Why do boats use nautical miles and why are they different than normal miles? Couldn't you just convert it to normal miles or km?

[Kwik Decision]

Just a few (maybe more than a few) years ago, when GPS was not a thing and we actually used charts, it was very handy that the minute scale on the right margin was also nautical miles. Approximate navigation could be done at a glance without looking for a scale of distance.

If you're using a Mercator projection chart, which for coastal cruising round the UK is the majority of them, then for accurate nav it is essential to use the latitude scale adjacent to the distance you're measuring. The chart scale changes from top to bottom, and if there is a distance scale printed, it will only be accurate at the latitude stated.

I've just had a look at a cross channel chart, to confirm what I learned many years ago, and yes, on this one 5 nm is 18 mm at St Malo, and 19 mm at Portsmouth.

 

[Refueler]

In post #11 you said it was a fixed length. Historically, it's not fixed, as your quote above demonstrates. It only became a standard distance because we made it so.

It gets worse than that .... the stated distance has changed depending on what application.

The Radar mile for years was based on 1800m but later adjusted to calculate based on 12.35 micro-second time to / from target per nm giving it 1852m

 

[Refueler]

If you're using a Mercator projection chart, which for coastal cruising round the UK is the majority of them, then for accurate nav it is essential to use the latitude scale adjacent to the distance you're measuring. The chart scale changes from top to bottom, and if there is a distance scale printed, it will only be accurate at the latitude stated.

I've just had a look at a cross channel chart, to confirm what I learned many years ago, and yes, on this one 5 nm is 18 mm at St Malo, and 19 mm at Portsmouth.

The variance of latitude scale on a mercator chart is a product of the mercator projection itself distorting the latitude as it moves further from the equator ... in theory - the scale would be infinity at the poles.
Its the reason different projection forms were developed for navigating in polar regions ... one of them being the Grid System. Another being Gnomonic ... Stereographic etc.

 

[AntarcticPilot]

But if you'd adjust the grid, they would?

No. Map projections ALWAYS distort distance, so measuring in kilometres simply doesn't make sense. Angular measurement is the only kind that avoids that - because you're measuring on the surface of a sphere.

 

[penberth3]

I always find it funny that when walking I use kilometres as UK Ordnance Survey maps are in 1km squares, when driving I use statue miles apparently based on Roman Miles, and at sea I use nautical miles.

It's good for numeracy skills. Kids today wouldn't understand!

 

[AntarcticPilot]

The variance of latitude scale on a mercator chart is a product of the mercator projection itself distorting the latitude as it moves further from the equator ... in theory - the scale would be infinity at the poles.
Its the reason different projection forms were developed for navigating in polar regions ... one of them being the Grid System. Another being Gnomonic ... Stereographic etc.

In Antarctica, the two usual projections are Polar Stereographic with a latitude of true scale at 71°S, and Lambert Conformal Conic with varying standard parallels; it's based on the IMW sheet system. The former is used for whole continent maps; the latter for maps of specific areas. In the Arctic, it's more complicated because of sovereignty issues, and I'm not aware of a general standard. When we published a map of the Arctic, we used Polar Stereographic with similar parameters to Antarctica, but North instead of S.

This (Coordinate reference systems for "antarctica") is a list of standard projections for Antarctica. I contributed a lot of it!

 

[capnsensible]

As a simple sailor, I'm gonna stick with normality.

 

[Refueler]

In Antarctica, the two usual projections are Polar Stereographic with a latitude of true scale at 71°S, and Lambert Conformal Conic with varying standard parallels; it's based on the IMW sheet system. The former is used for whole continent maps; the latter for maps of specific areas. In the Arctic, it's more complicated because of sovereignty issues, and I'm not aware of a general standard. When we published a map of the Arctic, we used Polar Stereographic with similar parameters to Antarctica, but North instead of S.

This (Coordinate reference systems for "antarctica") is a list of standard projections for Antarctica. I contributed a lot of it!

I was keeping it simple !!

I still have my Fathers 'computers' he used for trans polar flights .......

 

[Daydream believer]

And how many of those buses would it take to be as high as Nelson's Column?

I have often wondered about Nelson's column. Is the height based on the column that he stands on, or the overall height including Lord Nelson as well? It does make a difference to accurate measurement.

 

[Daydream believer]

As a simple sailor, I'm gonna stick with normality.

Normality?
Well what on earth are you doing contributing to this thread then?

 

[B27]

I can only suggest that you turn off your GPS and navigate with map, compass and chart. See how useful kilometers are. (face palm) IF the SI systems is so logical, ask why it was not based on the geometry of the earth? Just another arbitrary (see post 5--the French thought it was logical but measured poorly) basis, no better than any other.

I suggest going out with your friend and a chart, no GPS, and try navigating with kilometers. That should settle the debate. But as many have suggested, it's all arbitrary.

