Saturday, November 21, 2009

Lines In The Sea

Who says you can't draw a line in the water? Nations and private property owners have been doing it for hundreds of years, to define their property rights and defend their seaward borders. Today a whole series of overlapping and sometimes conflicting laws, decrees, and treaties exist that draw just such lines in the sea. Because you can't actually draw such a line, they are usually drawn with reference to something on land. In the United States, the following lines are drawn:

Mean higher high water. In areas with two high and low tides a day, this line marks the average of the higher of the two high tides over a period of several (usually 19) years. The State of Texas uses this line as the seaward boundary of private property; you may not build a fence below it, for instance.

Mean high water or shoreline. This is the average of all high waters over the 19-year period. Fourteen US states use this as the seaward property line; areas above this are called privately owned uplands.

Mean low water or coastline. The average of all low waters over the datum epoch period. Seven states use this as the private property line. Areas above it are called state owned tidelands or inland waters.

Mean lower low water. The average of each daily lowest of the two low tides. This is the baseline from which US territorial waters are measured seaward. Areas above this line are called state submerged lands.

Three-mile limit. This distance is the range of a shore-based cannon in the early 1700s, and thus served as a practical definition of a nation's seaward boundary. Although now largely obsolete, it still has some applications. Check out an online chart of southeast Alaska here and you can still see the "doughnut holes" created by the three-mile boundary.

Twelve-mile limit. This is the generally accepted seaward limit of a nation's laws, established by international convention. Waters landward of this line are called the territorial sea; waters seaward are called the high seas. Some nations claim an exclusive economic zone extending up to 200 miles off their baseline. Although marked on some nautical charts with a fish symbol, these zones include not only fishing but oil and natural gas extraction rights, as well as environmental law enforcement. The image above shows both the United Kingdom's twelve-mile limit and areas further offshore it has claimed.

Disputes in these boundaries still exist. The US refusal to recognize North Vietnam, and thus its twelve-mile limit, was cited (at the time) as a cause of the Gulf of Tonkin incident. Libya's attempt to claim the entire Gulf of Sidra as territorial waters led to conflict with the US twenty years later. Even today, both the US and Canada claim jurisdiction over potentially oil-rich areas of the Beaufort Sea.


  1. One common name given to the 3 mile limit off US territorial waters was the "Rum Line". After the Volstead Act (Prohibition) passed in the US, savy ship captains would offload their booze just outside this line to small fast boats. The most famous of these was a Capt. McCoy, but that is another tale for future blogs. To discourage this practice, the US congress in 1924 extended the territorial sea to 12 miles, making it harder small vessels to conduct their smuggling operations.
    Keep up the blog Capt. Earle- love it- TV

  2. It's amazing how much demand for, and our attitudes toward, mind altering and addictive substances drives political events, and thus law and history. Jimmy Carter might say that our "addiction to oil" is the whole reason behind the 200-mile exclusive economic zone. For an interesting take on the effect of drugs in general, and booze in particular, on the course of history, check out this edition of Dan Carlin's "Hardcore History" podcast: