Chart Datum: Understanding Depth Soundings

One of the most exciting maritime topics of all time is the subject of “chart datum”. Are you yawning yet? Chart datum is not an exciting topic, but it is interesting and informative. Chart datum is the tidal survey information that relates to the soundings and depth curves shown on a nautical chart.

The accuracy of charts published by the National Ocean Service (NOS) relies directly on the survey data collected. You might be surprised to know the source of some of the engineering and survey information that is represented on today’s nautical charts. You can find it in the title block of the chart. On many of today’s charts, the data is based on 1983 surveys.

Don’t be alarmed. Critical areas, such as channels, inlets and other “tight spots” are frequently monitored. As you might imagine, areas that are 100, 400 or 800 feet deep are not so critical and can go for longer periods without updating. It’s all based on risk and practicality. There is no point in spending the taxpayers’ money on areas that present no threat to mariners.

Remember, there is no such thing as a “sure thing”. If you are in a vessel with a 48-foot draft in an area showing soundings of 50 feet, it’s time to slow down. The soundings on a chart are not perfect. Where risk presents itself, we simply exercise a reasonable amount of caution. For example, when you see that a wreck or a shoal was “reported”, you should keep in mind that the word “reported” means that it was not surveyed yet.

Believe it or not, about sixty percent of soundings in U.S. inland waterways were determined through the use of a “lead line”. That’s right, the old fashioned way. The “lead line” was the state-of-the-art method prior to 1940. While it was a very accurate method of determining the depth, each time the lead line hit the bottom it was at a single point. More modern electronic methods report a continuous line of information.

Where lead lines were used, the survey vessel would follow courses that would be spaced according to the conditions of risk that mariners would face. In anchorages and channels, the spacing of the lines would be 50 meters (approximately 165 feet). On the open coast where the bottom was relatively even, the spacing would be 200 to 300 meters for depths up to 10 fathoms (60 feet). At 100 fathoms (600 feet) the spacing would be as much as four miles.

As electronic instruments became more affordable and more accurate, the lead line became a thing of the past. These new “continuous” data sources greatly improved the efficiency of the process of surveying “Davy Jones’ Locker” in the hopes that you would never become a visitor.

We are fortunate to live in the U.S. for many reasons, the least of which is that we live among the best charted waters in the world. However, you still have to keep your charts up-to-date. Each NOS chart has a revision date in the lower left corner.

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


World Wide Marine Training, LLC, is a U.S. Coast Guard Approved facility authorized to give examinations for captain’s licenses up to Master 200 Tons, Able Seaman up to Unlimited, STCW Basic Training, Radar, ARPA and other Endorsements. Please visit or call toll-free 866-249-2135.