SOUNDINGS

Isn't it nice of the National Ocean Service to mark-up our charts with the depth of the water? To put it into technical terms, those depths are actually called "soundings." They are recorded depths marked in the exact positions where they were determined by a survey.

On charts printed in the United States and the United Kingdom, soundings are represented in either feet or fathoms. Austria, Belgium, Chile, France, Germany, Holland, Italy, Norway, Portugal, Spain and Sweden all measure soundings in meters. Each chart for navigation indicates the "unit of measurement" of soundings in its title block.

Lets concentrate on the National Ocean Service charts that we use in the United States. The decision to represent soundings in fathoms versus feet involves the depth of the navigable waterway. Offshore charts are usually in fathoms, since a fraction of a fathom is seldom required. For near coastal and inland charts, "feet" is usually the measurement of choice in order to avoid printing "fractions" of fathoms.

With all of the numbers on our charts, how do we determine whether any given number indicates a sounding, a buoy number or something else? Lucky for us, there's a logical system to prevent that kind of confusion. Buoy numbers and letters are in quotation marks (i.e. "2" or "1AC," etc.). Other notations that might be confused with soundings are printed within a note, printed in parentheses, or printed in a style and size that is significantly different from a sounding.

So, now that we know how to determine the depth of the water, what do we do about the rising and falling tides? Surely, the soundings on our chart don't change with the tide! The soundings on U.S. charts indicate that they represent the depth of the water at a certain state of the tide. Generally, it is at or near the "low tide" level, but there are a few ways of calculating that level.

First of all, when we rely on a sounding to determine the depth of the water, we are relying on an average. There are generally two methods used to calculate the sounding represented on our charts. Before we get into that, I should point out that the sounding does not indicate "exactly" the lowest tide level. Since the low tides vary from day to day, we compromise by using an average. Accordingly, at times, the water level can be slightly lower than a sounding indicates.

For a long time, the soundings on charts represented the "average low tide." It was the average of all of the low tides throughout the 19-year tide cycle. Both the timing and the level of the tides repeat themselves on "nearly" an exact schedule every 19 years. The average of the low tides at any given point is called "Mean Low Water" (MLW) for that location.

In recent years, most of the soundings on U.S. charts have moved from "Mean Low Water" to "Mean Lower Low Water" (MLLW). Most locations have two high tides and two low tides each day. Each high and low is slightly different. The MLLW method includes only the "lower" of the two low tides in the average. In this way, we achieve a more accurate indication of what is expected to be the lowest tide.

Until next time, I wish you clear skies, fair winds, and calm seas!