[As a US engineer, our exams often featured problems that alternated between SI and US traditional units. They all worked out the same, no problem. Sometimes SI units were easier, and sometimes traditional units are easier to work with.

If you use Imray charts, they are mostly sensible scales, like 1:20,000 for the one in front of me, so 1mm =20 m or 50mm is a km.
Measure differences with a plastic ruler instead of taking your dividers to the scale.
Seems pretty useful to me.
And if you have charted objects with heights in metres, simple ratios of height to distance are easier if it's all metric.
Of course some that would be easier if your sextant read radians?

My pet hate, mapping apps which don't always put a scale on the screen or only a squitty short one.

In the real world, on my boat a knot is near enough half a metre per second if you want to guesstimate speed from watching the bubbles go by.

Back in the day, we used to set out speed log to read km/h, because it only read to 0.1 units and seeing a change of 0.1 km/h is more information than not seeing a change until you hit 0.1 knot difference.

 

[onesea]

Why can't we stick to the standard TV measurements.
Height= Nelson columns
length = London busses
area = football pitches or Isle of Wight
volume= olympic swimming pools
weight= elephants or jumbo jets.

London buses vary in length
“Double-decker bus: Typically range from 31 ft 2 in (9.5 m) to 36 ft 5 in (11.1 m) long. The maximum length for a rigid double-decker bus in the UK is 49 ft 3 in (15 m) for a three-axle bus and 44 ft 3 in (13.5 m) for a two-axle bus”

As do jumbo Jets although I always assume the standard for this is a 747

Elephants are a whole new problem:
The weight of an elephant depends on the species and the sex of the elephant:
African bush elephant: Adult males weigh an average of 6,000 kg (13,200 lb), while adult females weigh between 2,700 and 3,600 kg (6,000 and 8,000 lb).
African forest elephant: Adult males weigh between 2,700 and 6,000 kg.
Asian elephant: Adult males weigh between 1,800 and 4,500 kg (4,000 and 10,000 lb), while adult females weigh slightly less.
Borneo elephant: Adult male Borneo elephants can weigh as little as 2,950 kg (6,600 lb).


Do these standards need a standard?

 

[lustyd]

I’d never thought about it before, but is a nautical mile further at altitude and are knots faster in a jet plane?

 

[AntarcticPilot]

I’d never thought about it before, but is a nautical mile further at altitude and are knots faster in a jet plane?

I think aircraft use the standard international nautical mile, which is defined in metres - 1852m.

Technically you are right - a minute of latitude is longer at elevation. But the difference is small - the radius of the earth is approximately 6370 km; aircraft fly at about 10 km up. So a minute of arc at aircraft elevations is about 3m longer than at sea level.

 

[thinwater]

I think aircraft use the standard international nautical mile, which is defined in metres - 1852m.

Technically you are right - a minute of latitude is longer at elevation. But the difference is small - the radius of the earth is approximately 6370 km; aircraft fly at about 10 km up. So a minute of arc at aircraft elevations is about 3m longer than at sea level.

... But as you said in your first sentence, a nautical mile is a unit of distance, not one minute of the arc. That is only the traditional origin, and a useful close approximation.

 

[AntarcticPilot]

... But as you said in your first sentence, a nautical mile is a unit of distance, not one minute of the arc. That is only the traditional origin, and a useful close approximation.

For nautical use, it is still a minute of arc and changes in length with latitude. 1852m is actually a middle value in the range that it can take (I once calculated the range but it's about 10m different between equator and pole). It is a unit of distance in other fields (aeronautics being one) but for navigation it's a measure of arc.

 

[lustyd]

... But as you said in your first sentence, a nautical mile is a unit of distance, not one minute of the arc. That is only the traditional origin, and a useful close approximation.

But as he said, for navigation the other definition makes sense. The plane may fly further but navigationally it started and ended on the surface so the air speed is almost irrelevant since it travels surface miles.

 

[Slowtack]

On the Enterprise we use the metric system for navigation but of course we use the Klingon "uj" unit in general conversation for the convenience of Mr Spock....

 

[Sandy]

But the exact size of your "1 km square" actually varies significantly east and west of the OS central meridian (2° W)

Interesting, I always thought they used a standard 1 km square.

Does this mean that all the recorded kms walked in the past might be wrong?

 

[thinwater]

For nautical use, it is still a minute of arc and changes in length with latitude. 1852m is actually a middle value in the range that it can take (I once calculated the range but it's about 10m different between equator and pole). It is a unit of distance in other fields (aeronautics being one) but for navigation it's a measure of arc.

I would be interested in seeing a reference that codifies that difference. I'm a doubter when it comes to standard units. The Hydrographic Organization sounds nautical to me.

From NOAA:
"The measurement was officially set at exactly 1.852 kilometers in 1929 by what is now known as the International Hydrographic Organization."