Sunday, March 8, 2009

Using the Tide Tables

Predictions of tidal heights are published by the National Ocean Survey, theTideTables come in four volumes which are: Europe and West Coast of Africa (including the Mediterranean Sea), East Coast, North and South America (including Greenland), West Coast, North and South America (including the Hawaiian Islands), Central and Western Pacific Ocean and Indian Ocean. They contain daily predictions for 197 reference ports and difference data for about 6,000 stations.

Reference Stations
Table 1 lists the time and height of the tide at each high water and low water in chronological order for each day of the year at a number of places which are designated as reference stations. Depths are given in feet which can be easily and quickly converted to meters by multiplying by 0.3. All times in the Tide Tables are standard times, each page of Table 1 gives the central meridian of the time zone used. Adjustment are made for the use of daylight time or any other deviation from standard time at the locality concerned. Because the lunar or tidal day is a little more than 24 hours in length (an average of about 24 hours 50 minutes), the time between high or low tides is a little more than 12 hours. When a high or low tide occurs just before midnight, the next high or low tide occurs about noon of the following day, and the next one occurs just after midnight. When this happens three consecutive high or low tides may occur on three different dates, but the total interval may be no more than the average period of a lunar day, 24 hour 50 minutes. This means that on the middle of the three days, there is but one high or low water. Some of each month the tide becomes diurnal at some stations, which means, there is only one high tide and one low tide each lunar or tidal day.

Subordinate Stations
Secondary or subordinate stations are listed in geographical order in Table 2. Each subordinate station is given a number and its location is described, and its position in latitude and longitude given to the nearest minute. Data is then given which is to be applied to the predictions at a stated reference station (which is shown in bold type) to obtain the tidal information for the subordinate station. If there is more than one reference station on a page of Table 2, be sure to use the one printed above the subordinate station listing.

Determining Time of High or Low Tide
A separate time difference is tabulated for high and low water. Each time difference is added to or subtracted from the time of the high or low water at the reference station in accordance with its sign. When sitting for a exam watch out for changes of date, either forward or backwards, when the time difference applied. For example, if a high water occurs at a reference station 2200 on 8 March and the tide at the subordinate station occurs 4 hours later, then high water will be at 0200 on 9 March at the subordinate station. The height of the tide is found in several ways, depending on your local conditions. If the difference for height of high water is given, with feet tabulated as the low water difference, apply the high water difference in with its sign to the height of high water at in reference station. The height of low water will of course be the same as that at the reference station. If a difference for height of low as well as high water is given, each must be applied with its sign to the height of the corresponding tide at the reference station, adding the difference if its sign is plus (+) and subtracting if its sign is minus (-). If a ratio of ranges is given, the heights of the tides at the subordinate station can be found by multiplying the heights of both high and low tides at the reference station by the ratios. Any unusual conditions pertaining to a subordinate station are explained in footnotes on the appropriate page of Table 2.

Tidal Ranges
The mean tide level and the ranges of tide mean, plus spring, diurnal or tropic are listed in Table 2, but are seldom used except as a general interest. An explanation of them is given in the Tide Tables. The height of the tide at a specific time other than those tabulated in Table 1 or computed using Table 2 can be found by means of Table 3, which is normally used without interpolation. This table is easy to use and the instructions given below the table are very explicit.

Note: Interpolation is not done when using Table 3. The predictions of times and heights of tide are influenced by local conditions to the extent that they are not exact enough to make any interpolation for more precise values.

Other Information found in the Tide Tables
The Tide Tables also contain other information which might be of interest to a navigator.

1. The local mean time of sunrise and sunset is given in Table 4 for each two degrees of latitude. While this information is usually obtained from the Nautical Almanac.

2. Table 5 gives you a method for converting local mean time to standard time when you know the difference in longitude.

3. Table 6 gives you the times of moonrise and moonset for a few selected locations. The inside back cover of each volume gives data for the phases of the moon, apogee, perigee, and greatest north south declination and crossing of the equator. Also it will give you data on equinoxes and solstices.