Why do nautical miles linger on?

I recently had an exchange of correspondence with an acquaintance (a former RAF pilot) who tried to explain to me why most of the world of aviation still uses nautical miles and knots rather than kilometres and km/h.  The explanation went like this.

“Now navigation.  There are still lots of aircraft that are flown around the world that do not have sophisticated navigation aids and pilots need simple ways of mentally calculating navigational requirements.  One of the most common is based on the fact that 1 radian (the angle at the centre of a circle that is subtended by an arc equal to the radius) is approximately 60 degrees.  The navigational trick is known as the “one in sixty rule.”

Very simply put, if a pilot is 1 mile off track after 60 miles then the  error is 1 degree which gives a simple way of calculating the change needed to take out the error..  As an example, if the distance to be flown is 120 miles and after 60 miles the pilot identifies that he is 1 mile off track then he needs to turn 2 degrees to make good his destination (one to fly parallel to his track and one to close the destination).   Now this will work for any unit of measurement.  One banana off after 60 bananas is still an error of 1 degree.   The crunch it seems to me is that, as I understand it, the internationally agreed global positioning system is still based on latitude and longitude (all the GPS systems I have dealt with start with a very sophisticated lat/long model of the earth) and the angle subtended by one minute of arc at the earth’s surface on a latitudinal meridian is a nautical mile.  Now any maritime chart or aviation chart/map is overprinted with the lat/long grid so it is very easy to see 1 minute of arc and therefore see what 1 nm [sic] looks like irrespective of the scale of the map.   It makes using the one in sixty easier.  One could do it in kms but I think you would need to know the scale and use a ruler to measure kms so why make life difficult?.  As the Merecats would say – Simple.   Incidentally this also explains the dominance of using knots as a measurement of speed.”

This was my reply:

“Thanks for the explanation, which I think I understand. [Actually, I didn’t fully]

In your example, the deviation from course (1 nautical mile or banana) divided by the distance travelled (60) is in fact the sine of the angle subtended, and inverting this gives 0.955 degrees – i.e. roughly one degree off course. Presumably, this is good enough for travelling short distances. ( I couldn’t see the relevance of radians, but I note that one radian is 57.296 degrees, and this divided by 60 degrees gives 0.955). So far, so good. However, as you say, this relationship is independent of measurement units.

Turning to the latitude and longitude grid, surely this only works in a due north-south direction, as the parallels of latitude are shorter as you approach the poles. Measuring from my Philips world atlas, I calculate that 5 degrees along the equator (i.e. 300 nautical miles) in Brazil represents 554 km, which gives 1846 m per nautical mile (the SI definition of a nautical mile is 1852 m), whereas, measuring horizontally from British OS maps, one degree at latitude 50 degrees north (The Lizard) represents 1190 m. At 60 degrees north (Shetland) it is 940 m. So even at the scale of the UK, there is a considerable difference – and this ignores the related problem of portraying a curved surface on a flat map! (I also thought of introducing the Heisenberg Uncertainty Principle, but that would be a digression).

I suppose that if you have a pair of dividers to hand you could use the vertical scale of minutes of latitude to set the dividers to measure in nautical miles, but assuming that the map or chart has a scale in km, it would be just as easy to set it to measure in kilometres. Or use a standard scale rule calibrated in metres at the appropriate scale (such as architects and planners have used for 40 years). (The 1939 OS national grid, which I believe is used by the army, is based on kilometre squares).

(Incidentally, I gather from browsing internet sites that GPS can use decimal degrees as an alternative to degrees, minutes, seconds – so it is not necessarily dependent on minutes – hence, nautical miles are not essential to GPS systems. As I understand it).

So my conclusion is that the claimed advantage of using nautical miles is fairly weak. It must be primarily a question of resistance to change and the historical domination of the Americans in the aviation industry. Of course, changing the habits of a lifetime is always inconvenient at first, but I would have thought the long term advantages of a world-wide system, used and understood by all for all purposes, far outweighs the temporary inconvenience of a small minority having to adjust to change.

Incidentally, as NATO armies work in km, whereas air forces work in nautical miles (and feet for height?), what units do they use when they need to talk to each other about ranges, distances, heights etc – e.g. an OS map shows that a mountain is 782 m high, so what altitude do I need to fly at to clear it? (I seem to remember a Chinook helicopter flying into a hillside on the Mull of Kintyre – attributed to pilot error – could confusion over measurement units have had anything to do with it?).

Anyway, I have gone on too long. Hope this makes some sense.”

There was no reply from my correspondent – so I did a little more research.

It seems that the fundamental problem is that maps are flat, whereas the Earth is (roughly) spherical. So a co-ordinate map grid based on kilometre squares that is suitable for a relatively small land area – say, the UK – does not work when extended to a continent. (I am advised that navigators on ships crossing the Irish Sea have to make minor adjustments when they sail from the British National Grid area into the Irish grid area, as the latter has a different origin). For longer distances, navigators use latitude and longitude as a co-ordinate system in order to determine their position and their course.

However, what I still do not understand is why this should affect the units of distance used. There is no particular logic in dividing the Earth’s circumference into 360 degrees of longitude and then into 21 600 minutes (i.e. 360 x 60), and then using the distance that one minute represents at the equator (and only at the equator) as the basis for a unit of measurement.  Wouldn’t it perhaps be more useful to divide the distance from the equator to the poles by a convenient number – say, 10 000 – and then base measurements on that?  But, oh, I forgot: that’s exactly what the founders of the original metric system did.

