How to interpret a tide table
On the recently concluded trip to Lost Coast, we had to rely on tide
tables to keep track of which sections of coastline we had to pass at
what time. Since most of us are or were engineers, there was some
healthy debate on what the numbers in tide tables mean, and how to
interpret the data. Surprisingly, this information is not easily found
on the Internet, so here’s a quick and easy summary.
Shown below is a sample tide table entry (courtesy:
http://www.coos-bay.net/understanding-tides.html)
Saturday 2004-10-02
Sunrise 7:16 AM PDT, Sunset 6:55 PM PDT
Moonset 11:50 AM PDT, Moonrise 8:58 PM PDT
High Tide: 2.25 P.M. PDT 10.00
Low Tide: 9.08 P.M PDT -0.07
High Tide: 3.40 A.M. PDT 8.00
Low Tide: 9.00 A.M. PDT 3.00
As you can see, for each 24 hour period, we have two high tides, and
two low tides. Each of the tide entries has a time, and a number. The
time corresponds to the time at which the level of the ocean is at its
highest or lowest. The number is the level in feet from the mean low
tide level. So you should expect to see low tide numbers hovering
around the zero mark, as you can see above.
One additional point to note is that the tide movement is not uniform,
and the rate of change of sea level is not the same over the entire ~6
hour period between a high tide and a low tide. In the example above,
the first high tide is at 2:25 pm, and it’s quite high at 10.00 ft.
The next low tide is quite low at -0.07 ft. Since the tidal
differential for this period is higher than for the next, expect the
ocean levels to change quite rapidly at the mid point of the tidal
movement, ie, at around 5:30 to 6:00pm. In some tide tables, this is
shown as an additional data item, and if you’re going kayaking in
shallow waters, this rate of change and the time period when it occurs
is something you need to pay close attention to. In general, the most
rapid rate of change will occur mid-way between the high and low
points.
Hope this helped, and remember - never turn your back on the ocean.
Safe trails.