11 ms·

Using "Knot" because it's tied to longitude and latitude is the exact point the metric system is designed to avoid: needless and inaccurate attachments between

Using "Knot" because it's tied to longitude and latitude is the exact point the metric system is designed to avoid: needless and inaccurate attachments between system of units and specific phenomena that actually complicate rather than simplify and hinder interoperation with every other system. As you state: 1. a knot is only approximately equal to 1 min of latitude per hour (there are numerous cases where it's unusably wrong) 2. doesn't really help for longitudinal travel 3. maps are continuously projected to favor this mediocre form of navigation instead of something more accurate (see Mercator projection) 4. you could easily put kilometre lines on your charts instead of minutes of latitude and fix all the previous problems

Justin_K 10y ago
That's going to work great when the earth goes back to being flat.
Ultimatt 10y ago
No Mercator projection is used because its the least weird conformal projection (you draw a straight line its really straight in the real world). If you are trying to plot a bearing at the scale of the planet you want to be using Mercator unless you are going over the pole :P That the same projection is then used for everything even when you don't need to preserve local angles isn't the fault of Mercator. That you think km lines on an atlas would be better than lat/long makes you crazy. A decimal day might be useful but a decimal year is not. A fractional decimal day is a completely useless reference of time for doing quantum mechanics, but perfectly reasonable for people planning their work. No one would get rid of the concept of "a day". A map that maybe only shows tens or hundreds of km might be fine but representing the whole world with an equal area map with all grid reference based on km isn't useful to many people who use maps a lot, especially as the world isn't describable in whole numbers of km! But does have absolute and continuous angles from the gravitational centre.
- NickNameNick 10y ago
  Lines on a Mercator map aren't straight on a globe. They are constant-compass bearing, but that's not the same as straight. Also, as defined by the French academy of science ~1800, the meter is 1/10,000,000th the distance from the north pole to the equator, on the meridian through Paris.
  - kqr 10y ago
    Not sure what your metre comment is about, but it turned out that definition wasn't accurate enough to confidently state the speed of light, so the speed of light was set to 299792458 m/s and a metre is consequently the distance light travels in a 299792458th of a second. (The second is in turn precisely defined by particle physics.)
    - dalke 10y ago
      It's a reference to Justin_K's earlier statement "a Knot is standard for navigation world wide because the unit is standardized on earth's size". If that justifies knot then it also justifies kilometers, because it too was originally standardized on the earth's size. Neither are exactly the size as originally defined. A modern knot is 1852 meters. One minute of latitude is "about 1,855.325 m on the WGS 84 ellipsoid" says https://en.wikipedia.org/wiki/Nautical_mile https://en.wikipedia.org/wiki/Nautical_mile. The difference is 0.16%. A quarter of the Earth is: 1855.325m * 60 seconds/degree * 360 degrees/circle /4 = 10018755.0 = ~10018 km. The difference is 0.19%.
- deleted 10y ago
  [deleted]
- roywiggins 10y ago
  The shortest distance between two points on sphere is along the great circle that contains the points, and are the closest thing to a "straight line" you can have on a sphere. Straight lines on a Mercator projection aren't great circles, they're rhumb lines, and if anything they're a spiral on the sphere, not straight.
  - Retric 10y ago
    If you are holding a globe that a useful definition. However, that's not what navigators are doing. We can look at the stars and a compass both of which make rhumb lines far more useful.
    - OskarS 10y ago
      Rhumb lines may be useful for tiny vessels with only a compass and a map on board, but any commercial vessel, whether it is on the water or in the air, will follow the great circle route. It's the shortest and fastest way to go and it uses the least amount of fuel. Following rhumb lines just because they happen to be straight lines on Mercator projected maps would be silly. It's not the 1800's any more. We have computers and stuff, we don't have to rely on techniques developed for a time when your navigational tools consisted of a map, a compass and a sextant.
      - zentiggr 10y ago
        The passage of time doesn't diminish the usefulness of local navigation techniques... a Mercator projection will give you a bearing to steer to arrive at your destination, which can be followed by eye, autopilot, etc etc. Calculating a great circle arc and plotting it, following a bearing which may only change by a degree or two but isn't constant, is an unnecessary complication on short routes. Even commercial vessels will use those old techniques if they don't need to consider the longer range effects. Error correction in the human process is always a consideration. (Not even talking about data entry errors in GPS systems, but that Malaysia - Melbourne story was Funny.)
      - Retric 10y ago
- deleted 10y ago
  [deleted]
ChoHag 10y ago