In fact, until the Second World War, most aviation outside America and the British Empire actually did use the kilometre for distances (and, consistently, metres for height), and indeed, for domestic aviation, Russia still does.  It was only the post-war dominance of the USA in IATA and ICAO (supported of course by the British) that imposed nautical miles on an otherwise metric world.

Or have I missed or misunderstood something?  Can anybody help?  Above all, is there any hope of getting the situation changed?

Author: UK Metric Association

Campaigning for a single, rational system of measurement

36 thoughts on “Why do nautical miles linger on?”

  1. I’ve seen this claim about the indispensability of the nautical mile before on several occasions.
    It is quite telling that those who advocate it never take account of the fact that the alleged convenient relationship between it and the minute of arc in geographical co-ordinates, only holds in one direction except at the equator.
    This implies a lack of proper understanding of the underlying geometry.


  2. Dear All,

    Some time ago I proposed an angle measuring method that involved measuring a right angle (a quadrant) using the unit ‘quad’ with ‘q’ as its SI symbol. The following angle measurements relative to distances on any great circle route on the surface of the Earth would then apply:

    1 quad = 10 megametres (10 000 kilometres)
    1 milliquad = 10 kilometres
    1 microquad = 10 metres
    1 nanoquad = 10 millimetres

    It is interesting to note that the very first unit in the initial ‘decimal metric system’ was the quadrant; without the quadrant as the basis of measurement the metre could never have been defined at all.

    Pat Naughtin
    Geelong Australia


  3. There already is an angular unit of measure that was once part of the metric system called “gradian” or “grade”; symbol gon. A circle equaled 400 gon, thus a quadrant of that circle would be 100 gon.


    If we apply this to the earth and say the distance from the Equator to the North Pole is 100 gon and 100 gon is the same as 10 Mm, then each 1 gon increment would equal 100 km. The centigrade (cgon) would thus be equal to 1 km of distance. A metre would subtend an angle of 10 microgrades (µgon) or 0.01 milligrades (mgon).

    Whatever the advantages given by the RAF pilot is just a matter of what he was use to. It seemed easy to him because he adapted to it. Had he learned other methods and been equally proficient in other units, he might not see the nautical mile system as the most convenient or efficient.

    According to Wikipedia there is also advantages to doing mental math using gradians. The only disadvantage cited is inability to relate to the degree system using rounded numbers. But if you never need to refer back to the system based on degrees then there are no disadvantages.

    I can see why the gradian was dropped. There is no reason for SI to have a second unit of angle when they already have the radian. (There is also no reason to have degrees Celsius when kelvin would work fine in all applications.) Now-a-days with computers and GPS units there is no reason for anyone to do calculations by hand just as there is no need for people to use a slide rule. Thus the complications of any choice of units is highly reduced. It is most likely that with devices readily available to do the difficult calculations involving non-SI units, the need to change is not seen as a priority by most of the unconverted.

    I don’t really see the nautical mile as being an imperial or USC unit. It is in a a collection by itself. If the unit was called something other then “mile” then the relationship or connection with other imperial/USC units would not exist. In fact the nautical mile is a precise number of metres (1852 to be exact), which when converted to miles, feet or inches comes out to a number that never ends. It is more metric then imperial/USC.

    The main problem with nautical miles is that they are often confused with land miles and most people if not all who don’t connect with nautical terms would not think there is a difference between the two. If a country has a sea territory of 100 nautical miles (185.2 km), how many people would think it is the same as a 100 miles on land (161 km). The difference of 24 km is very significant.

    The difference between the nautical mile and the land mile is 243 m. Thus every 4 miles would be about 1 km difference.

    A change away from the use of nautical miles and the degree grid system would most likely occur when the US no longer is in charge.


  4. The suggestions for alternative angula units mentioned above are quite neat but we mustn’t get too carried away when relating them to distances on the Earth’s surface.

    Remember Earth is not a perfect sphere and if we take the circumference as 40 megametre there will be, on average, a 0.1% error and as much as 0.19% at the equator in an East-West direction. This doesn’t make any allowance for terrain.

    As far as SI is concerned it would go against the grain because units ‘accepted for use’ are there for historical reasons. The spirit of SI is not to invent new units that duplicate what is already there.


  5. There is a lot of history here. Suffice it to say that the means of navigation used by mariners over many centuries were adopted by aviators in the first decades of the last century. These are dead reckoning using for example knots and nautical miles, and celestial observation and astronomical tables using for example degrees, minutes and seconds. Means of navigation using radio waves became available from 1940 onwards and quickly supplanted traditional methods. But old habits die hard, and I noticed yesterday in BMI’s magazine “Voyager” that the cruising speeds of their fleet of aircraft were given in mph and knots only.

    However, there are signs that in some circles the present is now considered to be just as important as the past. For example, most airlines employ the same software to keep passengers informed about of the progress of their flight. This provides information as follows:

    Distance to destination (miles and km)
    Ground speed (mph and km/h)
    Head or tail wind speed (mph and km/h)
    Altitude (feet and metres)
    External air temperature (°F and °C)

    And on the question of change, I suspect the enemy is inertia. Our DfT can, of course, be relied upon to dust off the arguments used already to discourage consideration of changing road signs and also to produce a grossly inflated estimate of cost.


