There are numerous small localities along the coast where it is necessary at times to know the stage of the tide. Most of these places have no tide table available for the immediate vicinity, and if tidal differences and constants are used in conjunction with the table for a reference station, the results are not always very good. This is especially true if the local station is very distant from the reference station. The need for a tide table at a small port is usually not sufficient to warrant the time and expense necessary for making observations and computations for a set of tables by the harmonic method.
Puerto Armuelles, a small banana port on the Pacific coast of Panama about 250 miles from Balboa, was in need of a tide table due to the fact that work was about to begin on a new wharf, replacing one destroyed by an earthquake in July, 1934. This wharf was to be built on steel piles, and some of the details of construction would have to be done at low tide. Therefore, it was necessary to know beforehand the elevation of the tides within a foot or two on this work.
Records of the tides were made at Puerto Armuelles for some months several years ago on a graphical 7-day, water-stage recorder. The records for a period of six months were used in constructing the accompanying “Tide Chart.”
A chart of this kind affords a simple means for the tabulation of a tide table accurate enough for almost any practical use. By using a copy of the chart and a nautical almanac, a table can be made up for a year in very little time without making any computations.
With a record of the times and heights of high and low waters for several months, a similar chart can be made up for any place where the tides are of the semidiurnal type. This is the type of tide common to the Atlantic coasts of the United States, Canada, and Europe; the British Isles; the east and west coasts of Africa; the northern coast of South America; the Atlantic and Pacific coasts of South America below latitude 40 degrees south; New Zealand; the Bay of Bengal; Malacca Strait; and the Pacific coast of Panama.
To obtain data for the construction of the chart, the times and heights of all high and low waters were taken from the tide gauge record and tabulated on a form similar to U.S.H.O. Form for High and Low Waters, No. 92 (U. S. Coast and Geodetic Survey Form No. 138). Times of the moon’s upper and lower transits at Greenwich were taken from the Nautical Almanac and tabulated on the same form to the nearest tenth of an hour. Times of the high and low waters (75th meridian time) were tabulated to the nearest two- tenths of an hour as this was as close as the time scale on the gauge record chart could be read. After this was done, the reductions were made and the rest of the form was filled in. For a tide datum, mean low water was used as zero elevation.
From the above form, means were determined, on a separate sheet, of the times of the moon’s transits at Greenwich, and the times and heights of high and low waters following, for each hour between 0000-0100, 0100-0200, etc., for 24 hours. In determining these means, upper and lower transits were tabulated together as there was little difference in the heights of the tides following two successive (one upper and one lower) transits of the moon.
A table was then made up of the above means and the chart was plotted from it. Residuals show that the mean heights are usually correct to within a foot. In a few extreme cases there is a difference of between 1½ and 2 feet. The large differences occur during the exceptionally high and low equinoctial spring tides.
The chart was plotted by using a time scale of .6 of an inch to the hour. This scale is easily read to the nearest 5 minutes.
The height graph was first plotted on a scale of an inch to the meter, as the gauge record was metric. In the plotting for the final chart, the heights were reduced to feet by tracing the height diagram (in meters) on an equivalent scale in feet subdivided into halves. The mean low water elevation on the original gauge record (which was kept consistent) was found to be 2.39 meters. This elevation was shifted to agree with the zero on the final chart. Should the height scale on the original record be in feet, any scale large enough to be read to the nearest tenth of a foot is suitable for a similar chart. Over-all dimensions of chart within borders are 17X22 inches.
In making a set of tables, the times of upper and lower transits of the moon at Greenwich are taken from a nautical almanac and tabulated on any convenient form. This form should have six columns with the following headings: Date, time of moon’s transit at Greenwich, time of high water, height of high water, time of low water, and height of low water. (A specimen form is shown.) Each transit will be on a separate line, making two lines for each date. Opposite the times of transits, the times and heights of high and low waters will be taken from the chart and listed. A table for a month can be tabulated on an ordinary letter size sheet.
To save interpolation for the times of the lower transits of the moon, the American Ephemeris should be used instead of the abridged Nautical Almanac.
Specimen Tide Table for Eight Days in March, 1935, Made from Accompanying Tide Chart Lower transits of moon are shown in parenthesis |
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Date |
Time of Moon’s Transit (G.C.T.) |
Time of HAV. (75th Mer.) |
Height of H.W. (Feet) |
Time of L.W. (75th Mer.) |
Height of LAV. (Feet) |
March 5 |
(0 01) |
4 16 |
8.2 |
10 22 |
-0.9 |
|
12 27 |
16 35 |
8.3 |
22 45 |
-0.5 |
6 |
(0 53) |
4 55 |
8.4 |
11 00 |
-1.1 |
|
13 19 |
17 16 |
8.6 |
23 28 |
-0.7 |
7 |
(1 45) |
5 32 |
8.5 |
11 40 |
-1.2 |
|
14 11 |
15 55 |
8.7 |
— |
— |
8 |
(2 38) |
6 17 |
8.3 |
0 15 |
-0.7 |
|
15 (M |
18 35 |
8.4 |
12 25 |
-0.9 |
9 |
(3 31) |
7 00 |
7.8 |
0 58 |
-0.4 |
|
15 58 |
19 25 |
8.0 |
13 10 |
-0.6 |
10 |
(4 25) |
7 50 |
7.3 |
1 48 |
-0.1 |
|
16 53 |
20 15 |
7.5 |
14 00 |
0.0 |
11 |
(5 20) |
8 48 |
6.7 |
2 40 |
0.3 |
|
17 48 |
21 11 |
7.1 |
14 54 |
0.7 |
12 |
(6 15) |
9 50 |
6.2 |
3 40 |
0.8 |
|
18 42 |
22 12 |
6.7 |
15*55 |
1.1 |