FIGURE 22.-Tide Predicting Machine, designed in the Coast Survey, for predicting the times and heights of the tide for any port in the world and for any year. Astronomical equations involving as many as thirty-seven factors are solved mechanically to produce the tide in nature. Not shown in the view is a recent attachment which automatically types the predictions in the format of the Tide Tables.

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the predicted heights agreed within half a foot of the observed heights; and 59 percent agreed within one-tenth foot. Exact agreement cannot be expected between predictions and actual times of occurrence, since the times and heights of the tide will be modified by the prevailing meteorological conditions, which may not be normal. This is particularly true of the upper reaches of rivers subject to freshet conditions, and also of comparatively shallow bays with a small range of tide.

233. TYPES OF TIDAL DATA AVAILABLE

Besides the published tables and the tide predictions that are included with the Small-Craft charts, there are other types of tidal data that are available to the public on request. Those which are used frequently by many people are furnished in the form of processed material-state index maps; tidal bench-mark data; relation between sea level datum and hydrographic datums; and tables of mean and diurnal ranges and highest and lowest tides-while those required for a special purpose covering a specific period of time are furnished in the form of photostats of the original records-hourly heights; high and low waters; and monthly, yearly, and cumulative means.

2331. State Index Maps

Special index maps are available for each coastal state showing by symbol and numbered position the localities for which tidal bench-mark data may be obtained (see fig. 23). On the reverse side of these maps more specific information is given as to where the bench marks are located for each numbered position on the index map, both of which are to be used in requesting bench-mark data.

2332. Tidal Bench-Mark Data

It has been previously noted that Coast Survey practice is to establish at least three bench marks for any series of observations (see 2314). Data for such bench marks are published separately for each station in loose-leaf form that permits ready expansion and revision. These compilations include the descriptions of all bench marks at each tide station (for ready identification on the ground), and their elevations above the basic hydrographic or chart datum for the area-mean low water on the Atlantic and Gulf coasts and mean lower low water on the Pacific coast, or some special low-water datum. The date and length of the tidal series on which the bench-mark elevations are based are also given. An accompanying table shows the relations between the basic datum

FIGURE 23.-Index map of tidal bench marks in Massachusetts. The dots and numbers indicate the localities for which tidal bench-mark data are available. On the reverse side are the place names corresponding to the numbers.

and other tidal datums in general use-mean higher high water, mean high water, half tide level, mean low water, and mean lower low water. In addition, heights of observed or estimated highest and lowest water levels in relation to the basic datum are also included with the dates of occurrence of these abnormalities (see 2334).

2333. Relation Between Sea Level Datum and Hydrographic Datum

In the process of its extension, the precise level net of the country has come to include tidal bench marks at many localities in addition to those where tide observations originally established the Sea Level Datum of 1929 (see 2121 B). However, there are still about 900 stations out of a total of about 1,700 in conterminous United States where there are no spirit-level ties between the geodetic bench marks and the tidal bench marks, and consequently the elevations of the tidal bench marks referred to the Sea Level Datum of 1929 have not been established.

For each coastal state where spirit-level connections have been made, a table is available showing the relation between Sea Level Datum of 1929 and the hydrographic datum. By subtracting the tabular value at each station from the published elevation above mean low water (or mean lower low water), the bench-mark elevation above the Sea Level Datum of 1929 is obtained. From this the vertical relationship between a geodetic bench mark and a tidal bench mark can be derived (see 2121 в note 27).

2334. Mean and Diurnal Ranges and Highest and Lowest Tides

Also available to the public on request is a leaflet consisting of two tables (Table 1 and Table 2). Table 1 applies to the Atlantic coast and gives the mean range of the tide, together with the height of the highest tide above mean high water and the lowest tide below mean low water, as recorded at the various control stations through the year 1961. Table 2 applies to the Pacific coast and Alaska and furnishes similar information as Table 1, except that the diurnal range is given and the highest and lowest tides are referenced to mean higher high water and mean lower low water, respectively. Both tables also give the average yearly highest and lowest tides for each station.

2335. Hourly Heights and High and Low Waters

Initial processing of tide records from control tide stations consists of a complete and systematic scaling of the hourly heights and the times and heights of high and low waters. The hourly height records have three distinct and basic applications. They provide data for the computation of the basic tidal plane of mean sea level, from which is derived the datum for the level net of the country; for the computation of the harmonic constants used in the prediction and classification of tides; and for a compact and condensed file record for future reference and reproduction. From the high- and low-water records are obtained basic data on time and height relationships, including lunitidal intervals, ranges, diurnal inequalities, high- and low-water datum planes, half-tide level or mean tide level, and mean and extreme heights.

2336. Monthly, Yearly, and Cumulative Averages

For each control tide station, compilations are prepared of monthly averages obtained from the hourly height and high- and low-water records (see 2335). These compilations are further processed to show yearly averages, cumulative averages, and averages by successive and overlapping 19-year periods, each new year as available becoming the last year of a 19-year group. Included in these summarized compilations are also the dates and heights of the highest and lowest tides recorded each month. These extreme heights are generally the result of meteorological disturbances superimposed on the normal periodic tide, and are of particular value to the navigator and to the coastal engineer. During periods of extreme low water, the actual depths of water may be noticeably less than shown on nautical charts. The lowest tide to be expected, as referred to the chart datum, is therefore a necessary part of the tidal information included on the nautical charts of the Survey (see Part 2, 658 and 6581). Knowledge of the heights of extreme tides-high and low-that have occurred or that may be expected to occur in any locality is an indispensable factor in planning and designing waterfront structures, and the long-period tide records of the Bureau are in constant demand for this purpose.

Photostat copies of the hourly heights and high and low waters and of the monthly, yearly, and cumulative averages are available for public distribution at the cost of reproduction.