  6. Hi there
    I need to study your article carefully but surely the real question is why did we ever bother with metres when we already had a perfectly good and more accurate imperial metric (sic) system.
    There are 100 fathoms to the cable and 10 cables to the (original) nautical mile.
    Do the maths and you’ll discover the original nautical mile was 0.11% accurate against the earth measure it related to (the equatorial circumference).
    Compare that with the metre. Poor old Mechain made a right hash of it with his measurements around Barcelona and the original metre was 0.2 mm in error. Do the maths again and even the corrected version is still in error by 0.15%. Nautical miles win again.
    Jeremiah seems not to understand that relating the nautical mile to metres is because some latter day committee irrationally conceded to readjust a highly accurate measure to a whole number of a less accurate measure. And how does that stack up with Philh’s observation that the spirit of SI is not to duplicate what is already there?
    We also had an imperial land measure that was already metric – 100 links to the chain and ten chains to the furlong. So I look forward to your next article – “Why are horse races still run in furlongs”.
    The real answer to all of this is one of national rivalry. The French wanted their own land measurement – something that would be just a bit longer than the yard. But immediately you then have to concede that the yard was already of the convenient “order” of size. I don’t blame the French for that – in those days we were both imperial nations. What grinds my gears is when people nowadays start comparing the new against the old as if the new is better and the old wasn’t fit for purpose. Neither being true. Why not just allow people and organisations to use whatever measure they find most convenient? Yet in Britain we’ve made it a criminal offence to use the “wrong” units even between consenting adults.
    PS Your reference to the Chinook crash is unfortunate in its timing. It was likely due to a “positively dangerous” engine control system. Although seemingly “news” in the last few days, Private Eye has been defending the falsely discredited pilots (who died in the crash and became convenient scapegoats) for years, citing the control system as the most probable cause.


  7. One of the reasons why degrees are used for angular measure rather than grads is time zones. We are wedded to a 24 hour day, so having 15º time zones makes sense, but having 16.67 grad time zones is messy. Of course, the use of nautical miles follows on from minutes of arc.


  8. The aim of the meridian survey of the 1790’s by Delambre and Mechain was more political than scientific, although the surveyors may not have recognised this at the time. Those who commissioned the survey wanted a measure seen to be independent of national interests that would be accepted everywhere. In this they were not entirely successful, as many in the UK still see the metric system as essentially French. But the metre has now become the standard of length of every country of the world bar none, and the epic story of the meridian survey can take some credit for this.

    Mr Goodhand also asks, “Why are horse races still run in furlongs?” Well, as race goers may know and the rest of us may not, one furlong is 40 poles or 220 yards or about 200 metres (201.168 m exactly). Why change to a simpler and universal measure when you are familiar with the specialist units of your specialist field? Anglers, aviators, chefs, engineers, mariners, pharmacists, printers and a host of other groups have asked themselves this question, and most, but not all, have concluded that the long-term benefits of using the universal measurement system outweigh the short term costs of the changeover. Clearly, race horses and their owners are not yet convinced.

    For almost 800 years, it has been a criminal offence to use the ‘wrong units’ even between consenting adults. Recent objections to this old-established principle arose when the ‘right’ units ceased to be imperial and became international. But this is not the first time this has occurred. Some Scots objected to English units when these were imposed as a consequence of the Act of Union of 1707. Three centuries later, few dispute the need for that changeover. And a century from now, can we expect a similar verdict about the UK’s metric changeover (assuming it progresses beyond the current muddle)?


  9. When the Dutch aircraft museum was still at Schiphol Airport at Amsterdam I visited it a few times before flying to Ireland. There was a pre-war Fokker passenger plane and I looked in the cockpit. And lo and behold, the instruments were all metric. If they were able to fly in kilometres and metres in those years, there can be no special advantage in flying in feet, nautical miles and knots now. This retreat of the metric system from 1945 can only be blamed on the Second World War and its consequences.
    Gliding in metric countries has continued to fly the metric way: altitude and climbing/descent speed in metres, distance in kilometres and speed in km/h.
    When visiting a Navy Day some years ago, I saw that the instruments of their helicopters were as metric as possible. Only speed, distance and altitude were excepted. Pressure etc. was metric. The Dutch Navy uses metric and non metric where that is unavoidable. Fathommeters (I have no objection to that word) on ships were not set to fathoms or feet, but to metres.
    I find it strange that a purely agrarian unit of length, the furlong (or furrow long) became the ‘standard unit’ in horse racing. This unit was also used in some parts of pre-metric Holland, it was called the ‘voorling’ and was also about 200 m long.


  10. “….why did we ever bother with metres when we already had a perfectly good and more accurate imperial metric (sic) system.”

    Says what authority?

    “…. the original nautical mile was 0.11% accurate against the earth measure it related to (the equatorial circumference).”

    “….Compare that with the metre. Poor old Mechain made a right hash of it with his measurements around Barcelona and the original metre was 0.2 mm in error. Do the maths again and even the corrected version is still in error by 0.15%. Nautical miles win again.”

    The big problem with using the earth as a basis for any unit of measure is that it varies. The earth swells and contracts, which means each time you take a measurement your results will vary. It was OK in the dark ages, but this is the 21-st century and we need measurements that are accurate to the size of sub-atomic particles. Only the metre, by being defined relative to the speed of light meets the criteria. Barley corns, kings toes, etc just won’t do.

    “Jeremiah seems not to understand that relating the nautical mile to metres is because some latter day committee irrationally conceded to readjust a highly accurate measure to a whole number of a less accurate measure.”

    Like it or not the nautical mile IS defined relative to the metre and nothing else. It is exactly 1852 m precisely. Any instrument made today that uses nautical miles is made with the 1852 m definition in mind. If it weren’t for this definition, the nautical mile would vary as the earth varies. I don’t know where you get you idea that a measure based on the fluctuations of the earth would be “highly accurate”, but it is a good thing you don’t have a position of prominence when it comes to making decisions on units.

    “We also had an imperial land measure that was already metric – 100 links to the chain and ten chains to the furlong.”

    What inconsistency! Some units relate by powers of 10 others by factors of 3, 12, 36 and whatever. No wonder the world fled to the use of SI.

    “So I look forward to your next article – “Why are horse races still run in furlongs”.”

    Not everywhere. In Australia horse racing was one of the first sports to change. Now all races are in metres. I wonder though if even in the UK the race is set up for 200 m and is just called furlong. Who would be able to tell the difference?

    “… you then have to concede that the yard was already of the convenient “order” of size.”

    Convenient by whose definition?

    “… What grinds my gears is when people nowadays start comparing the new against the old as if the new is better and the old wasn’t fit for purpose.”

    The best answer for this nonsense is that if imperial was so great why didn’t the whole world adopt it? Why did they go for the metric system? Because the metric system is better. The old was fine for a stone age culture but not a technical culture.

    Funny how the world was technically backwards and primitive the entire time pre-metric measures were used. Yet, within a few years after the metric system was mandated in France in 1840 and was pretty much entrenched in Europe by 1850, that scientific discoveries took off and the industrial revolution began? Is it coincidence or did the invention of the metric system play a pivotal role? Why are the progressive nations of the earth using the metric system and the declining powers, such as the UK & US still desperately clinging to obsolete units?

    “…Why not just allow people and organisations to use whatever measure they find most convenient? Yet in Britain we’ve made it a criminal offence to use the “wrong” units even between consenting adults.”

    I have no problem with this as long you are OK with the right of businesses to produce its products in metric , even if it has to leave the home country to do so unemploying tens of thousands of people. Would you have a problem with companies discriminating against people who don’t know the metric system by not hiring them or letting them go if they are already employed?


  11. By the way, there is nothing inherent in the way GPS works or is defined that has to do with the degree of angle or the nautical mile. GPS is metric internally; the data fields of the 50 bit/s broadcast data (satellite orbit ephemeris, clock error, etc.) use the following units: metre, second, semi-circle, week, 1, second^-1, semi-circle/second, radians, and sqrt(metre). No degree and no nautical mile anywhere.
    [Reference: ICD-GPS-200C, 1993]

    The standardized GPS algorithm outputs its navigation solution in a simple Cartesian XYZ coordinate system known as earth-centred-earth-fixed (ECEF), which has its origin at the centre of gravity of the Earth (including its atmosphere). One of its axes coincides with the mean rotational axis of the planet, one axis goes through the IERS Reference Meridian (which btw passes 102.5 metres east of the Royal Observatory in Greenwich due to a historic measurement error). The unit of scale is 1 metre.

    The user interfaces of GPS receivers allow you to convert these XYZ ECEF coordinates into a range of other coordinates, including e.g. UK Ordnance Survey grid coordinates. For users of global polar coordinates, GPS receivers can offer several possible ways of expressing height. One of these is known as the WGS84 datum surface, and it approximates the shape of the Earth as an ellipsoid with 6378137 m radius at the equator and 6356752.3 m at the poles. There are several more sophisticated global “geoid” models (e.g. EGM96) which lead to a better match of 0 m height in polar coordinates and actual mean sea level, and which many GPS receivers also implement. You can also correct for tectonic plate movement, by converting to a coordinate system that is attached to you local plate (the one normally used in Europe is called ETRS89). Again, no degree or nautical mile in any of these post-processing steps!

    Finally, if (and only if) a polar coordinate output format is selected, many GPS receivers will also allow you to chose which unit of angle you want to see, e.g. deg, gon or rad, and whether you want your deg angles subdivided decimally or into minutes and seconds of arc. But that’s really just the very last step of the user interface and has nothing to do with how GPS works or is defined.

    So GPS technology is certainly no reason for sticking to nautical miles. GPS does not care whether you chose to divide the quarter circle into 90° or 100 gon. It is all just down to tradition.


  12. No one is suggesting we should still have a unit of length defined by some indeterminate natural measure. The nautical mile was fixed at 6080 feet. The updated international nautical mile is defined as 1852 metres. What’s key is that the imperial and metric measure of length are precisely related by 2.54 cm = 1 inch. As the metre is now defined in relation to the speed of light, both systems are locked down to an exact repeatable measure and we should all be happy using either as the mood takes us. [So airline pilots and air traffic controllers should be free to choose whichever unit they prefer? – Editor]


  13. Robert Goodhand’s concession that the nautical mile is not after all a “natural” unit is interesting as it contradicts its main claimed advantage – namely, that it corresponds to minutes of latitude (although not of longitude). It also undermines the argument that nautical miles somehow assist in navigation.


  14. I think my comment about the “spirit of SI” may have been misunderstood.

    It was intended as a reference to the principle of one unit for each type of measurement e.g. metre for length or distance, second for time, etc. In spite of appearances the km, mm are still metres – the prefixes are just for convenience.

    The metre is meant to *replace* the nautical mile not duplicate it.

    As far as angular measure is concerned I may have referred to SI incorrectly. The radian is a fundamental unit underlying geometry independently of any human method of counting. The series for sine, cosine etc only work when the angle argument is in radians. It’s a bit like the exponential function (e) in the calculus. A kind of “natural” number.

    The degree, grad etc are more arbitrary. Humans need an integral subdivision of the circle for practical purposes. We don’t cope very well with irrational numbers like pi.


  15. I’ve been a bit slow in responding to Derek’s comment on the 4th about the software used on flights to keep passengers informed of their progress. Within the last month I’ve flown with Virgin Atlantic who have their systems set on their flights to imperial only. This has been consistent on all flights I’ve taken with them since 2004.

    Ironically, I’ve also flown transatlantic with Delta (and at least one other US carrier) who show information in both imperial AND metric!


  16. Most USC educated pilots insist that knots and nautical miles are ideal
    measurements for aviation. So, let us scrutinise that “logical choice
    of measurements” and start with cruising heights of aeroplanes.
    Commercial airliners fly between 5000 and 11 000 m, or 15 000 to 33 000
    ft. Air traffic regulation requires pilots to denote 30 000 feet as
    flight level 300 and 11 000 feet as level 110, below that level actual
    numbers of feet apply. The reason for adding this extra tier of is to
    avoid unwieldy numbers that are all too easily misunderstood. Another
    claim is that 1000 feet are an ideal air traffic separation unit. If it
    is, metric countries are not privy to this fact. What is wrong with 300,
    400, or 500 m? Outdated measurements in aviation are an extra burden on
    metric educated pilots, the vast majority, because in emergencies they
    are prone to think in metric. How often and whether this has contributed
    to accidents is anybody’s guess?

    There is only one reason why the world has to fly in outdated
    measurements and it has nothing to do with customary being the “logical
    choice”. America loath to join an already 60% metric world after 1945,
    created ICAO (International Civil Aviation Organisation) in 1944
    (Chicago Convention) to foist its obsolete units on modern aviation. On
    behest of Uncle Sam and his reluctant allies, who flew in imperial
    dimensions, ICAO enshrined customary units as the preferred (what irony)
    measurements in aviation. European and other countries flying in metres
    had no choice, but to sign. To make that retrograde step palatable to
    metric nations a non-binding a preamble to metricate all aspects of
    aviation was added to the ICAO’s protocol.

    More then 60 years on theworld flies still in archaic units, but pilots are allowed to calculateload and fuel burn off in kg. Neither logic nor perceived advantages of
    archaic measurements, but America’s super power status after 1945 dictated events in aviation. Had it not been for concerted efforts of metric countries to put a stop to Uncle Sam’s measurement “imperialism” after 1945, UN agencies would have been forced to return to medieval times and work in mind boggling customary units today! All this proves is that intelligence does not always equate power.


  17. I thought eric’s reference to an annex to the ICAO protocol might be interesting, and indeed a google search revealed this webpage:

    Click to access an05_info_en.pdf

    I quote from it”

    “The question of the units of measurement to be used in international civil aviation goes back as far as the origin of ICAO itself. At the International Civil Aviation Conference held at Chicago in 1944, the importance of a common system of measurements was realized and a resolution was adopted calling on States to make use of the metric system as the
    primary international standard.”


    “In addition to the SI units the amendment recognized a number of non-SI units which may be used permanently in conjunction with SI units in aviation. These include the litre, the degree Celsius, the degree for measuring plane angle, etc. The amendment also recognized, as do the relevant ICAO Assembly Resolutions, that there are some non-SI units
    which have a special place in aviation and which will have to be retained, at least temporarily. These are the nautical mile and the knot, as well as the foot when it is used in the measurement of altitude, elevation or height only. Some practical problems arise in the termination of the use of these units and it has not yet been possible to fix a termination date.”

    So it appears that the mechanism is there for the current situation to be challenged. It presumably requires an alliance of states prepared to “take on” the Americans within the Council of the ICAO. China, Russia and France perhaps?


  18. A minor correction to eric’s points above.

    “Flight level” actually means the altimeter is NOT corrected for local variation for sea level pressure. Below a transition altitude, a correction (called QNH) is used, such that on the ground, the altimeter would indicate height above sea level. The pressure/altuitude relationship is based on a “standard atmosphere” true at about 45° latitude, and is NOT corrected for variation at the poles, equator, lapse rate, etc. Aircraft fly pressure contours, not real altitude. However, with QNH correction, it is referred to as altitude, but is still assigned in hectofeet by air traffic control. The transition rules for altitude vs flight level vary by country; for the US it is 18000 ft.

    In countries which fly metric (Russia, China, Mongolia, CIS States), 300 m separation is used in the same sense that 1000 ft separation is used in foot flight countries.

    Regional jets do not typically have “glass cockpits” and sophisticated, switchable instrumentation, long-haul jets do, and can accomodate the switch to metric altitude when flying into or over the above countries.

    A bigger barrier is the ATC equipment on the ground, which is often decades old and not so sophisticated. If it is set up for feet, I suspect it would have to be replaced with new equipment that could switch or be metric only. This is probably a major barrier to Europe or other metric countries switching if they have been using feet for aviation. More pilots would need additional training, but long-haul jets from the US could handle the transition better than local ground facilities. (We fly to Russia, China, etc and obey ATC in their airspace.)


  19. Somewhat belated, but still relevant to flight level/altitude. What I am talking about is that altitude should be expressed in flight levels above 10 000 feet. Maybe this article explains it?

    Flight level or altitude confusion occurs when a pilot is cleared to fly at a particular level and correctly acknowledges this clearance, yet levels at a different flight level or altitude.
    Flight level or altitude confusion is usually the result of the combination of two or more of the following factors:
    Read-back/hear-back error because of similar sounding phrases;
    Non-standard phraseology;
    Mindset tending to focus on two digits, e.g. “one zero” and thus to understand more easily “FLIGHT LEVEL ONE ZERO ZERO” when the clearance was to FL 110;
    Failing to question the unusual (e.g. bias of expectation on a familiar standard terminal arrival (STAR); and/or,
    Subconsciously interpreting a request to slow down to 250 kt as a clearance to descend to FL 100.


