:max_bytes(150000):strip_icc():format(webp)/109864226-58b9cb133df78c353c376473.jpg)
............................................................................................................................................
How Are Nautical Miles Measured?
The Development
of Nautical Miles and Nautical Charts
by Amanda Briney
A nautical mile is a unit of measurement used on water by
sailors and/or navigators in shipping and aviation.
It is the average length of one minute of one degree along a
great circle of the Earth.
One nautical mile corresponds to one minute of latitude.
Thus, degrees of latitude are approximately 60 nautical miles
apart.
By contrast, the distance of nautical miles between degrees
of longitude is
not constant because lines of longitude become closer together as they converge
at the poles.
Nautical miles are typically abbreviated with the symbols nm, NM
or nmi. F
or example, 60 NM represents 60 nautical miles. In addition to
being used in navigation and aviation, nautical miles are also used polar
exploration and international laws and treaties regarding territorial
water limits.
Nautical Mile History
Until 1929, there was not an internationally agreed upon
distance or definition for the nautical mile.
In that year, the First International Extraordinary Hydrographic
Conference was held in Monaco and at the conference, it was determined that the
international nautical mile would be exactly 6,076 feet (1,852 meters).
Currently, this is the only definition in use widely and it is
the one that is accepted by the International Hydrographic Organization and the
International Bureau of Weights and Measures.
Prior to 1929, different countries had different definitions of
the nautical mile.
For example, the United States measurements were based on
the Clarke 1866
Ellipsoid and the length of one minute of arc along a great
circle.
With these calculations, a nautical mile was 6080.20 feet (1,853
meters). The U.S. abandoned this definition and accepted the international
measure of a nautical mile in 1954.
In the United Kingdom, the nautical mile was based on the knot.
A knot is a unit of speed derived from dragging pieces of
knotted string from sailing ships.
The number of knots falling into the water over a given period
of time determines the knots per hour.
Using knots,
the U.K. determined that one knot was one nautical mile and one nautical mile
represented 6,080 feet (1853.18 meters).
In 1970, the U.K. abandoned this definition of the nautical mile
and now uses exactly 1,853 meters as its definition.
Using Nautical Miles
Today, one nautical mile still equals exactly the
internationally agreed upon measure of 1,852 meters (6,076 feet).
One of the most important concepts in understanding the nautical
mile though is its relation to latitude.
Because a nautical mile is based on Earth’s circumference, an
easy way to understand the calculation of a nautical mile is to imagine the
Earth being cut in half.
Once cut, the circle of the half can be divided into equal
portions of 360°. These degrees can then be divided into 60 minutes.
One of these minutes (or minutes of arc as they are called in
navigation) along a great circle on
Earth represents one nautical mile.
In terms of statute or land miles, a nautical mile represents
1.15 miles. This is because one degree of latitude is approximately 69 statute
miles in length. 1/60th of that measure would be 1.15 statute miles.
Another example is traveling around the Earth at the equator to
do this, one would have to travel 24,857 miles (40,003 km). When converted to
nautical miles, the distance would be 21,600 NM.
In addition to its use for navigational purposes, nautical miles
are also still significant markers of speed as the term "knot" is
today used to mean one nautical mile per hour.
Therefore, if a ship is moving at 10 knots, it is moving at 10
nautical miles per hour.
The term knot as it is used today is derived from the previously
mentioned practice of using a log (a knotted rope tied to a ship) to gauge the
speed of a ship.
To do this, the log would be thrown into the water and trailed
behind the ship. The number of knots that passed off of the ship and into the
water over a certain amount of time would be counted and the number counted
determined speed in “knots.”
Present-day knot measurements are determined with more
technologically advanced methods, however, such as mechanical tow, Doppler radar,
and/or GPS.
Nautical Charts
Because nautical miles have constant measurement following lines
of longitude, they are extremely useful in navigation.
To make navigation easier, sailors and aviators have developed
nautical charts which serve as a graphical representation of the Earth with a
focus on its areas of water.
Most nautical charts contain information on the open sea,
coastlines, navigable inland waters and canal systems.
Usually, nautical charts use one of three map projections:
the gnomic, polyconic and Mercator.
The Mercator
projection is the most common of these three because on it,
lines of latitude and longitude cross at right angles forming a rectangular
grid.
On this grid, the straight lines of latitude and longitude work
as straight line courses and can easily be plotted through the water as
navigable routes.
The addition of the nautical mile and its representation of one
minute of latitude make navigation relatively easy in open water, thus making
it an extremely important component of exploration, shipping, and geography.
Amanda
Briney
Professional
geographer, writer, and scholar
More
than 10 years of experience writing about a broad array of geographical topics
Holds
three university degrees and an advanced certificate in GIS
Experience
Amanda
Briney is a professional geographer and writer who contributed to ThoughtCo for
more than 10 years. She wrote countless articles on a wide range of topics such
as an introduction to the subject of geography, reviews of ecotourism,
discussions about environmental determinism, and the structure of Latin
American cities. The scope of her work also includes other formats such as
histories, guides, and fact sheets about many parts of the world. An ultimate
scholar, Amanda also contributes work to academic venues and the GIS Lounge, an
informational portal about geography.
Amanda
enjoys all aspects of geography and mapping but is especially interested in
examining natural landscapes through spatial analysis. As such, she holds a
certificate in Geographic Information Systems (GIS) from California State
University. She also attended Diablo Valley College where she studied air photo
interpretation and the formation of the Earth's landscapes.
Education
Amanda
Briney received a Master Arts (M.A.) in Geography from California State
University–East Bay. She also holds a Bachelor Arts (B.A.) in English and
Geography from California State University–Sacramento and a earned a
Certificate of Advanced Study in Geographic Information Systems (GIS) from
California State University.
ThoughtCo
and Dotdash
ThoughtCo is a premier
reference site focusing on expert-created education content. We are one of the
top-10 information sites in the world as rated by comScore, a leading Internet
measurement company. Every month, more than 13 million readers seek answers to
their questions on ThoughtCo.
For more
than 20 years, Dotdash brands have been helping people find answers,
solve problems, and get inspired. We are one of the top-20 largest content
publishers on the Internet according to comScore, and reach more than 30% of
the U.S. population monthly. Our brands collectively have won more than 20
industry awards in the last year alone, and recently Dotdash was named
Publisher of the Year by Digiday, a leading industry publication.
No comments:
Post a Comment