What is 1.18 metres different from UK to International?

[FullCircle]

It seems we no longer rule the waves......

And who can explain why the 0.18m gets missed off in 1995?

 

[sarabande]

the increase in distance between Europe and the Americas in the 20th century ? As redefined by new satellitic measurements in 1995 ?

 

[FullCircle]

the increase in distance between Europe and the Americas in the 20th century ? As redefined by new satellitic measurements in 1995 ?

Nil Points.....

 

[geoid96]

Is this the difference between Newlyn datum and LAT ?

 

[Wunja]

The Nautical Mile

In 1929 it was defined as 1852 meters
The Imperial NM is 6080 feet which is 1853.184 meters and was rounded down (presumably in 1995) to 1853 meters

(Wikipedia is my friend)

 

[FullCircle]

Is this the difference between Newlyn datum and LAT ?

Indeed not.

 

[FullCircle]

The Nautical Mile

In 1929 it was defined as 1852 meters
The Imperial NM is 6080 feet which is 1853.184 metersand was rounded down (presumably in 1995) to 1853 meters

(Wikipedia is my friend)

The International NM is indeed 1852 metres,
The statutory Instrument did indeed round it down - but why not to International Standard?

Because the datum is taken from a different place to the UK NM datum.


So where is the UK datum taken for a 6080ft NM?

 

[geoid96]

So where is the UK datum taken for a 6080ft NM?

I don't think there is a fixed datum as such. One nautical mile is the angular distance of 1 minute of arc on the earth's surface. As these differ slightly (6108 ft at pole and 6046 ft at equator), 6080 ft was adopted as being it's approximate value in the UK.

 

[Wunja]

So where is the UK datum taken for a 6080ft NM?

I'll do my Alan Davis QI impression.....

Greenwich?


I did learn something.... Aparently WGS84 is based on the Paris meridian because the Americans couldn't be ar5ed to change all their charts - despite it being agreed in 1884 by 25 countries (including USA) that it would be Greenwich.

 

[FullCircle]

I'll do my Alan Davis QI impression.....

Greenwich?


I did learn something.... Aparently WGS84 is based on the Paris meridian because the Americans couldn't be ar5ed to change all their charts - despite it being agreed in 1884 by 25 countries (including USA) that it would be Greenwich.

Ut-Urrrr!!!!!

You've thrown it all away on the Joker!!!!!

While you are looking for the datum point of one minute of arc.....


Perhaps you can also find out the total RANGE of distance in feet that the NM could be, if we took in the variations.....

 

[Simon_and_Tanja]

Is that one minute of arc along any great circle? If my schoolboy knowledge is accurate the longest great circle is around the equator and the shortest between the poles (presumably all meridians are roughly equal in length?) If the nautical mile was based on a datum measured somewhere in Great Britain this somewhat limits the range of circles. The resultant great circle will fall in a range from one aligned true north/south to one inclined no more than 39 degrees from north/south. Somebody, I am sure, can do the maths.
To accurately determine one minute to establish a datum would presumably require some clever celestial mathematics, with a flat horizon and an easily observed extraterrestrial object - rather rules out Greenwich.
The metre I seem to recall was intended to be a 40 000 000th of the great circle along the Paris meridian but they couldn't determine this accurately and the datum (a metal bar) is therefore wrong.
40 000 000 divided by 21 600 (number of minutes in a circle) = 1851.85

 

[FullCircle]

Is that one minute of arc along any great circle? If my schoolboy knowledge is accurate the longest great circle is around the equator and the shortest between the poles (presumably all meridians are roughly equal in length?) If the nautical mile was based on a datum measured somewhere in Great Britain this somewhat limits the range of circles. The resultant great circle will fall in a range from one aligned true north/south to one inclined no more than 39 degrees from north/south. Somebody, I am sure, can do the maths.
To accurately determine one minute to establish a datum would presumably require some clever celestial mathematics, with a flat horizon and an easily observed extraterrestrial object - rather rules out Greenwich.
The metre I seem to recall was intended to be a 40 000 000th of the great circle along the Paris meridian but they couldn't determine this accurately and the datum (a metal bar) is therefore wrong.
40 000 000 divided by 21 600 (number of minutes in a circle) = 1851.85

Very nice and undoubtedly accurate but barking up the wrong tree!!


So, if we assume that 6080ft is a nominal distance to set a standard, what are the range of distance which COULD form a NM, and, where is our datum taken from?

 

[Simon_and_Tanja]

Hmm peeked my interest.

The internet reckons:
"The British definition of the nautical mile originally related to the length on the surface of the Earth just south of Great Britain. It was not specified according to a calibrated measurement of the Earth, but chosen as exactly 800 feet longer than a statute mile, namely 6,080 feet. For disambiguation, this is sometimes called the "admiralty mile" after the British Admiralty. "

There is a long thread here http://www.ybw.com/forums/showthread.php?t=43306 which suggests a minute of lattitude at 49 degrees (just south of GB) was the derivation of a first stab at 6077ft which the Admiralty then rounded to 6080. but the number 49sounds like wishful thinking by some Americans to me!

Given that the statute mile is based on a statute defined in the basic imperial unit of length, which is a yard, which is the distance from Henry 1st nose to the tip of his outstretched arm, there you go! Your much vaunted datum is a somewhat arbitary number of multiples of part of a long decayed corpse.

 

[Master_under_Dog]

Now don't confuse me with science

Ahem! Extra Master T J Williams MRIN in his book on Coastal Navigation, what I have owned for a year or two now, says, and I quote:

"Since the earth is not a perfect sphere the length of 1' of arc varies from 6108 feet at the poles to 6046 feet at the equator. In the past an average length of 6080 feet was assumed for practical navigation. With the introduction of metrication the International Nautical Mile was adopted having a length of 1852 metres (6076.6 feet). This represents the length of a nautical mile measured in latitude 45 degrees. These small discrepancies need not concern the yachtsman as they have no significance in practical navigation."

So there you have it. Mr Williams says it need be no concern of ours. Back to my hot grog.

Michael

ps I note in passing that the average of 6108 and 6046 is 6077, not 6080. And 6076.6 is a damned sight closer to the average. Oh sorry, I am not to concern myself with that.

 

[geoid96]

ps I note in passing that the average of 6108 and 6046 is 6077, not 6080. And 6076.6 is a damned sight closer to the average. Oh sorry, I am not to concern myself with that.

Ah but the English Channel is not halfway between the equator and the North Pole. In practice the rate of change in the length of 1' arc between pole and equator is probably not linear, but if we take 51 deg N then:
1' arc = 6046+((6108-6046)*(51/90))=6081.13 ft.
at 49 deg N it would be:
1' arc = 6046+((6108-6046)*(49/90))=6079.76 ft.

 

[sailorman]

Ah but the English Channel is not halfway between the equator and the North Pole. In practice the rate of change in the length of 1' arc between pole and equator is probably not linear, but if we take 51 deg N then:
1' arc = 6046+((6108-6046)*(51/90))=6081.13 ft.
at 49 deg N it would be:
1' arc = 6046+((6108-6046)*(49/90))=6079.76 ft.

i would double check that new Furuno if i were you

 

[geoid96]

i would double check that new Furuno if i were you

but that will include the projection scale factor in the calculated distance and it's not applicable in this situation!