  20. Essentially knots and nautical miles only hang on for those who do global navigation (Naval & Air travel) both of whom are still reckoning by the sun and stars, and curvature of the earth. The original naval nautical mile was a minute of arc (N-S), and was measured from the moon and stars and was accurate enough at the time. Now it (the angular measure) isn’t accurate enough.

    However tha SI system does still have its problems because it doesn’t recognise Angle as a distinct Dimension. Thus we have N.m and m.N for both torque and work, yet one must be multipled by Angle to get the other! Perhaps a topic for a new thread? http://www.iso.org/iso/keeping.pdf [The SI unit for plane angle is the radian (rad). As it is defined in terms of pi, it is arguable a better unit than the degree, which is purely arbitrary, and hence the nautical mile, knot etc, which are based on dividing the Earth’s circumference by 2160, are equally arbitrary – Editor]


  21. Phil raises a good point above. The methods of sailing long distances and of fixing position by celestrial sights both involve spherical trigonometry. While a slight error, it has to assume a spherical earth to make the calculations manageable using tabular methods or hand calculators. On a spherical earth, 1 nautical mile is 1 minute of central angle, measured from the earth center, anywhere on the surface. A plane cutting the center, current position and destination forms a “great circle” on the spherical earth. (Note that lines of latitude are “small circles” except for the equator)

    Since the earth is ellipsoidal, some approximation is unavoidable. Past nautical miles have either been 1 minute average on a meredian (slightly over 1852 m) or 1 minute on a spherical earth with the same surface area as ellipsoidal earth (slightly over 1853 m). Rounding to either integer value (or switching) leads to minor errors compared to speed and bearing instrumentation on historical ships and airplanes (and navigating in a moving fluid).

    Great circle sailing consists of finding some intermediate points on the great circle route and sailing between them using plane sailing or mid-latitude sailing. The three together are known as “The Sailings” to navigators. The result is an arc specified by its central angle in degrees. It is easy to multiply by 60 for nautical miles, almost as easy by 111.12 for kilometers, or 69.05 for statute miles (with a calculator). All three are slightly wrong because the earth is an ellipsoid.

    Prior to electronic navigation, course holding and speed of both planes and ships had larger errors, and needed intermediate fixes and course corrections. The basic method is to assume a position, calculate an altitude of celestrial body and compare to an observed sextant altitude. The error in minutes leads to a Sumner line of position, whose perpendicular distance to the assumed position in nautical miles is the altitude error in minutes. Two or more Sumner lines of position lead to a fix.

    Obviously, modern GPS navigation is far better. The question is whether this is a useful backup method in the event of electronic failure. Small boats (crossing oceans) routinely carry a sextant and navigation tables to carry this out if required. The Navy used to on ships, I’m not sure if they still do. Is it important or a way to haze midshipmen? Debatable. Still, with a cheap plastic sextant and Table 35 (Ageton’s method) I could navigate a lifeboat to a port.

    However, I think those who criticize the unit should learn “The Sailings” and how to reduce a sextant altitude to a line of position before they decide the unit is useless.
    (All of this has no bearing on feet or meters for plane altitude; I think that is a matter of the ATC equipment installed in the country.)

    By the way, great circle sailing can be updated to great ellipsoid sailing using the series expansion known as Vincenty’s equations, widely used in geodesy and in GPS. I would suggest they are tractable on a computer or programmable calculator, intractable on a hand calculator or by tabular methods. You would either be there or miss before you determined the solution by hand.


  22. In answer to John Steele.

    The debate is about whether the nautical mile should be retained as a primary unit of distance for aerial and maritime navigation.

    The “sailings” method used in exceptional circumstances may invoke some reckoning of distance in the manner described, but does that justify the use of the nautical mile instead of meters in routine navigation?

    The circumference of an average great circle on Earth does happen to be very close to 40 000 km. Perhaps that too should be considered as having some merit.


  23. Philh,

    I think it depends on whether we retain degrees and minutes as the primary angular notation for latitude and longitude on maps. If we do, nautical miles play nicely with these units.

    If all the maps are changed to lat/long either in radians, or in grads, navigators would probably prefer to change. Grads of arc would approximate 100 km, and centigrads, 1 km. (New reduction tables in those units would be required)

    It is a specialty unit used in a narrow field and the CIPM seems to have accepted it. If they can’t accept it, they should demand absolute SI compliance of all disciplines, no astronomical units, liters, tonnes, hectares, barns, Angstroms, minutes, hours, days, angular degrees, minutes, and seconds, bars, mm Hg, etc.

