of sounding and at the tide gage, 

 allowance should be made in the reduction of the soundings. The 

 difference may generally be estimated from observations made at 

 several stations in the vicinity of the work, but when it has been 

 impossible to establish more than one tide station in the locality, the 

 following formula may be useful in estimating the velocity of a 

 progressive tidal wave and enable one_to obtain the approximate 

 difference in the time of the tide: r= -yj gd=5-Q7^d feet per second, 

 when ^ = 32.17 feet per second and d= depth of water for the average 

 cross section between stations, in feet. 



In order to convert feet per second into nautical miles per hour, 



aeoo 



multiply by ^QgQ =0.592, and we have 



r= 3. SGVc? nautical miles per hour 

 The time required for the tide wave is 



t = 



6080 



17.87 



60X5.672 V^^'V?" "''''''*^' P^"" ^^^*^^^^ ^^^^'• 



5280 15.51 



^^60X5.6727^/^71""^"'"*^' P^' '*^*"*' "'^'- 

 For convenience the following brief table is given : 



, Time required for the tide wave to travel 



256. For offshore areas where the continental shelf is broad and 

 the tidal wave approaches parallel to the coast the tide will arrive off- 

 shore earlier than inshore, and for an accurate reduction of soundings 

 a time correction will be necessary. This time correction can be 

 applied from the shore outward by taking sections where the time of 

 the tide averages 15 minutes, 30 minutes, 45 minutes, 1 hour, etc., 

 earlier than at the tide station. These sections can be determined 

 by means of the above table, giving the time required for the tide 

 wave to travel at different depths. 



257. Height Allowance. — When the tide station used for deriving 

 the tide reducers is made to cover an area over which the range of 



