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?

[zoidberg]

Let's not even start on Irish Miles....

 

[AntarcticPilot]

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?

The 1 km squares are defined on a map, not on the surface of the earth. The OS uses a transverse Mercator projection, so distance measurements are only exact in along the central meridian (2°W). Because the whole of the UK is not too far from the central meridian, the distortion isn't great - but it exists. Further, grid north is not the same as true north (true north is indicated on OS sheets, I think).

 

[billskip]

Let's not even start on Irish Miles....

Don't they depend on how long the car is?

 

[Momac]

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

If there were any deviations from level ground on the route of a walk and you measured off the map then yes .

 

[Jerbro]

This is how long a nautical mile is here. If I ever forget, I motor by to check…

 

[B27]

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.

For nautical use, worrying about small differences is pretentious most of the time.
How often do you need to talk about a distance and need to care whether it's 100NM as in 185.2km or 100 minutes on the chart? and what would it actually mean if the first of those 100NM were longer or shorter than the later ones along a great circle?

I've worked a little with satellites and stuff, in their world for every mile of altitude, an orbit is 2pi more miles.

I enjoy sailing more when there are enough waves that you sail maybe 2km to the NM anyway...

 

[B27]

Let's not even start on Irish Miles....

Best not, he's a good chap, I sailed with him ages ago.

 

[AntarcticPilot]

For nautical use, worrying about small differences is pretentious most of the time.
How often do you need to talk about a distance and need to care whether it's 100NM as in 185.2km or 100 minutes on the chart? and what would it actually mean if the first of those 100NM were longer or shorter than the later ones along a great circle?

I've worked a little with satellites and stuff, in their world for every mile of altitude, an orbit is 2pi more miles.

I enjoy sailing more when there are enough waves that you sail maybe 2km to the NM anyway...

Of course - but my work was in mapping and intimate knowledge of datums, projections and the figure of the earth was all part of it. I had to understand how maps distort reality - which all flat maps do; the map-maker's skill is in choosing projections and parameters that minimise the distortion for users of the map.

 

[Refueler]

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."

The reason a figure was assigned and agreed was based on various where a defined physical length was required. It ONLY applies where such physical length is required.
As I mentioned earlier - that figure was not universally applied - in early days of radar - the rounded figure of 1800m was used .... but as years moved on and computers / chips and so on took over - the figure was revised to agree with the 12.35 time factor for a radio pulse to travel out and back to complete the standardised 1852m range.

The Nautical Mile for navigation purposes remains as 1 minute of arc on earths surface.

 

[Daydream believer]

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....

But that is illogical. Spock did not initially know about the Klingons when he was born,as they arrived in a later episode. Bit like the Borg. So would have used either earth values, or Vulcan ones. Mind you, a few earth creatures I know, have had a dose of the cling-ons, so perhaps he had heard of them.

 

[Boathook]

Let's not even start on Irish Miles....

Pubs per mile or miles per pub ?

 

[Boathook]

This is how long a nautical mile is here. If I ever forget, I motor by to check…
View attachment 185173

I've always wondered why they put a measured distance there. Tides running at nearly 4 knots so either I'm doing 8 knots or zero knots, all GPS speed whilst the log carries on at 4 knots ...

 

[Alicatt]

I've always wondered why they put a measured distance there. Tides running at nearly 4 knots so either I'm doing 8 knots or zero knots, all GPS speed whilst the log carries on at 4 knots ...

On the inland waterways it's what we have to use, speed over the ground in km/h, though the flow should be less than 3km/h, the Rhine can get a bit of a sprint on and manage a flow of 8km/h in places

 

[Refueler]

I've always wondered why they put a measured distance there. Tides running at nearly 4 knots so either I'm doing 8 knots or zero knots, all GPS speed whilst the log carries on at 4 knots ...

Measured Miles are I am sure you realise are designed to be made in both directions to average out and negate the tide / current flow.

 

[B27]

The reason a figure was assigned and agreed was based on various where a defined physical length was required. It ONLY applies where such physical length is required.
As I mentioned earlier - that figure was not universally applied - in early days of radar - the rounded figure of 1800m was used .... but as years moved on and computers / chips and so on took over - the figure was revised to agree with the 12.35 time factor for a radio pulse to travel out and back to complete the standardised 1852m range.

The Nautical Mile for navigation purposes remains as 1 minute of arc on earths surface.

A radar mile is a unit of time.
Like a light year is a unit of distance perhaps!

For practical navigation purposes afloat, is there ever a significant difference between a minute of latitude and 1852 metres?

 

[Refueler]

A radar mile is a unit of time.
Like a light year is a unit of distance perhaps!

For practical navigation purposes afloat, is there ever a significant difference between a minute of latitude and 1852 metres?

Read my post again slowly and digest .....

12.35 microsecs .... is the radar mile which means in that time = 1852m x2 (it is x2 as the radar pulse goes out and back again)

Difference ? Not to create any problem in general navigation ... of course not.

 

[thinwater]

The reason a figure was assigned and agreed was based on various where a defined physical length was required. It ONLY applies where such physical length is required.
As I mentioned earlier - that figure was not universally applied - in early days of radar - the rounded figure of 1800m was used .... but as years moved on and computers / chips and so on took over - the figure was revised to agree with the 12.35 time factor for a radio pulse to travel out and back to complete the standardised 1852m range.

The Nautical Mile for navigation purposes remains as 1 minute of arc on earths surface.

As a practical matter, yes, I agree. Many good reasons. But is there a reference for this statement? I swear to God, I am not trying to be argumentative, but I am a writer and I like to know what I know is supportable fact. The below (from Wiki, but the same information is many places) is pretty weird, but there is nothing I have found that says navigation uses minutes instead. The difference, of course, does not matter.

This from Wiki

Both the United States and the United Kingdom used an average arcminute—specifically, a minute of arc of a great circle of a sphere having the same surface area as the Clarke 1866 ellipsoid.[25] The authalic (equal area) radius of the Clarke 1866 ellipsoid is 6,370,997.2 metres (20,902,222 ft).[26] The resulting arcminute is 1,853.2480 metres (6,080.210 ft). The United States chose five significant digits for its nautical mile, 6,080.2 feet, whereas the United Kingdom chose four significant digits for its Admiralty mile, 6,080 feet.

In 1929 the international nautical mile was defined by the First International Extraordinary Hydrographic Conference in Monaco as exactly 1,852 metres (which is 6,076.12 ft).[1] The United States did not adopt the international nautical mile until 1954.[27] Britain adopted it in 1970,[28] but legal references to the obsolete unit are now converted to 1,853 metres (which is 6,079.40 ft).[29]

This from OED
A unit used to measure distances at sea, (now) spec. (in full international nautical mile) a unit fixed in 1929 at exactly 1852 metres (approx. 6076 feet, 1.151 statute miles).
In Britain the nautical mile was formerly fixed by the Admiralty at 6080 feet (approx. 1853 metres).

 

[Boathook]

Measured Miles are I am sure you realise are designed to be made in both directions to average out and negate the tide / current flow.

I feel that the flow changes to quickly there, especially at my speeds !

 

[MontyMariner]

The earth is an oblate spheroid ... best way to describe it as a pear shape

Hence the expression 'The world has all gone pear shaped'

 

[Refueler]

there is nothing I have found that says navigation uses minutes instead

When was last time you took a sight ... run the calculation ... plotted ?? Until you move the position lines to intersect - all calculations are based on angular ..
Actually even the movement of position lines to cross at desired 'clock time' are based on the minute of arc Nm ...