Nautical mile

[trays]

Can some please remind me why the length of a nautical mile varies?

Ray

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[Guest]

A nautical mile is the distance covered on the surface of the earcth by one second of arc. As the earth is a distorted sphere the length varies.

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[tcm]

It doesn't vary. But on a chart using a Mercator projection, the vertical axis is more and more exaggerated the further north you go. This is why Greenland looks massive on maps of the world. The scale on the right is the one to use.

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[MedMan]

You are technically right, but to avoid the problems caused by the distortion of the earth the Nautical Mile has been standardised as 1,852 meters or 6,076 feet.

What I guess trays is refering to is the fact that, on a small scale chart, a nautical mile is not the same length at the top of the chart as it is at the bottom. That is because of the distortion inherent in the Mercator Projection. The greater the latitude the greater the scale. That is why Greenland looks so huge on Mercator Charts of the Northern Atlantic.

Wright improved on the projection developed by Mercator and wrote a book which provided tables and explanations. The following intuitive explanation is his. Make a globe out of a spherical balloon. Place it in a larger glass cylinder. Blow it up slowly so that it first touches at the equator. Continue blowing it up slowly. Each point on the balloon globe is blown up until it is pressed against the wall of the glass cylinder. Higher latitudes are blown up more. The final result is a Mercator projection. Wright explained that the scale factor varied with the secant of the latitude.

If you would like to know more, look <A target="_blank" HREF=http://www.geocities.com/yachtretreat/>http://www.geocities.com/yachtretreat/

 

[MainlySteam]

Was and is still generally considered to be the length of 1 minute of arc on any great circle of the earth. Because the shape of the earth is not a perfect sphere that is not a very sound definition for exactness. Most nations standardise it as an International Nautical Mile being 1,852 meters, which is the figure proposed by the International Hydrographic Bureau. One cannot escape the metric system even when it is most unexpected.

John

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[tome]

Actually, metrics has an even greater bearing. The distance from the equator to the pole was arbitarily chosen as 10,000km hence the derived length of the metre.

This yields a distance of 1851.852 metres to an arc of 1 minute (== 1 nautical mile), which is rounded by convention to 1852 metres.

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[MainlySteam]

Thanks Tom - I didn't know that is how the 1852 came about. Did it on a calculator and it works out just as you say.

John

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[tome]

John, whilst you have your calculator out...

Look at the keys 852 and you'll see they are the central vertical row.

That's how I used to teach our commercial navigators to remember 1852 (metres in a mile).

Regards
Tom

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[MainlySteam]

I see what you mean. Smart!

John

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[bob_tyler]

Am I VERY oldfashioned.

To me, for all practical purposes is 2000 yards or 10 cables.

Much easier to calculate mentally than 1852mtrs

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[richardandtracy]

Very old fashioned..?

Old fashioned enough to claim that a cricket pitch is 2 perches long?

Regards

Richard.


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[AndrewJ]

don' u mean one minute of arc. (60 miles to a degree, 60 minutes to a degree = 1 mile/1 minute, 60 seconds to a minute =1/60th of a nm =101feet)or our my calculations wrong?

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[NigeCh]

Even more very old fashioned..?

Surely a cricket pitch length between the stumps is the breadth of the Saxon strip-acre??? ie 22 yards exactly.

At the poles a nautical mile is 6,108.8' and at the equator it is 6,046.5 feet ... It's all because Napoleon's mathematicians didn't know the difference between a sphere and an oblate speroid.


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[Guest]

= 1 minute of arc of latitude

Latitude increases to infinity at the poles in theory !!!! But anyway if you look at a chart in different regions you will see a difference in the masurement assuming same scale. Even on the same chart you can measure the lat scale at the bottom ..... now place the same dividers not changed at the top scale and see the measurement ....

<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Guest]

No nm

*

<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[tome]

Oblate Spheroid I wish

If it really were we wouldn't have to worry about local datums! It's more akin to a King Edward potato in reality, but the spheroid is the closest we come to a mathematical model.

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[Guest]

When I was at college ...

I was told that the metre was based on the wavelength of a particular matter .... can't remenber what exactly what ... but never heard about the equator - pole theory ..........


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[tome]

You've got me puzzled here Nigel, latitude cannot be more than 90 degrees in any spheroid model.

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[Guest]

Actually

6080 feet for fixed calculations, variable as per latitude in reality.


<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !

 

[Guest]

I only repeat what Plymouth Maritime told me .... anyway its acedemic bulls--t about the infinity we all know that.

The basic thing is 1nm = 1 min of Latitude QED nothing else.

<hr width=100% size=1>Nigel ...
Bilge Keelers get up further ! I only came - cos they said there was FREE Guinness !