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Old 10-14-2003
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Using Tide Tables

 
Sailors voyaging in unfamiliar waters can save themselves this kind of trouble by simply consulting the proper tide tables.
 
With just a click of the mouse you can see the tide reference stations displayed on your electronic chart, and with a click on the desired station you can see the tides displayed for any day of the year. With this kind of convenience it's easy to see why most of us carry the tide tables in book form for emergency use only. For this reason, it's important to understand what tables are available and how to use them. If you are staying in your local area for a daysail, the tide information in the daily newspaper or on a local calendar may be all you need. However, if you are going cruising for several weeks, you will need tide tables for your cruising area. And remember, all tables are published for the current calendar year and need to be replaced on a yearly basis.

Besides the government tide table publications, there are several commercial publications available such as Reed's Nautical Almanac or Eldridge's that you may want to consider. Reed's is a complete and easy-to-use reference book for tides, currents, aids to navigation, celestial tables, and weather information. There is a Reed's Almanac for both the East and West Coast of the US in addition to a Caribbean volume. Eldridge's has tables for tides and currents for the East Coast of the US only and covers the Northeast especially well. Like Reed's, it also contains a light list and weather information.


Graphic tide tables, like this one for Rockport, MA, allow you to see the approximate height of the tide for any time between high and low water.

In 1995 the National Ocean Service (NOS) stopped publishing the tide and current tables. Instead it makes the data available to private publishers and distributors. The NOS tidal information tables are much larger and easier to read than the privately published tables and are somewhat less expensive, but contain only tide data. You will also need to buy the tidal current tables that are published separately. Tide tables for various parts of the world are published in four volumes. These volumes are:

 East Coast of North and South America (including Greenland).
 West Coast of North and South America (including Hawaiian Islands) and Alaskan Supplement
 Europe and West Coast of Africa and Mediterranean
 Central and Western Pacific Ocean and Indian Ocean

Each volume has five common tables:

 Table 1 contains a complete list of the predicted times and heights of the tide for each day of the year at a number of places designated as reference stations. Boston, Key West, and San Francisco are examples of points where detailed information is given.
 Table 2 gives tidal differences and ratios for a large number of subordinate stations, which are used to modify the tidal information in the reference stations. From these tables, the tide at any point along the coasts can be calculated.
 Table 3 provides information for finding the approximate height of the tide at any time between high water and low water.
 Table 4 is a sunrise-sunset table organized in five day intervals for various latitudes from 76 degrees north to 60 degrees south.
 Table 5 provides an adjustment to convert the local mean time of sunrise and sunset to zone or standard time.

The US East Coast and West Coast volumes also contain tables for moonrise and moonset, conversion from feet to centimeters, estimated tide prediction accuracies, a glossary of terms, and an index to tide stations. Each table is preceded by a complete explanation and sample problems are given where necessary. The inside back cover of each volume contains a calendar of critical astronomical data to help explain the variations of the tide during each month and throughout the year.

Tide Predictions For Reference Stations  For each day of the year, the time and height of each high and low water are listed in chronological order for all reference stations. Although high and low waters are not labeled as such, they can be distinguished by the relative heights given immediately to the right of the times. The heights are given in both feet and centimeters. If two high tides and two low tides occur each tidal day, the tide is semidiurnal. Since the tidal day is 50 minutes longer than the civil day, any given tide occurs later each day. Because of the later times of corresponding tides from day to day, certain days have only one high water or only one low water. Times are given using the 24-hour-clock system and are in local standard time. When in Daylight Saving Time, you must add one hour to the given time to obtain the Daylight Saving Time.

Tide Predictions For Subordinate Stations  For each subordinate station listed, the following information is given:

 Number  The stations are listed in geographical order and assigned consecutive numbers. Each volume contains an alphabetical station listing, correlating the station with its consecutive number to assist in locating the entry in Table 2.
 Place  The list of places includes both subordinate and reference stations with the reference stations appearing in bold type.
 Position  The approximate latitude and longitude are given to assist in locating the station on your chart.
 Differences  The time and height differences to be applied to the predictions for the reference station, are shown in capital letters above the entry. Time and height differences are given separately for high and low waters. Where differences are omitted, they are either unreliable or unknown.
 Ranges  This is the difference in height between high water and low water for the tides indicated.
 Mean tide level  This is the average between mean low and mean high water, measured from chart datum.

The time difference is the number of hours and minutes to be applied to the reference station time to find the time of the corresponding tide at the subordinate station. This interval is added if preceded by a plus sign (+) and subtracted if preceded by a minus sign (-). The results obtained by the application of the time differences will be in the zone time of the time meridian shown directly above the difference for the subordinate station. Special conditions occurring at a few stations are indicated by footnotes on the applicable pages. In some instances, the corresponding tide may fall on a different date at reference and subordinate stations.


On-line tide tables, like the one above for James Island Creek near Charleston, SC (obtained from www.harbortides.com), offer precise, year-round tidal information for most geographic locations.

Height differences are shown in a variety of ways. For most entries, separate height differences in feet are given for high water and low water. These are applied to the height given for the reference station. A plus sign indicates that the height is to be added to that given in the reference station while a minus sign means that it is to be subtracted. Remember that on occasion you may end up with a negative height of tide, meaning that the actual depths will be less than that indicated on the chart. In many cases a ratio is given for either high water or low water, or both. Ratios are identified by an asterisk and are given as a decimal fraction. The height at the reference station is multiplied by this ratio to find the height at the subordinate station. For a few stations, both a ratio and difference are given. In this case the height at the reference station is first multiplied by the ratio, and the difference is then applied. Special conditions are indicated in the table or by footnote.

If you are not completely familiar with using published tide tables, you should solve the examples and problems given in Bowditch and Chapman's Piloting or see if you arrive at the same values as the computer program. Don't wait until your electronics fail to learn how to use these tables. On a dark and stormy night it may take too long.

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