Gen Nav Questions

[Haroon]

Hi

Q.1. The shortest distance between 2 point of the surface of the earth is:

a) a great circle <-- Correct
b) the arc of a great circle
c) half the rhumb line distance
d) Rhumb line

(a) marked correct. Whats wrong with (b)?


Q.2. Deviation is:

a) an error to be added to magnetic headings
b) a correction to be added to magnetic heading to obtain compass heading
c) a correction to be added to compass heading to obtain magnetic heading
d) an error to be added to compass heading to obtain magnetic heading

(c) marked correct. But it depends whether the deviation is east or west. You may have to subtract, so what does it mean?

Thanks

[Genghis the Engineer]

Have you tried understanding the subject rather than just question spotting? It's usually wisest.

But (1) Arc is a section, great circle is the line between two points. (2) You can add a negative number - technically that's still adding, even if in certain light it's subtraction.

G

[paco]

In question 1, probably answer b) is not as correct as the "proper" answer?

In question 2, c is the only possible answer even with the unfortunate use of the word "added" as opposed to "applied".

[what next]

Hi!

1b is the correct answer. Even Mr. Wikipedia knows this :-) (Great-circle distance - Wikipedia, the free encyclopedia (http://en.wikipedia.org/wiki/Great-circle_distance) see the second pragraph!)

A great circle on Earth is 21600NM long and certainly not the shortest distance between the two points. But the segment of that circle (and a segment of a circle is called an arc) which connects the points is!

[paco]

A fair point, but we are dealing with JAA questions here! :) Reality has nothing to do with it.

[Haroon]

Hi, Thanks for your time gentlemen

Have you tried understanding the subject rather than just question spotting? It's usually wisest

Yes I have tried understanding the subject and its not question spotting. My understanding is that:

Great circles are straight lines that go all the way around the center of the earth. The equator is a great circle. Meridians of longitude are great circles.

The shorter arc of a great circle between two points is the shortest surface-path between them. The length of the shorter arc is taken as the distance (the great-circle distance) between two points on a surface of a sphere.

Quoting Oxford: Great Circle is a circle on the surface of the earth whose centre and radius are those of the earth itself is called a Great Circle. It is called 'great' because a disc cut through the earth in the plane of the Great Circle would have the largest area that can be achieved.

The shortest distance between two points on the Earth's surface is the shorter arc of the Great Circle joining the two points. Given two points on the Earth's surface, there will be only one Great Circle joining them (unless the points are diametrically opposed).


In question 2, c is the only possible answer even with the unfortunate use of the word added as opposed to applied

b) a correction to be added to magnetic heading to obtain compass heading

c) a correction to be added to compass heading to obtain magnetic heading

Disregarding the "added" phraseology, both options appear to be the same (to me). By applying the correction you can obtain either of the values i.e. magnetic from compass or compass from magnetic. So whats the catch?

1b is the correct answer. Even Mr. Wikipedia knows this :-) (Great-circle distance - Wikipedia, the free encyclopedia see the second pragraph!)

Thanks, as that also confirms

Regards

[paco]

For question 2, deviation and variation allow you to go from compass to magnetic and magnetic to true, respectively. Of course you can go the other way, but that is the official progression. True is, after all, what you really want to know.

[Haroon]

Thanks Phil. Got your point. The question is flying oriented and not math oriented i.e. we are interested in getting the magnetic heading after applying the deviation.

[paco]

That's it - because deviation is individual to an aircraft, and variation changes so much around the world, the only constant is True.

[Capt Pit Bull]

Deviation is by definition a correction applied to Compass to yield magnetic.

However as pilots these days the most common task is to calculate the required compass heading to fly to achieve a desired magnetic heading.

BUT it hasn't always been that way. These days we generally already know where we are. GPS or radio bearing from VORs don't require us to correct for deviation. But if you imagine having to fix your position by taking bearings on things, perhaps by ADF or RADAR or even visually (after all, a lot of this stuff has it's origins in Sea naVigation).

If you spent enough time position fixing like that you would find your most common task became converting compass into magnetic rather than vice versa. At which point:

- defining deviation as the correction to be added to compass to yield magnetic,
And
- defining west deviation as negative and east as positive.

(and along similar lines, variation as the value added to magnetic to calculate true)

Makes it quick and easy to do the arithmetic. No need for all the 'helpful' memory joggers that people have subsequently come up with. When I say helpful I am being sarcastic, since I'm from the UK and we have westerly deviation, the 'west is best' saying tends to make people think that by definition westerly deviation or variations are positive when in fact the reverse is the case.

This is very typical of aviation where we have an unfortunate tendency to avoid anything perceived as difficult. I say 'perceived' because adding a negative number is in the primary school maths syllabus. it's hardly degree level. So instead of teaching something that is actually pretty simple anyway, it is dumbed down even further, in the pursuit of 'flight without formulae', such that there is no longer understanding and instead rote knowledge.

C + D = M
M + V = T
East positive, West negative (for D and V)

honestly, isn't that simpler than 4 rhymes?

[Haroon]

honestly, isn't that simpler than 4 rhymes?

Absolutely :D

Thanks Capt Pit Bull. Now the question makes perfect sense to me.

