Secondary Station Adjustments Instructions

The publication of full daily predictions is necessarily limited to a comparatively small number of stations. These stations are referred to as "reference stations". Tidal current predictions for more than 2500 other locations can be obtained by applying certain differences to the daily predictions for the reference stations.

These pages provide a listing of the more than 2500 "subordinate stations" for which such predictions can be made, the differences and ratios to be used, and a link to the appropriate reference station. The stations in the listing are arranged geographically to make it possible to find stations which are available for an area you are interested in.

Since all tidal current stations are located offshore, many of them are named for the channels, rivers, and inlets they are located in, or for cities, towns, or navigational points they are located near. Some personal knowledge of the area you are interested in may be necessary to determine which station(s) are most appropriate for your use.

Depths: Although current measurements may have been recorded at various depths in the past, the data listed here for most subordinate stations are mean values determined to have been representative of the current at each location. For that reason, no specific current meter depth for those stations are given. Beginning with the Boston Harbor tidal current survey in 1971, data for individual meter depths were published and subsequent new data may be presented in a similar manner.

Since most of the current data in these pages came from meters suspended from survey vessels or anchored buoys, the listed depths are those measured downward from the surface. Some later data have come from meters anchored at fixed depths from the bottom. These meter positions were defined as depth below chart datum. Such defined depths in these pages will be accompanied by the small letter "d".

Minimum Currents: The user may note that at many locations the current may not diminish to a true slack water or zero speed stage. For that reason, the phrases, "minimum before flood" and "minimum before ebb" are used in these pages rather than "slack water" although either or both minimums may actually reach a zero speed value at some locations.

Maximum Currents: Near the coast and in inland waters, the current increases from a minimum current (slack water) for a period of about 3 hours until the maximum speed or strength of the current is reached. The speed then decreases for another period of about 3 hours when minimum current is again reached and the current begins a similar cycle in the opposite direction. The current that flows towards the coast or up a stream is known as the flood current; the opposite flow is known as the ebb current. Speeds of the current at reference stations are listed as positive values for floods and negative values for ebbs. These pages list the average directions of the maximum floods and maximum ebb currents. The directions listed are given in degrees, true, reading from 000 at north to 359 and are the directions toward which the current flow.

Differences and Speed Ratios: These pages contain time differences by which the user can compile approximate times for the minimum and maximum current phases at the subordinate stations. Time differences for those phases should be applied to the corresponding phases at the reference station. It will be seen upon inspection that some subordinate stations exhibit either a double flood or a double ebb stage, or both. In those cases, a separate time difference is listed for each of the three flood (or ebb) phases and should be applied only to the maximum flood (or ebb) phase at the reference station. The results obtained by the application of time differences will be based upon the local time meridian. Differences of time meridians between a subordinate stations and its reference station have been accounted for.

The speed ratios are used to compile approximations of the daily current speeds at the subordinate stations and refer only to the maximum floods and ebbs. No attempt is made to predict the speed of the minimum currents. These ratios are multiplied to the corresponding maximum current phases at the reference station. As mentioned before, however, some stations may exhibit either a double flood or a double ebb, or both. As with time differences, separate ratios are listed for each of the three flood (or ebb) phases and should be applied only to the daily maximum flood (or ebb) speed at the reference station. It should be noted that although the speed of a given current phase at a subordinate station is obtained by reference to the corresponding phase at a reference station, the directions of the current at the two places may differ considerably. These pages list the average directions of the maximum current phases at the subordinate stations.

Example Tidal Current Calculations

For Cape May Channel, the time and speed adjustments listed in the tables are:

Minimum       Minimum         Speed
Before Flood  Before  Ebb     Ratio
Flood         Ebb           Flood Ebb
-1 14  -1 30  -1 11  -0 45   1.1  1.8

and the reference station is Delaware Bay Entrance. If the times and speeds listed in column 1 are the minimum and maximum tidal currents for a day at Delaware Bay Entrance, column 2 are the time corrections, and column 3 are the speed corrections; column 4 will be the predicted currents at Cape May Channel. These values are computed by adding or subtracting the times in column 1 to the adjustments in column 2; and by multiplying the speeds in column 1 by the ratios in column 3.

     (1)                 (2)        (3)         (4)
  Times     Speed                            Times     Speed
0114  0425   1.3    -1 14  -1 30   *1.1    0000  0255   1.4
0736  1055  -1.3    -1 11  -0 45   *1.8    0625  1010  -2.3
1351  1650   1.2    -1 14  -1 30   *1.1    1237  1520   1.3
1958  2316  -1.3    -1 11  -0 45   *1.8    1847  2231  -2.3