    I admit there are several of those I can do without. but I assume they listen to how hard the experts in the field whine, and have some process for deciding. There is a rationale for both the value and continued use of the nautical mile. Even if it is not rock solid, it is more solid than any argument for the continued use of the International Mile (or in the US, in many States, the Survey Mile)

    However, I would point out the CIPM does accept it and Table 8, footnote d gives a rationale approximating mine. I don’t think we on a forum will have much impact if people who actually navigate (and make maps) oppose the change. [This is all very interesting, but we seem to be talking past each other. What nobody has explained is why the co-ordinates used for global navigation (which are only dimensionally consistent in the special case of north-south direction) should determine the units of length used for measuring distance and speed. They are independent. – Editor]


  24. Isn’t the dominance of imperial units in aviation post-WWII not due to any American conspiracy to impose imperial measurements, but because airlines worldwide in the immediate post-war era overwhelmingly used war-surplus DC-3s, with imperial instrumentation? [Not really. The Convention on International Civil Aviation was signed in Chicago in December 1944 – see http://www.icao.int/icaonet/arch/doc/7300/7300_orig.pdf. IATA was founded in April 1945 in Havana, Cuba – see http://www.iata.org/about/history.htm – Editor]


  25. Well, I attempted to explain it (three up, i think):
    “What nobody has explained is why the co-ordinates used for global navigation (which are only dimensionally consistent in the special case of north-south direction) should determine the units of length used for measuring distance and speed. They are independent. – Editor]”

    Spherical trigonometry is complex, however, the basic triangle solved is from the point of origin to destination, and the nearer pole. The three sides are the two co-latitudes and the oblique great circle cutting origin and destination. The three angles are the meredian difference, and the forward and reverse initial bearings. All the sides are measured as arcs, and the distance from origin to destination is calculated as an arc, seen from the center of the earth. It may be specified in degrees and minutes, or all minutes. In “all minutes” it is nautical miles (recognizing that spherical earth is an approximation).

    I do not understand what you mean by inferring nautical miles and degrees are only consistent in a north-south direction. (other than circles of latitude being small circles except for the equator). You do have to solve the spherical triangle, not just take differences in latitude and longitude.

    But, if you want navigators to love distances in kilometers, change the angular units of latitude and longitude to grads, and reprint all the maps.

    [I think we can all understand (in principle) the trigonometry behind the argument (we all learned about Pythagoras at school). But it STILL doesn’t explain why the unit of distance has to correspond with minutes of a great circle. It sounds like special pleading from a profession that wants to stick to what it is used to – rather than use the same system as everybody else – Editor]


  26. Good debates all, but math and logic not withstanding…I believe the real deterrent for changing to SI for marine and air navigation are cost and good old “reluctance to change mentality”. Unless it saves someone time and/or money, people don’t want to make a change, that costs them time or money. And that’s for all people, not just the Yanks.


  27. There is always the perception that it will be a large cost initially, but this may be unwarranted as accurate cost estimates are seldom done. Future cost benefits are never factored in thus the costs of not changing can exceed the cost to change, if there is a cost to change.

    If everything is computerized, the costs to maintain the old system can be hidden when the computer can be programmed to display in any combination of units. With computers, the cost to change could be zero as all that would be required would be a few seconds to change a setting to prefer metric units.

    Consider that through the 1990s as shops upgraded their weighing scales to dual as they needed to be replaced anyway, that on 2000-01-01 there would be no cost to switch the scale to metric mode simply by pressing a switch or changing a setting. After that date as scales wear out or when they are naturally upgraded they can be replaced with metric only scales. Thus the costs can be zero if planned properly. Resistance and refusal to change is what adds costs.

    The fact is, once everyone is working in one standard, supported system, the costs and errors (errors are costly too), become zero.


  28. Okay this is pretty old, just came across it, but here goes!

    “Above all, is there any hope of getting the situation changed?”

    Why do you think the situation needs to be changed? The aviation world works in knots for speeds, nautical miles for navigational distances, metres for horizontal separation between traffic, and feet for height and altitude. There is no problem with this; it’s not as if these matters intersect or confuse the average person’s daily lives, so why impose a change for purely dogmatic reasons – “my unit of measurement is better than yours!”. Who cares? Changing would be expensive – manuals and charts reprinted, re-training necessary, instrumentation changed. And for what? Pilots are used to thinking in knots and feet, as are ATC, etc. It doesn’t cause issues but switching, especially without the proper training, really could lead to problems – 130 now becomes a dangerous speed where previously it was a safe one, for example.

    There’s a terrible amount of misinformation in these comments. Fundamental misunderstandings about flight levels, for example. Also this: “Outdated measurements in aviation are an extra burden on metric educated pilots, the vast majority, because in emergencies they are prone to think in metric.” No, pilots, when flying, do not think in metric. We think in knots, feet, and nautical miles. That is what we are trained in from day 1.


  29. @BenK says: –
    “No, pilots, when flying, do not think in metric. We think in knots, feet, and nautical miles. That is what we are trained in from day 1.” Do they really? Do you really think a French or German pilots have a ‘natural thinking reaction in feet and miles’? I would need an awful lot of convincing of that theory.
    Why change?
    You give a very good example of why change. Why should pilots from every country in the world bar 1 1/2, have to learn a new (old, out of date) system of measures used only in flying?
    The American pilots have to learn metres for horizontal distance and in most cases all airfield dimensions. Many already know and understand them. Every other nation has to learn feet and miles ‘just because that is how we do it, because that is how we have always done it, and that is how we are going to keep doing it’.
    If that is not a pointless exercise in stupidity, when there is a sensible world standard, recommended by ICAO since before the war (the first time I have seen that), then I need a new definition for that word.
    One day the big bang will come.
    A little off topic but in the same range, at what point in launching a missile do the operators change from ‘thinking in miles, yards and feet’ to actually firing the missile or field gun, calibrated only in metres? That point I always ponder when someone gets killed due to ‘an error’ somewhere in the non-system.


  30. Ben,

    Even though you think we should keep miles in aviation because that is the way it has always been, you are wrong. We need to change to a uniform system of units that everyone understands. You think because these pilots are trained to use feet from the first day that they get a feel for them, that is very wrong. It doesn’t imply that if your instruments are calibrated in a set of units, you are given instructions in those units, you can read off the units from the instruments, that you have a gut feeling for those units.

