Version 1. - May 2006
090 M would be equivalent to 100 T). When the variation is west the magnetic amount be the greater than the true amount. This is easily remembered by:
"Variation east, magnetic least.”
Example: Information in the margin of a topographic map of Adelaide dated 1980 shows the magnetic variation is 6 15' eastwards, plus 02' annual change easterly. Magnetic variation in the year 2005 will therefore be 6 15’ plus 25 x 2 minutes = 50 minutes, which gives a total of 7 05’ east, which is close to 7 . In 2005 a magnetic bearing of 030 M would therefore give a true bearing of 037 T (remember – variation is east, so the magnetic figure is least). In practice, magnetic variation is often ignored unless very precise flying is required as in a competition task.
On topographic maps and VTC charts, the magnetic variation is written in the margin. On VNC, ERC and WAC charts, which cover much more terrain, the magnetic variation can vary significantly across a single chart, and so is indicated by isogonals which are lines across the chart joining points of equal magnetic variation. (The isogonal which joins points of zero magnetic variation is called an agonic line.)
Magnetic deviation means any local error caused by something affecting the compass reading, such as nearby metal objects or metallic ore deposits in the ground. Fixed compasses in ships and planes are corrected for this. To minimise magnetic deviation, hand held compasses should be used a few metres away from cars or other large metal objects.
Some navigation terms are intended for powered aircraft, which from inside the cockpit can appear to be heading in a particular direction but are typically blown a bit off course by a crosswind factor, so that the resulting track is a slightly different direction on the map. Balloons by comparison have zero airspeed as they fly with the wind rather than through it. They also do not have a nose or front end to aim or ‘head’ towards something. So in a balloon the direction the balloon is actually moving towards is both its heading and its track.
Your bearing (eg, from an aerodrome), is the magnetic direction from that point towards your position.
Example: ‘5 miles northeast Mildura’ means your position is 5 miles from Mildura aerodrome towards the northeast.
ARE YOU UP TO DATE? New regulations and procedures may apply from time to time. Check on the ABF website that you have the latest version of these study notes.
© Australian Ballooning Federation Inc
ABF Pilot Training Manual Part 8