navigation using true or magnetic
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navigation using true or magnetic
Hi
very simple question that I'm having trouble getting my head around.
With regards to navigation using true or magnetic north: On a commercial operation covering hundreds of miles would a GPS usually be set to navigate via true or magnetic N?
I would think that it would be True N as this would enable us to programme specific coordinates along lines of long and lat to form waypoints. But if this is the case then what role does magnetic N in navigation over long distances?
maybe i've got my head screwed on backwards about all of this, but any pointers would be appreciated.
very simple question that I'm having trouble getting my head around.
With regards to navigation using true or magnetic north: On a commercial operation covering hundreds of miles would a GPS usually be set to navigate via true or magnetic N?
I would think that it would be True N as this would enable us to programme specific coordinates along lines of long and lat to form waypoints. But if this is the case then what role does magnetic N in navigation over long distances?
maybe i've got my head screwed on backwards about all of this, but any pointers would be appreciated.
Generally speaking magnetic north is all that is used except when operating close to the magnetic poles.
I think perhaps you have misunderstood the application of magnetic north vs true north. Using magnetic north you can still plot the same waypoints using latitude and longitude and they have the same coordinates regardless of whether you are using true or magnetic north. The difference comes when measuring the angle (bearing) of the course between waypoints. You can measure that angle reference true north or you can measure it with reference to local magnetic north. Given that basic aircraft compass systems point to magnetic north, it makes sense to use magnetic where you can. To convert the bearing to true north is simply a matter of adding or subtracting the local variation which is the difference between mag north and true north at your location.
I think perhaps you have misunderstood the application of magnetic north vs true north. Using magnetic north you can still plot the same waypoints using latitude and longitude and they have the same coordinates regardless of whether you are using true or magnetic north. The difference comes when measuring the angle (bearing) of the course between waypoints. You can measure that angle reference true north or you can measure it with reference to local magnetic north. Given that basic aircraft compass systems point to magnetic north, it makes sense to use magnetic where you can. To convert the bearing to true north is simply a matter of adding or subtracting the local variation which is the difference between mag north and true north at your location.
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The base system used depends on the installation, though normal references via cockpit instruments are by magnetic north.
For example, the 744 has no magnetic compasses other than the standby compass. The FMS derives courses and headings from the IRUs, which are referenced to True North, plus computed Magnetic Variation. The computed Magnetic Headings/Courses are then displayed in the cockpit.
True Headings/Courses are displayed in the Polar regions or when selected by the pilot.
For example, the 744 has no magnetic compasses other than the standby compass. The FMS derives courses and headings from the IRUs, which are referenced to True North, plus computed Magnetic Variation. The computed Magnetic Headings/Courses are then displayed in the cockpit.
True Headings/Courses are displayed in the Polar regions or when selected by the pilot.
Polar nav
Airbus aircraft can be fitted with a polar nav button that changes the Nav System display from the default Mag to True (and grid).
Used above 78N (IIRC) and below 60S.
I'm sure Boeing have similar.
Otherwise it's out with the astrocompass....
Used above 78N (IIRC) and below 60S.
I'm sure Boeing have similar.
Otherwise it's out with the astrocompass....
Last edited by compressor stall; 13th May 2015 at 01:23.
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Thanks for the responses guys. It makes sense to me that the cockpit instruments are aligned to magnetic north. But if this is the case then what is the purpose of true north when it comes to navigation?
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True North = TRUE north
Magnetic North = wherever the magnetic pole happens to be, and it's not always where you think it is!
If maps used Magnetic North instead of True North, then they'd have to be re-printed quite frequently. Instead they stay the same, and just have a "declination" change made, that you add or subtract to magnetic north to get true north.
Magnetic North = wherever the magnetic pole happens to be, and it's not always where you think it is!
If maps used Magnetic North instead of True North, then they'd have to be re-printed quite frequently. Instead they stay the same, and just have a "declination" change made, that you add or subtract to magnetic north to get true north.
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All the procedures designers and chart makers dream of the day that all airplanes have IRUs, or their equivalent. Then, everything would be done with true courses and headings, and magnetic variation would be a nuisance of the past.
