DIFFERENCES. This section of the table contains the time and the height differences of high and low water between the subordinate station and the reference station. Where dif-ferences are omitted, they are unreliable or not known.

To obtain the times and heights of tides at a subordinate station on any date, you must apply the differences to the times and heights of the tides at the reference station for the same date.

Time Difference. The time difference is the hours and minutes you apply to the reference stations time of high and low tide. The time dif-ference is added to (+) or subtracted from () the reference stations time, depending on the sign preceding the time difference.

Height Difference. The height differences in figure 6-6-5 are applied to the height of high and low water at the reference station. Height dif-ferences are usually given in feet. However, height differences may be given using ratios. Ratios may be shown for the high tide, low tide, or both. In this case, you must multiply the ratio by the tide height listed for the reference station to find the height of the tide(s) at the subordinate station. Height differences may also appear in the table (fig. 6-6-5) as a combination of a ratio and a height measurement. In this case, multiply the corresponding tide at the reference station by the ratio; then apply the height difference.

RANGE. Two ranges are shown for subor-dinate stations. Mean range is the difference, in height, between the high tides and the low tides, Spring range is the annual average of the highest semidiurnal range, which occurs semimonthly (twice a month), when the Moon is in its new or full phase. The Spring tide range is larger than the mean range where the type of tide is either semidiurnal (two high tides and one low or two low and one high). The difference between the Spring tide range and the mean range is of no practical significance where the type of tide is diurnal (one high and one low tide a day). Where the tide is chiefly diurnal, the table gives the diurnal range, which is the difference in height between mean high water and mean low water.

NOTE: For stations where the tide is chiefly diur-nal, time differences, height differences, and ratios are intended primarily for predicting the higher high and lower low waters. When the lower high water and higher low water at the reference station is nearly the same height, the correspond-ing tides often cannot be obtained satisfactorily by tidal differences.

MEAN TIDE LEVEL. The mean tide level is the plane between mean low water and mean high water. Dashes are entered when the data is unreliable or unknown.

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