Newbie Nav Question
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Pompey till I die
Joined: Nov 2006
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From: Guildford
Newbie Nav Question
Without using a map, estimate the approximate distance (in nautical miles) between Aberporth airfield (N5207 W00433) and RAF Valley (N5315 W00432)
I'm also presuming that there are 60 minutes in a degree, and 60 seconds in a minute ? Is that true ?
Joined: Aug 2006
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From: UK
Correct
Yes, that is correct.
1 nm is the arc subtended at the earths circumference by a angle of 1 minute at the earths centre.
So if you considered the earth a perfect sphere ( it is not, but the error is not significant) then the distance (circumference) around the centre of the sphere, is 360 degrees X 60 minutes = 21600 nm
Or as you put it 21600 nm divided by (360 degrees x 60 minutes) = 1 nm.
But for longitude this relationship only holds true at the equator, as you move North or South a distance of 1 minute of longitude is now smaller than 1nm.
Very bady explained without a diagam, but if you are calculating distances between latitudes you can use the rule 1 degree = 60nm
For Longitude it only holds at the equator, otherwise its 1 degree = 60nm*cos(latitude)
Hope that hasn't confused you
So in your example you are effectively moving due North and the calculation you have done is correct. If the 2 co-ords were due east/west of each other then the calculation would not be correct.
1 nm is the arc subtended at the earths circumference by a angle of 1 minute at the earths centre.
So if you considered the earth a perfect sphere ( it is not, but the error is not significant) then the distance (circumference) around the centre of the sphere, is 360 degrees X 60 minutes = 21600 nm
Or as you put it 21600 nm divided by (360 degrees x 60 minutes) = 1 nm.
But for longitude this relationship only holds true at the equator, as you move North or South a distance of 1 minute of longitude is now smaller than 1nm.
Very bady explained without a diagam, but if you are calculating distances between latitudes you can use the rule 1 degree = 60nm
For Longitude it only holds at the equator, otherwise its 1 degree = 60nm*cos(latitude)
Hope that hasn't confused you
So in your example you are effectively moving due North and the calculation you have done is correct. If the 2 co-ords were due east/west of each other then the calculation would not be correct.
Last edited by JP1; 28th February 2007 at 12:36. Reason: mean't degrees not minutes
The Original Whirly
Joined: Feb 1999
Aviation Qualifications: CPL
Posts: 4,327
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From: Belper, Derbyshire, UK
Alex, sorry - my answer was ambiguous....
AFAIK, that sort of calculation is CPL level.
Can't remember! I don't think so. But whizzwheels definitely are.
Do you think they would be mean enough to put an East/West longitude calculation (at a latitude other than at the equator) in the exam paper?
Are calculators allowed?
Joined: Feb 2002
Posts: 93
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From: Hampshire, UK
In practical terms - On the CAA charts (Lamberts Conformal Conic Projection) measure the distance betwixt the two points with your dividers and then read off the measured disance using the Latitude scale (use the latitude scale alongside the approx. area of your journey)
And just to confuse matters .....
In Navigational terms: You can use a Haversine formula to gain the great circle distance - Thus :-
Hav(Dist) = Hav((90-LatA)~(90-LatB))+Hav(longA-LongB) x Sin(90-LatA) x Sin(90-LatB)
If you want to use Excel to help the sums for you you could use:-
60*SQRT((LatA-LatB)^2+(COS(((LatA+LatB)/2)/RadConv)*(LongA-LongB))^2))
(Where RadConv = 180/PI()) to do the radian/degrees conversion)
And just to confuse matters .....
In Navigational terms: You can use a Haversine formula to gain the great circle distance - Thus :-
Hav(Dist) = Hav((90-LatA)~(90-LatB))+Hav(longA-LongB) x Sin(90-LatA) x Sin(90-LatB)
If you want to use Excel to help the sums for you you could use:-
60*SQRT((LatA-LatB)^2+(COS(((LatA+LatB)/2)/RadConv)*(LongA-LongB))^2))
(Where RadConv = 180/PI()) to do the radian/degrees conversion)
Joined: Dec 1999
Posts: 402
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From: Wilmslow and North Yorks
When i did the exams (roughly 820 years ago 
) there were no calculators allowed, you had to use a "whizz-wheel". Now though.........who knows, you don't even have to sit a morse exam anymore!!
) there were no calculators allowed, you had to use a "whizz-wheel". Now though.........who knows, you don't even have to sit a morse exam anymore!!
Joined: Nov 2003
Posts: 636
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From: notts
Nav calculations
For the PPL navigation exam you are provided with 1:500,000 southern chart. Use of a standard navigation scale ruler and a protractor is all that is required. On such a chart scale may be considered the same in all directions. However to answer your question - 1 minute of arc on a great circle = 1 nm but only on a great circle. You are not allowed a calculator but must use the whiz-wheel for all calculations (or long hand if your good at sums but to get the same answer as the CAA use the whiz-wheel for that is what they have used).
You will not be required to scale using change lat/long. You will be required to identify a position by reference to lat and long figures. Seconds may be given as 60ths of a minute or in divisions of a hundred. Count the digits given to be sure.
You will not be required to scale using change lat/long. You will be required to identify a position by reference to lat and long figures. Seconds may be given as 60ths of a minute or in divisions of a hundred. Count the digits given to be sure.