Unfortunately, though, the earth is not a cube.
dredmorbius 10y ago
The SI unit of length, the meter, was originally defined according to Earth measurements, as 1/10,000,000 of one half a meridian. This was supplanted by reference bar measurements in 1889 and 1927, by a count of emitted light wavelengths in 1960, and as the distance traversed by light in a vacuum in a specified time interval in 1983. (So space is measured by time.) The metric system has its own foibles, one of which is being based on the number of digits a phenotypical abundant primate on a minor planet of a backwater solar system happens to have. It has certain conveniences, but isn't the hallmark of unquestionable rationality some would like to think it is.
- raattgift 10y ago
  The critical feature of SI is that it directly and explicitly incorporates a single positional numbering system used in scaling all quantities. Exact definition of quantities, while an important feature, has almost always been subordinate to that goal. Arguendo, let's assume that you are right that the decimal numeral system arose because of the typical number of human fingers. Note, we do not have to agree for the sake of argument that the place-value system is intimately tied to decimal (and indeed in our history, the latter predates the former by several millennia). That there are ten values per position is therefore parochial, but not as parochial as you think: almost every vertebrate has five digits per extremity (and essentially all of them embryologically[1]), and there are huge numbers of decapod arthropods extant and in the fossil record, and superorder Decapodiformes is an important group of cephalopods. So if we use base ten because of the number of our fingers (this is far from proven, unfortunately; there are many examples of very different ways of counting and doing arithmetic on human fingers), it is reasonable to think that many plausible civilization-building non-primates would be liable to use similar representational systems. Unfortunately that conflicts with human history, in which there has been a wide variety of number representation... [1] even in non-tetrapods: https://images.sciencedaily.com/2016/08/160817142550_1_540x360.jpg https://images.sciencedaily.com/2016/08/160817142550_1_540x3... Some harder-core evodevo: http://www.scientificamerican.com/article/why-do-most-species-have/ http://www.scientificamerican.com/article/why-do-most-specie...
  - Coincoin 10y ago
gchadwick 10y ago
> a knot is only approximately equal to 1 min of latitude per hour (there are numerous cases where it's unusably wrong) Whilst it isn't precisely equal it's certainly precise enough for navigational purposes. What are the numerous cases of which you speak (i.e. give me a sea navigation scenario where the current definition of a nautical mile is unusuably wrong, may be easier to come up with an air one but again I think you'll struggle). Certainly as a sailor (who routinely navigates yachts off-shore) 1 nm = 1 knot = 1 minute of latitude is very useful, especially when combined with a mercator projection. Lines of constant bearing are straight which is a very useful property. For longer ocean passages where you may want to sail a great circle route to save distance other projections are available. Yes you could use kilometre lines but what's your positioning system going to be? It won't map well to lat/long so you'll need to come up with an entirely new coordinate system. Given basically all charts available (and the surveying data behind them) are done in a lat/long system (though many different datums exist) it's a huge job to switch to the new coordinate system for little benefit. The current system of units also works very nicely for celestial navigation. Measure angle (A) of body above the horizon. (90 - A) * 60 = distance in nm between you and the point on the earth that body is directly overhead. Have you actually got any experience of navigating by sea or air or are you just assuming the system of units used is ill thought out and useless and everyone's crying out to use metric instead?
- logfromblammo 10y ago
  > Yes you could use kilometre lines but what's your positioning system going to be? It won't map well to lat/long so you'll need to come up with an entirely new coordinate system. Will you really? Why not use UTM [0] + UPS [1] or MGRS [2]? [0] https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_... [1] https://en.wikipedia.org/wiki/Universal_polar_stereographic_coordinate_system https://en.wikipedia.org/wiki/Universal_polar_stereographic_... [2] https://en.wikipedia.org/wiki/Military_grid_reference_system https://en.wikipedia.org/wiki/Military_grid_reference_system
  - gchadwick 10y ago
    Well indeed such coordinate systems exist but you've still got the cost of translating a large body of cartography into them. Plus as pointed out by another commentor the number of KMs around the earth is not a round number, so you need to do some fudging to get a metricised spherical coordinate system (and you want a spherical coordinate system because the 60th of a degree = 1nm equivilance is very useful for simplfying the mathematics required for navigation).
7952 10y ago
Surely in this context a minute is used as an angular measurment. I am not sure that km is directly comparable as it implies assumptions about projection.
todd8 10y ago
Well, at least an inch isn't defined that way. It is exactly 2.54cm.

