Using map coordinates
 

Anyone know how to use them by manual calculation?

You are at airport A located at 33’20”N 108’02”W and are flying to airport B located at 35’55”N 107’58”W. Approximately how many miles is it from airport A to airport B?

Thanks!

Use the following (and if you can do it in your head, you're awesome):
Dlon = longitude2 - longitude1
Dlat = lattitude2 - lattitude1
a = (sin(Dlattitude/2))^2 + cos(lattitude1) * cos(lattitude2) * (sin(Dlongitude/2))^2
c = 2 * atan2( sqrt(a), sqrt(1-a) )
d = R * c (where R is the radius of the Earth in appropriate units - result will be same units)

This is from code, so where you see '*' think "multiplied by" or "times".

N35*55' minus N33*20' equals 155nm of north-south distance. The two airports are separated by 4' of longitude, which at 34* of latitude is 3.31 nm, (cosines of 34). So, very slightly more than 155nm

Hint: one minute of latitude equals one nautical mile

GF

The two answers above remind me of the old bull and young bull joke. They're standing at the top of the hill looking down at the herd of cows. Young one says hey look at all those cows! Let's run down the hill and :D one of them. Old bull says, why don't we walk down and :D them all.

GF takes the wise, simple approach and could definitely get the answer in his head if desired.

Whoever posed this question was definitely looking for you to figure out it was essentially a northerly course from A to B and have you get the difference on the latitude minutes which equates to NM.

So long as the two points are roughly N/S of each other, then the simple rule of one nm per minute of latitude totally works. But that's the only time it works, so it's an optimization of a specific case rather than a general solution to the question "Using map coordinates - Anyone know how to use them by manual calculation?" as the OP asked.

Personally, I favor the finger/thumb method - you know, finger on one coordinate and thumb on the other one in an EFB app which then tells you the bearing and distance. But the OP asked for the manual calculation :-)

I think the point of the question was recognizing the fact it was a north-south problem and did the person see that and apply a basic bit of navigation knowledge. You could do the n-s part, apply the cosine of the latitude to the number of minutes from the E-W distance and scale it out on an HSI, but really?

GF

As GF just said, the question targeted a specific aspect of Lat/Long knowledge. It wasn't designed to get someone to develop a multi-level trigonometry method that applies to any two sets of coordinates. If you really think the answer you provided was the best option for answering the question, then you probably haven't taken enough FAA tests.:D

You're probably right. I *have* written enough navigational software where the problem is common to the point that memorizing the general solution is an occupational hazard :-)

I must have slept through the FAA exams that referenced a "Map.";)

Map= Obtained at AAA or DMA, (and used for Land Navigation).
Chart= Used for Aeronautical or Nautical Navigation, (and each use different projections).

My first FI would of slapped me if I ever asked him for a map.

I do admit I couldn't solve that equation without using artificial assistance.

Carry on

:confused: Not quite sure why I appear to be the target of your vocabulary lesson. The "M" word was used by the OP and I'm sure originated with him and not the test or textbook he referenced. Considering the question he posed and the way he did it, I felt the terminology error was beyond the scope of the situation.