View Full Version : Calculating Distance without scale
24th Jun 2012, 06:31
How do you calculate the distance between INGO VOR (N6350 W01640) and Sumburg VOR (N5955 W 00115) on the given figure, without any scale. Normal trigonometry doesnt work here. Answer: 494 NM
24th Jun 2012, 06:43
Do you mean, make the calculation using just the chart? Cannot help you there.
Otherwise, formulas here (http://williams.best.vwh.net/avform.htm#Intro), with worked examples at the end!
24th Jun 2012, 07:17
Use the scale at about halfway along from the latitude scale next to the 10w.
Then work it out using a bit of paper or a pair of dividers
24th Jun 2012, 07:20
Haversine formula. I used to have it on an old programmable calculator which was barred from ATPL exams. Or depending on the projection and distances involved use the latitude scale.
Sorry Mad Jock postings crossed.
24th Jun 2012, 07:56
This is a plotting type problem and as Mad Jock says you should be using a pair of dividers. You are using the fact that anywhere on Earth 1 degree of latitude (measured up the meridian of longitude) is 60 nm and 1 minute is 1 nm.
I personally would set my dividers to say 5 degrees = 300 nm and step from INGO to SUM. You would get a full 5 degrees, then close the dividers to SUM then measure the difference - much easier shown than described.
NB. This is not a departure problem and unless you are using proper spherical trigs (not a JAA requirement) then you will only get an approximate and probably wrong answer.
24th Jun 2012, 10:19
Thankyou everyone for the feedback
Use the scale at about halfway along from the latitude scale next to the 10w
mad_jock, can you please elaborate a little bit. If I have understood correctly then measuring from 60N along 10W meridian halfway (till 62.5N) is about 6.cm.
That means 6cm for 2.5deg or 150nm. 1cm = 23nm
Ingo to SUM = 21.5cm
23 x 21.5 = 494 (quite precise)
Is it just a fluke or that's what u mean. For measuring distances on other parts of the charts, at which point do you establish the scale?
24th Jun 2012, 17:51
Roughly you are going from N64 to N60 so what MJ is saying is measure around N62 using the W010 meridian as good practice.
This is a Lamberts chart and scale is ONLY correct at the 2 standard parallels which might not even be actually on the chart. On a Lamberts whether you measure at N62 or N55 (if possible) would not make a fat lot of difference.
However, if you were ever to plot on a Mercator (not in JAA) it is VITAL you measure distance using scale mid-way along a track, there would be a BIG difference in using scale at say N40 for a flight around N20.
If you follow my original post calculations aren't even required.
25th Jun 2012, 03:33
Great! Thats loud and clear now
26th Jun 2012, 10:40
The distortions due to the projection which Richard correctly describes should be less than 1%. When the UK CAA used these charts 10+ years ago the bigger problem was the distortion caused by photocopying, hence you use a meridian close to or at the centre of track as described above, working on the idea that the meridian and the marked track will have been distorted by similar amounts. In real life, of course, you would never use photocopied charts.
26th Jun 2012, 11:40
In real life, of course, you would never use photocopied charts.
I wouldn't be so sure about that. You also get the same effect when printing out briefing packs. And then someone starts jamming the GPS signal and you have to wag it with no ground based NAV aids either in range or working because the locals have nicked the copper.
26th Jun 2012, 12:50
love it! Africa?
26th Jun 2012, 13:36
And middle east 500 mile leg with no radio coms with center, no nav aids, no GPS.
Found out later israel was practising bombing Iran.
Round djibouti its an all to common occurance. I presume the west coast is just as bad.
Folk don't believe me when I say I used to think the theory was a pile of useless crap but 12 years on quite alot of it has turned out to be useful.
We used to dream of photocopied maps.....
27th Jun 2012, 03:06
distortion caused by photocopying, hence you use a meridian close to or at the centre of track
Thanks for putting that piece in the puzzle Alex
27th Jun 2012, 08:42
Not a problem.
Some of the stuff you are being asked to learn may seem useless just now and may very well be if your career goes in certain directions.
I suspect PACO has far more entertaining storys about navigating round the world with a broken pencil, bit of wet seaweed and a condom as his only nav aids. At least I had access to a pair of dividers.
But it just shows you though 2.5 million dollar aircraft, honeywell EFIS avionics and we were twiddling a pair of dividers and looking out the window at FL240 for an hour and half trying to spot roads. I might add I was quite chuffed when we did eventually pick up a VOR we were only 3 degree off the radial that I predicted so 6 miles off track.
The condoms get all messy, but the spread of my fingers is 9 inches.... :)
Nice when it all works out, isn't it?
27th Jun 2012, 12:09
Yep. And to be honest after the first time when similar things have happened since, but no where near as far as that leg, it really has been a none event.