True North = TRUE north
Magnetic North = wherever the magnetic pole happens to be, and it's not always where you think it is!
If maps used Magnetic North instead of True North, then they'd have to be re-printed quite frequently. Instead they stay the same, and just have a "declination" change made, that you add or subtract to magnetic north to get true north.
Magnetic North = wherever the magnetic pole happens to be, and it's not always where you think it is!
If maps used Magnetic North instead of True North, then they'd have to be re-printed quite frequently. Instead they stay the same, and just have a "declination" change made, that you add or subtract to magnetic north to get true north.
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I wasn't very clear, sorry, yes, things would change with respect to "north" if we used magnetic north for our maps (rather than changing the lines on the map).
Also, VORs would have to be aligned differently every so often.
Also, VORs would have to be aligned differently every so often.
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Good stuff from SWARM here: https://youtu.be/O7SJrG_7Sy8
Go to the 4:00 point to see the actual movement of the Mag North Pole.
Go to the 4:00 point to see the actual movement of the Mag North Pole.
Crossing the Atlantic is done in true. Presumably other oceans too.
The only time we measured True and steered True was over the Polar region.
Only the Navigator had some idea of where they were at any given time, ( Flt. engineers had to be able to do sums, co-pilots had to be able to write and the Captain had to know someone who could read ) and even the Navigator only really knew where they had been, i.e. at his last "fix", 'cos plotting position lines from whatever source, Astro, Loran, Consol, NDB or whatever, took time to get down on paper by which time the aircraft had moved on, so immediate present position was always an estimate. VOR/DME was the nearest one could get to exact "present position" I guess, but there were none of those over the Atlantic ( or any Ocean, and few deserts )
The whisky compass (i.e. magnetic) is slightly wrong most of the time, by a predictable (but varying) amount - but it is cheap, light, simple, requires no power, and has worked for 1,000 years (except around the Poles).
The basic deal is that aviation is a "system" - controllers, charts, plates, runway headings, radio beacons, etc, etc. That has to work, universally, for any type of airplane.
From a Piper Cub with just a whisky compass, to a Cessna with a gyroscopic compass (that drifts, and therefore needs a whisky compass for periodic correction), to airliners with IRU's or GPS units - that may happily work with no magnetic reference at all (unless something goes wrong and needs cross-checking).
Busy controllers (as just one example) can't be handling TWO systems - giving 747s True vectors, and the little guys Magnetic vectors.
So the system can't abandon magnetic North just yet, even though many specific planes can (or already do, effectively). And as aterpster says, that will be the ideal (except for the gals/guys in Piper Cubs).
As others have said - there are remote spots in the Polar regions, where magnetic doesn't work - Magnetic "North" may be W, E or even S of your position, in grid and True terms.
The commercial systems can adapt (mostly) - and the little guys pull out the charts and pencil and do some math (or these days, may have an "app") and correct for the error that way.
The basic deal is that aviation is a "system" - controllers, charts, plates, runway headings, radio beacons, etc, etc. That has to work, universally, for any type of airplane.
From a Piper Cub with just a whisky compass, to a Cessna with a gyroscopic compass (that drifts, and therefore needs a whisky compass for periodic correction), to airliners with IRU's or GPS units - that may happily work with no magnetic reference at all (unless something goes wrong and needs cross-checking).
Busy controllers (as just one example) can't be handling TWO systems - giving 747s True vectors, and the little guys Magnetic vectors.
So the system can't abandon magnetic North just yet, even though many specific planes can (or already do, effectively). And as aterpster says, that will be the ideal (except for the gals/guys in Piper Cubs).
As others have said - there are remote spots in the Polar regions, where magnetic doesn't work - Magnetic "North" may be W, E or even S of your position, in grid and True terms.
The commercial systems can adapt (mostly) - and the little guys pull out the charts and pencil and do some math (or these days, may have an "app") and correct for the error that way.
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I can't agree more with you, pattern_is_full.
Having left the profession few years ago as a skipper on a 744, and if nothing has dramatically changed since then, I always used "magnetic" on any route and any ocean crossing (it was the company SOP)...maybe it was just luck, but I always found the destination. Sometimes, just for curiosity, I changed from magnetic to true for a while, to have an idea of the local variation (being "lazy " to check the map), but then I reverted to magnetic.
And as you say, ATC never gives a command like, e.g. "Turn right heading 040 TRUE"...it has no sense at "normal latitudes"..., after all, the runways are "magnetic", so the final approach course, the courses leading to the approach and back to the enroute system.
The TRUE system has a role, in the unlikely event, today, (at least in the commercial aviation) of a complete failure of the navigation system so we should go back to the basics and plotting therefore the route on a TRUE referenced chart while considering the local variation.
Having left the profession few years ago as a skipper on a 744, and if nothing has dramatically changed since then, I always used "magnetic" on any route and any ocean crossing (it was the company SOP)...maybe it was just luck, but I always found the destination. Sometimes, just for curiosity, I changed from magnetic to true for a while, to have an idea of the local variation (being "lazy " to check the map), but then I reverted to magnetic.
And as you say, ATC never gives a command like, e.g. "Turn right heading 040 TRUE"...it has no sense at "normal latitudes"..., after all, the runways are "magnetic", so the final approach course, the courses leading to the approach and back to the enroute system.
The TRUE system has a role, in the unlikely event, today, (at least in the commercial aviation) of a complete failure of the navigation system so we should go back to the basics and plotting therefore the route on a TRUE referenced chart while considering the local variation.
Last edited by vincenzino montella; 10th Jan 2016 at 21:42.
If crossing the Atlantic Eastbound on a more Southerly track then normal, i.e. heading more for the French Atlantic Coast towards Brest, before turning northwards, I would often find myself off track towards the South.
On one occasion I was trying to understand the reason why, and the Skipper - who had been a Coastal Command Sunderland Captain in the days when they had to understand navigation techniques not normally of concern to mere pilots - said that he had experienced errors in this area, too, and was of the opinion that the charted Magnetic Variation in this area was incorrect, maybe the nicely curved isogonal lines in that area should show more of a kink. Maybe ?
On one occasion I was trying to understand the reason why, and the Skipper - who had been a Coastal Command Sunderland Captain in the days when they had to understand navigation techniques not normally of concern to mere pilots - said that he had experienced errors in this area, too, and was of the opinion that the charted Magnetic Variation in this area was incorrect, maybe the nicely curved isogonal lines in that area should show more of a kink. Maybe ?
Wouldn't surprise me.
The Mid-Atlantic Ridge is constantly pushing up new material from the Earth's mantle, some (~6% by weight) of which may have magnetic properties (iron). An extra-large "burp" of such might result in a local magnetic anomaly under the ocean.
The outflow also "records" reversals in the Earth's magnetic field over the millennia, also forming anomalies:
http://earthref.org/drupal/sites/ear...anomalies2.jpg
(Big Image): http://www.geology.sdsu.edu/kmlgeolo..._anomalies.jpg
And the charted declination is generally based on a model, rather than direct point-by-point measurements (which would be hard to do consistently every 90 days in mid-ocean anyway).
Plus, our old friend the solar wind pushes on the magnetosphere, and can cause brief distortions (cf: geomagnetic storm).
The Mid-Atlantic Ridge is constantly pushing up new material from the Earth's mantle, some (~6% by weight) of which may have magnetic properties (iron). An extra-large "burp" of such might result in a local magnetic anomaly under the ocean.
The outflow also "records" reversals in the Earth's magnetic field over the millennia, also forming anomalies:
http://earthref.org/drupal/sites/ear...anomalies2.jpg
(Big Image): http://www.geology.sdsu.edu/kmlgeolo..._anomalies.jpg
And the charted declination is generally based on a model, rather than direct point-by-point measurements (which would be hard to do consistently every 90 days in mid-ocean anyway).
Plus, our old friend the solar wind pushes on the magnetosphere, and can cause brief distortions (cf: geomagnetic storm).
Plus, our old friend the solar wind pushes on the magnetosphere, and can cause brief distortions (cf: geomagnetic storm).
BAA Aurora Section: A Jam-Jar Magnetometer
