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millermilla
6th Apr 2004, 22:05
Hiya,

When planning a route on the half mil chart from say town A to town B I draw a straight line and measure the distance. To get the true track I mark the middle of A - B and align my protratctor in the middle and read off the heading which works fine. To mark on my chart a 10 degree off track line I use the same meathod of aligning the protractor in the middle of true track and minus or add 10 degrees and mark the fan line originating from B and extending to A. Now when I move the protractor directly over B to double check the heading or give the radial from B which should be the same as track true this works fine but when I check the 10 degree off track line I only get a radial of about 7 degrees. This is the meathod ive used throughout my PPL and beyond. Am I doing it correctly????? The difference between measuring the heading half way along track and over the destination confuses me and I can understand why the fan line heading differs.

FlyingForFun
7th Apr 2004, 08:18
To get the true track I mark the middle of A - B and align my protratctor in the middle and read off the heading I can't quite picture what you're doing here. When I measure the track, I make sure the protractor is lying along my route (or the extended route, if necessary), and lined up with a known north-south line, usually a line of longitude. Then I measure the angle between the line of longitude and my route (or extended route, if the route doesn't cross such a line). How do you know where true north is relative to your route, if you are measuring at the middle of your track? I suspect I've misunderstood you somewhere....

(I do agree, by the way, that measuring as close to the middle of your track as possible is desirable.)To mark on my chart a 10 degree off track line I use the same meathod of aligning the protractor in the middle of true track and minus or add 10 degrees This does not sound right to me.... but since I don't quite understand the way you are measuring your track angle, it's possible I'm wrong (and I'm sure someone else will be quick to say so if that's the case).

The fan-lines should be measured and drawn from each end of your route. The middle point of your route is not relevant to drawing fan lines. Line the protractor up with your route such that the point of the protractor is at one end of the route, and the route itself is along the 0 degrees line. Then mark off 10 degrees and 350 degrees (or minus 10 degrees, if you prefer), and draw a line from the end-point of your route to each of these points in turn. The repeat, for the other end-point of your route.

I'm not sure I've explained that very well - it's much easier to show than to explain in words. Definitely something that would best sorted out by having a chat with your instructor, I would have thought.

FFF
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homeguard
8th Apr 2004, 14:37
millermilla

The reason that you have been taught to measure your track at the halway point is to find the mean of your 'great circle' track - the shortest distance between to points on the surface. When drawn on the map or chart it is called a rhumb line ( wavy line). It is not necessary to measure halfway on a due north/south track for there are no map making angular errors along a line of longitude. If you check a line of latitude using a straight edge you will notice that the line of latitude is concave to the equator. Therefore the errors are greatest East/West and zero north/south. Draw a track East/West for a length of your ruler, say 100nm. Measure from the eastern end aligning your protractor as normal to north. then do the same from the western end. The track that you measure halfway is the mean of the two measurements.

Remember the Earth is round but your map is flat. It is not possible to replicate exactly from one to the other. Your map though is mathematically arranged to be good enough for relatively short distances but you need to follow some simple rules such as the one to which you have referred.

There is no need to do the same when marking drift lines. You are simply marking a line +/- of track, whether from the start point of the track to abeam your first pinpoint or from your pinpoint to close with the next waypoint, whatever technique that you use. You do so only to help you assess during flight an estimated drift from track, dead reckoning. Again, usually good enough.