REDUCTION OF CURRENT RECORDS 57 
149. Inferred values.—When tabulating strengths and slacks for the purpose of 
reduction it is desirable in general to have an even number of items for each current 
phase in order to partially balance the effect of any diurnal inequality either in the time 
or in the velocity of the current. This is especially important for very short series of 
observations and for localities where the diurnal inequality is relatively large. To 
accomplish this purpose it is usually better to infer an additional phase item rather than 
reject an observation already obtained, especially in a very short series. Sometimes 
when the observations have commenced just after, or have terminated just before, 
one of the principal phases, the plotted velocity curves may be easily extended to 
include the needed extra item. Inferred values may also be obtained by comparison 
with predicted currents at a reference station or with simultaneous observations at 
some other current station. A value roughly inferred from the sequence of the other 
observations at the same station may be used when necessary. Inferred values should 
be enclosed in parentheses to distinguish them from the actual observations. 
150. Time comparison.—The times of the current phases at the station for which 
the reduction is being made (the subordinate station) may be referred either to the 
corresponding phases at a current reference station or to the times of high and low water 
at a tidal reference station. If no suitable reference station is available, the current 
phases may be referred directly to the moon’s transits. In general, predictions for 
the reference station are to be preferred to actual observations. This is especially true 
when the currents are referred to times of high and low water, since irregularities in 
observed tides arising from meteorological causes may have no counterpart in observed 
current velocities. 
151. Reference to tides.—Figure 28 illustrates a reduction with reference to a tide 
station. The times of the high and low waters at the reference station are taken to the 
nearest tenth of an hour and entered in the column provided in Form 451. Differences 
between the times of the tide and the current phases are obtained and entered.in the 
columns of current intervals, the symbol H or L at the top of each column indicating 
whether the reference is to a high or low water. The signs (++) and (—) are used to 
indicate whether the current follows or precedes the tide to which it is referred. Usually 
the current will be referred to the nearest high or low water, but all references in any 
one column must be to like phases of the tide in order to be comparable. In case that 
one tidal phase is much better defined than the other, it is permissible to refer all the 
currents to that phase rather than using both high and low water. In localities where 
there is considerable diurnal inequality it is desirable that the greater flood strength 
and greater ebb strength be referred to the lower low water and the higher high water 
rather than to the lesser tides. The columns of differences are summed and averaged, 
the means being usually carried to two decimal places. 
152. Greenwich intervals.—By referring the several current phases at all stations 
to the times of the moon’s transit of the meridian of Greenwich, the results are directly 
comparable, and relations between the times of the current in different localities may 
be obtained simply by taking the differences between their Greenwich intervals. After 
having obtained the average difference between each current phase and the tidal phase 
at the reference station as described in the preceding paragraph, this difference is applied 
to the Greenwich interval for the tidal phase to which it is referred in order to obtain 
the corresponding Greenwich current interval. A multiple of the tidal period 12.42 
hours may be included or rejected when necessary. The mean current hour (MCH) is 
the Greenwich interval for the flood strength as modified by the intervals for the other 
current phases. It is computed by taking the average of the intervals for the following 