    People from metric countries who are trained from childhood to think in metres only have a gut feeling for metres and everything else they just deal with on a individual basis. Even Americans who think in miles are not savvy to nautical miles. To them a mile is a mile is a mile. They don’t know or care to know there is a big difference between the two.

    The same goes for decimal inches in US machining versus fractional inches used elsewhere in US life. American machinists always rely on charts to get a feel for the decimal inches by comparing them to the nearest fraction. This makes them inefficient and prone to errors. Metric machinists use the same decimal millimetres on the job as they do in their home life. No need for charts, no confusion, less errors, less cost to themselves and others.

    A uniform standard for everyone is what works the best, even in aviation. Once China overtakes the US as a world power, they will push the aviation community to fully metricate.


  31. @BenK.

    You say.. ****Why do you think the situation needs to be changed?****(with reference to measurement).

    What is the advantage of using two systems of measurement? Sure it works OK and it ain’t broke. But so was the horse and cart, but it was soon replaced by the horseless carriage, the mother car.
    Metric is the measurement of science and technology, and both of these are predominate in aviation. Most modern aircraft can be configured, or switched from USC/Imperial to metric. The older ATC technology is restricted to USC/Imperial, but as new generations of this technology comes on line, the conversion to metric becomes more probable.

    I’m not a pilot, but I know that pilots are human, and although you say, for you, this mixture of units doesn’t cause issues, it does for other pilots.

    From the link below..
    Distance Across The Ground

    World-wide, the nautical mile (nm) is the standard for measuring the distance an aircraft travels across the ground. That’s nice and easy and it makes me happy.

    Other lateral measurements are a mess. Most of the world measures runway length in meters while North America uses feet. Most of the world measures airport visibility in meters. North America? Not nautical miles, not meters, but statute miles! Huh?? Not to worry, North America changes back to feet when measuring Runway Visual Range (runway visibility measured with a laser), while the rest of the world sticks with meters. Confused? I sure am, and I do this for a living!

    See link.. http://aerosavvy.com/metric-imperial/


  32. Back in the early 1980s, I was on a plane from Vancouver to Toronto. The chap in the seat next to me and I got talking, and he was from Transport Canada (which set Canada’s aviation standards). As I was involved in Canada’s then-recent metric conversion in the construction industry, I asked him what were the chances of the aviation industry converting. “Very good,” he replied, adding that he was returning from a conference that was going to start the process, that the USA was committed to metric conversion, and that this would be the catalyst for change. Thirty years later, we are still waiting for that catalyst to take effect…


  33. British measurements including the nautical mile will survive despite the noise being raised by the EU and the rest of “foreigners”. Once we kick the EU out of the UK maybe we can undo the half-baked, half-metrication mess in industry. There is nothing wrong with British units, just bunch of foreigners wanting to impose their stupidity on us English. And as a pilot and navigator, I abhor the fact that the nautical mile was shortened to be an exact number of meters rather than feet. Not only will we scrap metrication in Britain but undo this interference with the nautical mile and our culture. Nelson and Drake must be tuning in their urns at what we have allowed our traditional enemies to impose on us. Now is not only the time to dump the EU but also kill metrication. England Rules OK!
    Editor: Metric Views will post an article shortly on issues related to the forthcoming EU referendum.


  34. Reply to previous comment: When a “navigator” crosses the Irish Sea say from Holyhead or Liverpool to say Wexford, Dublin or Belfast they do not use the OS grids of either country, rather he or she uses a British Admiralty Chart issued by the Hydrographic Office in Taunton and plots both GPS and visual fixes from the various lighted objects set up for the use of mariners. Back in the 1960s, I brought a small ship across primarily using D/F fixes on the BBC and RTE Athlone broadcast transmitters supplemented by visual sightings.
    The error outside Rosslare was less than one nautical mile but by then I had plenty of experience of “inshore navigation”. However, not something for the novice navigator to attempt. (dja.)


  35. @OldBlueBear:

    Oh dear, you must be stuck in some 19th century time warp. You say: “There is nothing wrong with British units, just bunch of foreigners wanting to impose their stupidity on us English.” Do you honestly think that 95% of the world’s population are really ‘stupid foreigners’? That’s pretty arrogant, to say the least. As for them imposing the metric system upon us, may I remind you that it was some enlightened British politicians almost half a century ago who decided, all by themselves, to convert this country to the world’s measuring system.

    Perhaps you should go back to your unheated, candle-lit (electricity is metric, you know), thatched cottage and live in your own non-metric fantasy world … and see how long you survive.


  36. @OldBearBlue,

    If you go back to the foot-based nautical mile, you will be navigating alone as the US adopted the International nautical mile (1852 m) in 1954. Admittedly, even before that, the US and UK had slightly different nautical miles, because the earth is an ellipsoid and there are many places to choose the minute of latitude defined as a nautical mile.

    If you bring back a primary physical standard for the foot (or yard) and pound, you will again be alone as we define ours off the meter and kilogram (since 1893, although tweaked in 1959 by a consensus of six English-speaking nations).

    You are aware most of the world is metric, not just the EU, right?


Leave a Reply

Fill in your details below or click an icon to log in:

WordPress.com Logo

You are commenting using your WordPress.com account. Log Out /  Change )

Facebook photo

You are commenting using your Facebook account. Log Out /  Change )

Connecting to %s

%d bloggers like this: