3 ms·
This is only one half of what actually happens while navigating; you have true north and magnetic north - see https://theprepared.com/wp-content/uploads/2017/08
by _s 4y ago
This is only one half of what actually happens while navigating; you have true north and magnetic north - see https://theprepared.com/wp-content/uploads/2017/08/Magnetic-North-Pole.png https://theprepared.com/wp-content/uploads/2017/08/Magnetic-... to visualise.
The difference between them depends on where you are on the globe - it could even be 180 if you happen to stand in the middle of the two!
Magnetic compasses always point to magnetic north, but with earth being molten metals - magnetic north likes to move around a bit, so we work out the difference between the two and apply it to the directions we track.
For example, where I am - the difference is around 11 degrees. In order to head directly west, I can't just let my compass point north and go directly perpendicular port - I have to point north plus 11 degrees, and then go. My actual track will be 281 degrees magnetic, not 270.
What's more interesting is that most navigation charts have all their directions and bearings in "true" north, with the variance also charted, whereas when you actually fly you track the magnetic headings plus/minus the variance. If you go long enough - the variance also changes so you have to take that into account when planning your route.
When you take off / land, runway headings directly corresponding to your heading are a good sanity check - alarm bells go off if you're meant to be flying west but come across Runway 09 in front of you (the opposite!), not to mention we still rely on magnetic compasses - you don't want to be coming into land at a track for 281 degrees and see 27 painted on it; chances are something went wrong somewhere.
- adolph 4y agoAt what distance a flight would you use a great circle route? How is a continuous compass heading derived for a GC? Example, San Francisco US, SFO (37°37'08"N 122°22'31"W), to Granada Spain, GRX (37°11'19"N 3°46'38"W) would go north of Newfoundland Canada: http://www.gcmap.com/mapui?P=SFO-GRX%0D%0A&MS=wls&DU=mi http://www.gcmap.com/mapui?P=SFO-GRX%0D%0A&MS=wls&DU=mi
- kortilla 4y agoThe first non-stop flights to Hawaii from mainland US were part of a competition put on by Dole leaving from Oakland. The 1st place plane took off after the 2nd place plane. This happened because the navigator of the 2nd place flight didn’t take into account the great circle distance and flew straight headings.
- captaincrowbar 4y ago"The first non-stop flights to Hawaii from mainland US" (looks at map of Pacific) Hang on ... where did the earlier ones stop?
- drpixie 4y agoIn practice, there's not a specific distance but planning convenience. For short manually planned tracks (eg VFR planning), most people will plan from one visually helpful waypoint to another, and measure the heading at the mid-point of each route segment. The midpoint approximates the GC heading, and anyway the discrepancy is (very) small on short segments. For long tracks (eg IFR interstate or intercontinental flights), most plans comply with standard routes, between nominal waypoints, and the route segments together may approximate GC tracks. Between each waypoint, one normally flies a constant heading. This is imperfect, but variations due wind and traffic are far bigger than losses due not quite conforming to a GC track.
- bonzini 4y agoMadrid-Los Angeles indeed goes all the way north of Newfoundland, even northern than the great circle. The exact route taken over the Atlantic may vary from day to day depending on weather conditions. https://www.flightradar24.com/data/flights/ib6171#2ebdae12 https://www.flightradar24.com/data/flights/ib6171#2ebdae12
- lsh123 4y ago“If you go long enough…” Press GPS direct button and don’t bother ;) ;) ;)