Marine Navigation Charts

What would we do without our marine navigation charts? We would probably spend a lot of time running aground, getting lost and taking "round about" paths to our destinations. You could always use your GPS; but what if you have no battery power or satellite signals. Then What?

You might wonder why I used the term "marine navigation charts." It is because nearly all of the navigation theory on which mariners rely is common to both aviation and land travel. All three use the equator and the prime meridian as relative points from which we measure direction. They all rely on the level of the sea (sea level) as a relative point from which to measure altitude, elevation and depth. 

There was a time when mariners had almost no printed information from which to plot a course. It was common for men of the sea to learn from their fathers or other elder seamen. They relied on known courses between "points of land" and other visible landmarks.

When in doubt, mariners used a "lead line" to determine the depth of the sea. The "lead line" gets its name from the seven pound lead weight attached to a hemp line. The lead weight usually had a "cup shaped" under side where material from the bottom would accumulate. From that small sample of the bottom, seamen could determine the condition of the bottom for anchoring.

It was not until the first century A.D. that chart making began to evolve. Marinus of Tyre, thought to be of Greek extraction, is believed to be the first man to draw charts for mariners at that time. 

While not as precise as current day charts, the work of Marinus was the foundation on which much of chart making was built. His charts included meridians and parallels, which we use for coordinates today. 

In 1795, Alexander Dalrymple was appointed the first hydrographer of the Royal Navy. The British Admiralty published its first marine navigation chart in 1801. Their work was from their own surveys. Dalrymple's attention to detail and his demand for precision were a model for the high standards that exist in chart making today.

Since the days of Marinus, chart making has come a long way. In addition to the coastline and soundings (depths), there are many other features shown on today's charts. Things have been added like bottom contours, tide & current information, land features, and compass variation, just to name a few.

In the early 1940's, the advent of electronic navigational aids was responsible for a new layer of information on some charts. What is commonly called a "LORAN overlay" is a colored grid that is superimposed on some charts. This grid relates to the TD's (timing difference lines) that are used in LORAN (LO-ng RA-nge N-avigation).

Most modern day charts of navigable waters in the US are published by the National Ocean Survey (NOS) of the National Oceanic and Atmospheric Administration (NOAA) of the US government. They perpetually compile and maintain a database of information from which charts information is developed.

There is a wealth of information available to both guide us and to keep us safe on the water. Yet, I continue to see mariners referring to roadmaps, restaurant place mats, and realtors' drawings of waterfront areas. Go figure!

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