NAUTICAL MILES AND NAUTICAL CHARTS - 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. 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. 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. 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.

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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.

https://www.thoughtco.com/what-are-nautical-miles-1435097

TRACKING A PLANE - Standard international practice is to monitor airspace using two radar systems: primary and secondary. Primary radar detects and measures the approximate position of aircraft using reflected radio signals. Secondary radar, which relies on targets being equipped with a transponder, also requests additional information from the aircraft - such as its identity and altitude. All commercial aircraft are equipped with transponders (an abbreviation of "transmitter responder"), which automatically transmit a unique four-digit code when they receive a radio signal sent by radar. Once an aircraft is more than 240km (150 miles) out to sea, radar coverage fades and air crew keep in touch with air traffic control and other aircraft using high-frequency radio.

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Tracking A Plane

How do you track a plane?

The disappearance of Malaysia Airlines flight MH370 has raised the disturbing question of how a modern aircraft packed with communications equipment can apparently vanish without trace.

Evidence has emerged to suggest that the Boeing 777-200ER aircraft flew on for hours after being reported missing.

The plane, with 239 people on board, disappeared from air traffic control screens at approximately 01:20 local time on Saturday 8 March - just an hour after leaving Kuala Lumpur bound for Beijing.

The investigation is now focusing on the actions of the air crew, as officials believe that the plane was deliberately diverted, with key communications systems switched off.

So, how do you track a plane and what do we know about the movements of flight MH370?

Air traffic control - standard international practice is to monitor airspace using two radar systems: primary and secondary.

Primary radar -based on the earliest form of radar developed in the 1930s, detects and measures the approximate position of aircraft using reflected radio signals. It does this whether or not the subject wants to be tracked. 

Secondary radar, which relies on targets being equipped with a transponder, also requests additional information from the aircraft - such as its identity and altitude.

All commercial aircraft are equipped with transponders (an abbreviation of "transmitter responder"), which automatically transmit a unique four-digit code when they receive a radio signal sent by radar.

The code gives the plane's identity and radar stations go on to establish speed and direction by monitoring successive transmissions. This flight data is then relayed to air traffic controllers.

However, once an aircraft is more than 240km (150 miles) out to sea, radar coverage fades and air crew keep in touch with air traffic control and other aircraft using high-frequency radio.

How did flight MH370 vanish?

The Boeing 777 disappeared from air traffic control screens when its transponder signal stopped. The last definitive sighting on civilian radar showed the aircraft flying north east across the Gulf of Thailand.

The final radio message received by air traffic control - "Alright, goodnight" - suggested everything was normal on board.

Military radar shows that the plane then turned and headed west across Malaysia towards the Andaman Sea.

On 15 March, satellite data emerged to suggest the plane could be somewhere on an arc stretching either north up to to central Asia, or south, to the Indian Ocean and Australia.

Can't planes be tracked with GPS?

Yes, but while GPS (Global Positioning System) is a staple of modern life, the world's air traffic control network is still almost entirely radar-based.

Aircraft use GPS to show pilots their position on a map, but this data is not usually shared with air traffic control.

Some of the most modern aircraft are able to "uplink" GPS data to satellite tracking services, but handling large volumes of flight data is expensive and such systems are usually only used in remote areas with no radar coverage.

The satellite data which suggests flight MH370 flew on for several hours are basic 'pings' sent by the plane, and so far only help to identify two very approximate flight corridors north and south.

Over the next decade, a new system called ADS-B (Automatic Dependent Surveillance Broadcast) is expected to replace radar as the primary surveillance method for air traffic control

ADS-B will see aircraft work out their position using GPS and then relay data to the ground and other planes.

But, as with existing secondary radar, ADS-B coverage does not extend over the oceans.

ADS-B is already used by flight-tracking websites, but the Malaysian aircraft disappeared from these at the same time it vanished from air traffic control screens.

Could other data systems provide clues?

When Air France flight 447 crashed into the mid Atlantic in 2009, its onboard data system - Aircraft Communications Addressing and Reporting System (ACARS) - gave investigators an early insight into what had gone wrong.

ACARS is a service that allows computers aboard the plane to "talk" to computers on the ground, relaying in-flight information about the health of its systems.

Messages are transmitted either by radio or digital signals via satellites, and can cover anything from the status of the plane's engines to a faulty toilet.

This provides ground crews with vital diagnostic information, allowing maintenance to be carried out more quickly.

In the Air France case, ACARS highlighted faulty speed readings, which caused the air crew to become disorientated.

The final message from the ACARS onboard the Malaysian plane came at 01:07 and investigators believe the system was then deliberately shut down.

Turning off ACARS is no easy feat, requiring a person with technical knowledge to climb down through a trapdoor into the plane's hull to remove circuit breakers.

What about the 'black boxes'?

The mystery of flight MH370 may only ever be solved when the aircraft's "black box" flight recorders are recovered.

However, retrieving them from the sea is not easy. In the case of Air France flight 447, it took nearly two years.

If under water, the boxes emit ultrasonic signals - but these signals have a limited range, and search crews may not detect them unless close to the crash site.

Black boxes - described by aviation reporter Stephen Trimble in The Guardian as "one of the most galling anachronisms of modern aviation technology" - are not currently equipped with any form of GPS location transmitter.

https://www.bbc.com/news/world-asia-pacific-26544554