Justin_K 10y ago

That's going to work great when the earth goes back to being flat.

Ultimatt 10y ago

No Mercator projection is used because its the least weird conformal projection (you draw a straight line its really straight in the real world). If you are trying to plot a bearing at the scale of the planet you want to be using Mercator unless you are going over the pole :P That the same projection is then used for everything even when you don't need to preserve local angles isn't the fault of Mercator. That you think km lines on an atlas would be better than lat/long makes you crazy. A decimal day might be useful but a decimal year is not. A fractional decimal day is a completely useless reference of time for doing quantum mechanics, but perfectly reasonable for people planning their work. No one would get rid of the concept of "a day". A map that maybe only shows tens or hundreds of km might be fine but representing the whole world with an equal area map with all grid reference based on km isn't useful to many people who use maps a lot, especially as the world isn't describable in whole numbers of km! But does have absolute and continuous angles from the gravitational centre.

NickNameNick 10y ago
Lines on a Mercator map aren't straight on a globe. They are constant-compass bearing, but that's not the same as straight. Also, as defined by the French academy of science ~1800, the meter is 1/10,000,000th the distance from the north pole to the equator, on the meridian through Paris.
- kqr 10y ago
  Not sure what your metre comment is about, but it turned out that definition wasn't accurate enough to confidently state the speed of light, so the speed of light was set to 299792458 m/s and a metre is consequently the distance light travels in a 299792458th of a second. (The second is in turn precisely defined by particle physics.)
  - dalke 10y ago
    It's a reference to Justin_K's earlier statement "a Knot is standard for navigation world wide because the unit is standardized on earth's size". If that justifies knot then it also justifies kilometers, because it too was originally standardized on the earth's size. Neither are exactly the size as originally defined. A modern knot is 1852 meters. One minute of latitude is "about 1,855.325 m on the WGS 84 ellipsoid" says https://en.wikipedia.org/wiki/Nautical_mile https://en.wikipedia.org/wiki/Nautical_mile. The difference is 0.16%. A quarter of the Earth is: 1855.325m * 60 seconds/degree * 360 degrees/circle /4 = 10018755.0 = ~10018 km. The difference is 0.19%.
deleted 10y ago
[deleted]
roywiggins 10y ago
The shortest distance between two points on sphere is along the great circle that contains the points, and are the closest thing to a "straight line" you can have on a sphere. Straight lines on a Mercator projection aren't great circles, they're rhumb lines, and if anything they're a spiral on the sphere, not straight.
- Retric 10y ago
  If you are holding a globe that a useful definition. However, that's not what navigators are doing. We can look at the stars and a compass both of which make rhumb lines far more useful.
  - OskarS 10y ago
    Rhumb lines may be useful for tiny vessels with only a compass and a map on board, but any commercial vessel, whether it is on the water or in the air, will follow the great circle route. It's the shortest and fastest way to go and it uses the least amount of fuel. Following rhumb lines just because they happen to be straight lines on Mercator projected maps would be silly. It's not the 1800's any more. We have computers and stuff, we don't have to rely on techniques developed for a time when your navigational tools consisted of a map, a compass and a sextant.
    - zentiggr 10y ago
      The passage of time doesn't diminish the usefulness of local navigation techniques... a Mercator projection will give you a bearing to steer to arrive at your destination, which can be followed by eye, autopilot, etc etc. Calculating a great circle arc and plotting it, following a bearing which may only change by a degree or two but isn't constant, is an unnecessary complication on short routes. Even commercial vessels will use those old techniques if they don't need to consider the longer range effects. Error correction in the human process is always a consideration. (Not even talking about data entry errors in GPS systems, but that Malaysia - Melbourne story was Funny.)
    - Retric 10y ago
deleted 10y ago
[deleted]

ChoHag 10y ago

Unfortunately, though, the earth is not a cube.

