In order to safely navigate inshore waters, it is important to be able to predict the tides. This is accomplished with the use of tide tables. They can be found in various formats and contain varying amounts of infomation.
The National Ocean Service (NOS) collects and publishes tidal data for various geographical areas. These publications contain data for all reference stations and subordinate stations in a given area.
The data for all points in all areas is given as relative to a reference plane called Mean Lower Low Water (MLLW).
To predict the tides at a point that is not a reference station, one uses a combination of data from a reference station and data from a table of 'tidal differences' for that reference station. The table of tidal differences actually tells the difference between a subordinate station's tides and that of it's 'parent' reference station.
The sample tables used here are of a relatively simple format and are readily available at most marinas, sporting stores, boating stores, and even some general stores along the coast. These tables are highly abbreviated as compared to the NOS tables and almost always contain the data for a single reference point only. Some also include data for a limited number of subordinate stations.
The chart used in the following exercise is an example of a graphic calendar format. It illustrates the movement of the tides with the moon quite nicely. The reference point for this data is Portland, ME.
A Practical Example
NOTE: It is ILLEGAL to handle baby lobster, in any state, without proper authorization. Even licensed fishermen may not handle them. For this example, then, we assume I have said authorization.
Let's say it's early January 1998 and I need to do a census of baby lobster for The Lobster Conservancy in an area at Kittery Point, ME that month . The baby lobster will only be accessible at low tide during a spring tide, when the water is lower than the average low tide. Let's also assume that I need a low of at least 1 foot below MLLW in order for the right amount of sea bottom to be exposed for a good census.
First, I consult a tide table for that month (click here to open it) and look for times when a low tide is the lowest of the month. I speed up the process my remembering that spring tides occur on or around the full and new moon each month so I'll start there.
A full moon occurs on the 12th and a new moon on the 27th. Examining the low tides on days surrounding these dates I see that during the full moon there are actually two low low tides, January 11 at 4:14 pm with a low of 1.0 foot below MLLW, and January 12 at 5 pm, also with a low of 1.0 foot below MLLW.
Following the same logic for days around the new moon, I determine that January 30 has a lowest low tide at 6:38 pm with a low of -1.7. I also notice that the days from January 28 (second low tide) to January 31 all have low low afternoon tides; even lower than the full moon dates.
So now I know when to go do the census, right? Not quite. The data I have read is for the reference station at Portland, ME, not the subordinate station at Kittery Point. There could be huge differences in the time and height of tides between the two places, so corrections must be made.
The next step is to turn to the table of tidal difference which this particular table has (not all do). There I see two valuable pieces of information; the entry for Kittery Point, and a footnote. Below is the entry from the tidal differences table and my translation of what that data means.
Finally we calculate the corrected figures (only corrected data for January 12 and 30 are shown):
In our example, I really don't need to correct the tidal height since I'm only interested in relative changes in height. If I were in a boat on the water, however, the tide's height would be very important to me.
The differences in time and height in this example are not very significant, however, they can be very significant at other subordinate stations. For example the time difference for the subordinate station at Richmond, ME on the Kennebec River can be as much as 3 hours!
Here is an example of a tabular format. What is the reference station for this table? Can you spot the day that only has one high tide? How does this data compare with that given in the calendar format?