similar ranges in amplitude. The periods of tsunamis are always much shorter than tidal 

 periods. The periods of storm effects overlap the periods of both tsunamis and astronomical 

 tides but are less regular. Most of the instruments which record analog records have since 

 been replaced as standard instruments by digital gages which record the tide on a punched 

 paper tape at 6-minute intervals. 



3. Tide Observations and Tide Record Analysis. 



Standard tide records are routinely analyzed by tabulating tlie time and height of each 

 high and low astronomical tide and each hourly value. Most tide records compiled by the 

 NOS are used for defining tidal datums and the harmonic constants used for tide prediction. 

 Thus, a systematic analysis procedure is required for all records. Detailed instructions for 

 tide gage operation and data analysis are given in a manual published by U.S. Coast and 

 Geodetic Survey (1965). According to this manual ". . . the general trend of the curve, 

 rather than the individual peaks . . . ." are to be considered in tabulating highs, lows, and 

 hourly values. Thus, perturbations with periods of 2 hours or less should be smoothed in 

 determining the high and low waters and hourly values. Perturbations with periods of several 

 hours, such as that shown in the record for Atlantic City (Fig. 3), are included in tlie 

 tabulations. It is necessary to filter short-period oscUlations with periods less than 2 hours 

 from the hourly records because they cannot be properly represented, and including them in 

 the record would lead to errors in determining the harmonic constants used for tide 

 prediction. To include the effects of storm surges with durations exceeding 2 hours in the 

 records would facUitate the determination of the extreme high and low water level, and 

 because large storm surges are not common or very regular, it would introduce no more than 

 a trivial error in the determination of harmonic constants. Barbee (1965) presents an 

 additional discussion of the more recent types of tide gages adopted by the NOS. 



An average of all hourly tidal heights in a given month is taken as the average sea level for 

 that month. A 12-montli average is taken as tlie average sea level of the year. 



4. Perturbations in Tide Records with Periods Much Longer Than the Tidal Period. 



Tide records include many perturbations with small amplitudes and periods much longer 

 than those of the astronomical tide. These perturbations are more easily recognized if the 

 astronomical tides are removed from the record by some type of numerical filter. The most 

 effective filtering process is to subtract the predicted astronomical tide from the observed 

 tide. The difference between the observed average daily sea level (average of the 24 hourly 

 values) and the predicted daily mean at five tide stations is plotted in Figure 5. The 

 differences between the observed and predicted tides along the Atlantic and gulf coasts tend 

 to be less variable from May to September than during the remainder of the year. The 

 positive difference between observed and predicted mean daily sea level for the year results 

 from a slow rise in mean water level relative to the land, that is neglected in tide predictions. 



Figure 6 illustrates the annual cycle of sea level and the yearly variability in this cycle. 

 Each plotted point represents the average tide departure from the estabhshed local sea level 



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