dredmorbius 10y ago

The SI unit of length, the meter, was originally defined according to Earth measurements, as 1/10,000,000 of one half a meridian. This was supplanted by reference bar measurements in 1889 and 1927, by a count of emitted light wavelengths in 1960, and as the distance traversed by light in a vacuum in a specified time interval in 1983. (So space is measured by time.) The metric system has its own foibles, one of which is being based on the number of digits a phenotypical abundant primate on a minor planet of a backwater solar system happens to have. It has certain conveniences, but isn't the hallmark of unquestionable rationality some would like to think it is.

raattgift 10y ago
The critical feature of SI is that it directly and explicitly incorporates a single positional numbering system used in scaling all quantities. Exact definition of quantities, while an important feature, has almost always been subordinate to that goal. Arguendo, let's assume that you are right that the decimal numeral system arose because of the typical number of human fingers. Note, we do not have to agree for the sake of argument that the place-value system is intimately tied to decimal (and indeed in our history, the latter predates the former by several millennia). That there are ten values per position is therefore parochial, but not as parochial as you think: almost every vertebrate has five digits per extremity (and essentially all of them embryologically[1]), and there are huge numbers of decapod arthropods extant and in the fossil record, and superorder Decapodiformes is an important group of cephalopods. So if we use base ten because of the number of our fingers (this is far from proven, unfortunately; there are many examples of very different ways of counting and doing arithmetic on human fingers), it is reasonable to think that many plausible civilization-building non-primates would be liable to use similar representational systems. Unfortunately that conflicts with human history, in which there has been a wide variety of number representation... [1] even in non-tetrapods: https://images.sciencedaily.com/2016/08/160817142550_1_540x360.jpg https://images.sciencedaily.com/2016/08/160817142550_1_540x3... Some harder-core evodevo: http://www.scientificamerican.com/article/why-do-most-species-have/ http://www.scientificamerican.com/article/why-do-most-specie...
- Coincoin 10y ago

gchadwick 10y ago

> a knot is only approximately equal to 1 min of latitude per hour (there are numerous cases where it's unusably wrong) Whilst it isn't precisely equal it's certainly precise enough for navigational purposes. What are the numerous cases of which you speak (i.e. give me a sea navigation scenario where the current definition of a nautical mile is unusuably wrong, may be easier to come up with an air one but again I think you'll struggle). Certainly as a sailor (who routinely navigates yachts off-shore) 1 nm = 1 knot = 1 minute of latitude is very useful, especially when combined with a mercator projection. Lines of constant bearing are straight which is a very useful property. For longer ocean passages where you may want to sail a great circle route to save distance other projections are available. Yes you could use kilometre lines but what's your positioning system going to be? It won't map well to lat/long so you'll need to come up with an entirely new coordinate system. Given basically all charts available (and the surveying data behind them) are done in a lat/long system (though many different datums exist) it's a huge job to switch to the new coordinate system for little benefit. The current system of units also works very nicely for celestial navigation. Measure angle (A) of body above the horizon. (90 - A) * 60 = distance in nm between you and the point on the earth that body is directly overhead. Have you actually got any experience of navigating by sea or air or are you just assuming the system of units used is ill thought out and useless and everyone's crying out to use metric instead?

logfromblammo 10y ago
> Yes you could use kilometre lines but what's your positioning system going to be? It won't map well to lat/long so you'll need to come up with an entirely new coordinate system. Will you really? Why not use UTM [0] + UPS [1] or MGRS [2]? [0] https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_coordinate_system https://en.wikipedia.org/wiki/Universal_Transverse_Mercator_... [1] https://en.wikipedia.org/wiki/Universal_polar_stereographic_coordinate_system https://en.wikipedia.org/wiki/Universal_polar_stereographic_... [2] https://en.wikipedia.org/wiki/Military_grid_reference_system https://en.wikipedia.org/wiki/Military_grid_reference_system
- gchadwick 10y ago
  Well indeed such coordinate systems exist but you've still got the cost of translating a large body of cartography into them. Plus as pointed out by another commentor the number of KMs around the earth is not a round number, so you need to do some fudging to get a metricised spherical coordinate system (and you want a spherical coordinate system because the 60th of a degree = 1nm equivilance is very useful for simplfying the mathematics required for navigation).

7952 10y ago

Surely in this context a minute is used as an angular measurment. I am not sure that km is directly comparable as it implies assumptions about projection.

todd8 10y ago

Well, at least an inch isn't defined that way. It is exactly 2.54cm.