[Wirbelsturm]

Cadburys
Dairy
Milk
Very
Tasty

Amazing how these things stick in your memory! :}

[Haroon]

B
G
S
:ok:

[Flaymy]

Interesting. My friend paco is quite right that this is JAA and the right answer is what they say it is. However do note that in reality b is the correct answer to the first question, and a is wrong.

A great circle is what it says: a circle, the complete 360 degrees. It might contain two given points, and one can be drawn between any two chosen points on the Earth's surface which will contain the shortest route between those points, but it is not the shortest route between those points (unless they are antipodes, when all bets are off because there are an infinite number of equal routes). That is the shorter arc of that same great circle between those two points.

Always worth a query if you suspect they actually have the answer wrong, as you believe in this case.

As for the second question, do not forget that deviation can be expressed as + or - as well as E or W; in JAA exams it often is. In this case it is always added to compass bearing to find magnetic bearing, as adding a negative is equivalent to subtracting the same magnitude of positive number.

[Alex Whittingham]

I'm going to go slightly against the consensus here. These are terrible questions, I am almost certain they are not from the JAA CQB. If they are 'school questions' the school involved should be ashamed.

The correct answer to the first one is (b) for the reasons given above by Flaymy and others, the correct answers for the second one are both (b) and (c) because it is equally possible to add (or subtract) deviation to magnetic heading to get the required compass heading or to apply deviation to compass heading to find magnetic heading.

[Capt Pit Bull]

If you interpret the question as being about the arithmetic (in otherwords ignoring the sign of the deviation) then (b) is only correct if the deviation is westerly and (c) is only correct if the deviation is easterly.

However if you interpret that the question is asking about the fundamental relationship, including the sign of the deviation, then (c) is always correct.

[Alex Whittingham]

And if you interpret the question as being about the fundamental relationship, and accept that you can add negative numbers, as Ghenghis points out above, you get (b) and (c). Bad question.

[Capt Pit Bull]

Sorry Alex, with correct arithmetic including number signs applied, (b) is incorrect. Fine question; tests understanding instead of rote.

[custardpsc]

I imagine the point of the compass question was to see if you knew the difference between deviation and variation. Deviation being a correction applied to a compass to correct the realities of a compass installation and variation, ie the difference between magnetic and true. Deviations are positive and negative NOT e/w - look at a deviation card to see they are normally given +/- on at least 4 headings (so a correction on a easterly direction would be a N/S correction if that were the case)

Assuming you knew the difference between variation and deviation that left you with distinguishing between whether deviation was a correction or an error. Since it is a correction the answer can only be c)

The great circle one is clear. It does actually say 'ON surface of the earth' so the arc one is wrong as that penetrates the surface, the other two are distractors related to rhumb line being shortest distance on one type of map projection.

[keith williams]

The problem with question 2 isn’t just related the mathematics.

The McGraw-Hill Dictionary of Scientific and Technical Terms (see the link below) gives the following definition:


http://www.answers.com/topic/compass-deviation (http://www.answers.com/topic/compass-deviation)

(navigation) The difference between the readings of a compass which is without mechanical defects and is held motionless in space, and the same instrument when it is installed in the same geographic position but is mounted on a ship or aircraft; deviation is a systematic error which is compensated by placing iron bars in places about the compass; residual deviation errors are calibrated and noted on a card so it can be used by the pilot or navigator.

[Haroon]

the arc one is wrong as that penetrates the surface

can you throw some light on this please

[keith williams]

I suggest that anyone who wants to debate question 1 should read these:


http://en.wikipedia.org/wiki/Great_circle (http://en.wikipedia.org/wiki/Great_circle)

To prove that the minor arc of great circle is the shortest path connecting two points on the surface of a sphere, one has to apply calculus of variations (http://en.wikipedia.org/wiki/Calculus_of_variations) to it.



http://mathworld.wolfram.com/GreatCircle.html (http://mathworld.wolfram.com/GreatCircle.html)


The shortest path between two points on a sphere (http://mathworld.wolfram.com/Sphere.html), also known as an orthodrome, is a segment of a great circle.


http://en.wikipedia.org/wiki/Great-circle_distance (http://en.wikipedia.org/wiki/Great-circle_distance)



Through any two points on a sphere which are not directly opposite each other (http://en.wikipedia.org/wiki/Antipodal_point), there is a unique great circle. The two points separate the great circle into two arcs. The length of the shorter arc is the great-circle distance between the points. A great circle endowed with such a distance is the Riemannian circle (http://en.wikipedia.org/wiki/Riemannian_circle).

Or for those wishing to concentrate on the definition of an arc

http://en.wikipedia.org/wiki/Arc


In geometry (http://en.wikipedia.org/wiki/Geometry), an arc is a closed (http://en.wikipedia.org/wiki/Closed_set) segment of a differentiable (http://en.wikipedia.org/wiki/Differentiable)curve (http://en.wikipedia.org/wiki/Curve) in the two-dimensional plane (http://en.wikipedia.org/wiki/Two-dimensional_manifold); for example, a circular arc is a segment of the circumference (http://en.wikipedia.org/wiki/Circumference) of a circle. If the arc is part of a great circle (http://en.wikipedia.org/wiki/Great_circle) (or great ellipse (http://en.wikipedia.org/wiki/Great_ellipse)), it is called a great arc.