Tuesday, April 03, 2012

Pilot Chart Admiration

Stuart Florida
N 27 12.86 W 080 15.46

I've been reading a biography of Thomas Edison (more on that at a future date).  One thing that stands out from the book is the skill and ingenuity that practitioners of earlier centuries.   An outstanding example of that is the oceanic pilot chart.   I'll try to explain this highly technical subject in words that ordinary people can understand and appreciate.

Below is a pilot chart of the Caribbean Sea and Gulf of Mexico for the month of April. Such charts are published for each month for every ocean (Download them free here).   The data on the charts have been gathered mostly by mariners keeping century after century of detailed ship's log entries.  It was a monumental effort.

The close up below shows a familiar area, Southern Florida and the Keys.

The principle data on the chart shown are the wind roses. For example, the wind rose near Key West means that the average wind is North at force 4 15% of the time, NE at 4 22% of the time, E at 4 35% of the time, SE 4 15%, S 3 5%, SW 3 4%, W 4 5% and calm 3%.  Wind speeds are on the Beaufort Scale.  In mathematical terms, this is a probability distribution in two axes.  However, presentation in this form means that non-mathematical people can read it, understand it and make good use of the data.  Edward Tufte, author of the classic book The Visual Display of Quantitative Information would be proud of what they did.  Note that people like us who spent months in Boot Key Harbor are well familiar with that pattern of winds.

Much has been said in recent years about climate versus weather in the context of global warming debates.  In this case, there is no ambiguity.  The data on the pilot charts shows climate, not weather.  It is long term averages, valid over decades and even centuries.

Sailors who plan ocean passages find weather forecasts useless.  At best, the weather forecast might tell what it might be like in the first 4 days of a 60 day voyage.  For the other 56 days, the sailor depends on climate data, not weather.

In addition to wind, the pilot charts give other highly relevant information.  Current roses, temperatures, the frequency of gales, predominant size and direction of waves, and so on. For example, the green line near the Florida Keys shows the location and average speed of the Gulf Stream.  Passage planners need to factor all that information into their plans.

Compare this with the animated view of ocean currents I posted recently here. Modern sattelites gather the data in a form that computers can use, and the rest of us admire as art.  The ancients however did an excellent job of gathering the data by hand and presenting it in a way that a navigator on a sailing ship could use in his cabin.

Of course, this is the modern age.  Instead of a big chest full of rolled up pilot charts, we use computers nowadays.  I have a PC app, called Visual Passage Planner, that incorporates all the pilot chart data.  In addition, I describe Tarwathie's sailing characteristics, and our preferences on the speed versus comfort scale.  Then we can ask the program, "To go from A to B in the month of April, what is the optimum route?"  It gives the answers readily.  The picture below shows some results from this program; a route from Cape Canaveral to Charleston, SC, and one from Cape Canaveral to Beaufort, NC.  In the first route it recommends hugging the coats.  In the second it is telling me to sail out to the center of the Gulf Stream.  Those are based on both wind and current climate data.  Cool.

Of course, it is overkill for Tarwathie because we do mostly coastal sailing.  We can listen to weather forecasts and we can modify our plans according to weather forecasts.  It is entirely foreseeable that the next generation of GPS chart plotters will incorporate the pilot chart data and downloaded weather data, and even ship traffic data and give us continuously updated suggested optimal routes. That may be amazing, but I think that the labors of the pre-computer mariners was much more impressive.

I think that the ideas represented by oceanic pilot charts, and the skill of their execution are superb examples of good engineering, and good seamanship.  My hat goes off to those who contributed to them.


  1. If I remember my flying history, weather forcasters would only "predict" the weather for six hours in advance. The next sixteen hours they will provide an "outlook". Don't even try to get the weather three days in advance. Ken

  2. peter starrenburg2/25/2013 2:00 PM

    Dick and Libby,

    a very helpfull blog, very detailed and very good links.
    Thank you very much


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