4 ms·

This is pretty sad. For at least decades, maybe even centuries, there have existed compasses that let you rotate the "north" indicator to match the current loca

This is pretty sad. For at least decades, maybe even centuries, there have existed compasses that let you rotate the "north" indicator to match the current local magnetic declination. Adjusting as you travel is no different than the pressure adjustments that pilots make for altimeters. There is no good reason we couldn't have all of the airports using geographic north. That one is far more resistant to change, with continental drift and Earth axis wobble being far slower than magnetic pole movement.

aeternus 8y ago
There is a good reason. An adjustable compass like that would be much more prone to user-error. Someone may rotate it too much causing very large discrepancies and land on the wrong runway.
- dnadler 8y ago
  Same can be said of altimeter adjustments which would literally cause you to fly into the ground, but that isn't stopping us from allowing manual adjustments.
  - aeternus 8y ago
    Barometric pressure can fluctuate every day, and by relatively large amounts causing errors of 500ft+ so there, the 'price' of manual adjustment is worth the gain in accuracy. I'm not convinced the same can be said for magnetic adjustment.
repiret 8y ago
Ease and reproducibility of the measurement is more important than its absolute accuracy. Whisky-ball magnetic compasses are cheap and easy to build, and have been throughout aviation's history, and don't require access to an accurate geomagnetic survey to use. Its interesting that you mention pressure adjustments, because: * Above 18,000 feet in the US, you don't make pressure adjustments - because its more important to know that your neighbors in the airspace are measuring their altitude the same way you are than it is to know exactly how far off the ground you are. In other parts of the world the threshold is lower, sometimes much lower. * Airspeed is also measured with a pressure instrument, whose accuracy is affected by the density of the air, which in turn is affected by temperature and altitude. But the speed limits in the regulations, and the speeds ATC might tell you to fly, are all based on the uncorrected output of the pressure instrument - "indicated air speed", as its called. You only try to adjust for altitude and pressure when using the airspeed for navigation.
- snuxoll 8y ago
  Sectional charts also display magnetic variation, there's no reason to change the instruments when you can do basic math.
toomanybeersies 8y ago
Magnetic declination changes quite a lot as you travel the kind of distances that aircraft travel. Pilots would have to periodically adjust their declination while flying. Declination is also not linear or varying only by latitude/longitude, but it also varies by area due to magma flows and iron deposits. Take a look at this map of global declinations for reference: https://en.wikipedia.org/wiki/File:World_Magnetic_Declination_2015.pdf https://en.wikipedia.org/wiki/File:World_Magnetic_Declinatio...
jonahhorowitz 8y ago
If you buy maps from the USGS [0], they have the magnetic north and true north indicated on them so you can adjust your compass. I didn't realize how rapidly my maps got out of date. [0] - https://www.usgs.gov/products/maps/topo-maps https://www.usgs.gov/products/maps/topo-maps
- emmelaich 8y ago
  For naviagtion, charts are updated via [0] Notices to Mariners . It is of course the responsibility of the mariner to mark/attach the notice to the chart. [0] https://en.wikipedia.org/wiki/Notice_to_mariners https://en.wikipedia.org/wiki/Notice_to_mariners

aeternus 8y ago

There is a good reason. An adjustable compass like that would be much more prone to user-error. Someone may rotate it too much causing very large discrepancies and land on the wrong runway.

dnadler 8y ago
Same can be said of altimeter adjustments which would literally cause you to fly into the ground, but that isn't stopping us from allowing manual adjustments.
- aeternus 8y ago
  Barometric pressure can fluctuate every day, and by relatively large amounts causing errors of 500ft+ so there, the 'price' of manual adjustment is worth the gain in accuracy. I'm not convinced the same can be said for magnetic adjustment.

repiret 8y ago

Ease and reproducibility of the measurement is more important than its absolute accuracy. Whisky-ball magnetic compasses are cheap and easy to build, and have been throughout aviation's history, and don't require access to an accurate geomagnetic survey to use. Its interesting that you mention pressure adjustments, because: * Above 18,000 feet in the US, you don't make pressure adjustments - because its more important to know that your neighbors in the airspace are measuring their altitude the same way you are than it is to know exactly how far off the ground you are. In other parts of the world the threshold is lower, sometimes much lower. * Airspeed is also measured with a pressure instrument, whose accuracy is affected by the density of the air, which in turn is affected by temperature and altitude. But the speed limits in the regulations, and the speeds ATC might tell you to fly, are all based on the uncorrected output of the pressure instrument - "indicated air speed", as its called. You only try to adjust for altitude and pressure when using the airspeed for navigation.

snuxoll 8y ago
Sectional charts also display magnetic variation, there's no reason to change the instruments when you can do basic math.

toomanybeersies 8y ago

Magnetic declination changes quite a lot as you travel the kind of distances that aircraft travel. Pilots would have to periodically adjust their declination while flying. Declination is also not linear or varying only by latitude/longitude, but it also varies by area due to magma flows and iron deposits. Take a look at this map of global declinations for reference: https://en.wikipedia.org/wiki/File:World_Magnetic_Declination_2015.pdf https://en.wikipedia.org/wiki/File:World_Magnetic_Declinatio...

jonahhorowitz 8y ago

If you buy maps from the USGS [0], they have the magnetic north and true north indicated on them so you can adjust your compass. I didn't realize how rapidly my maps got out of date. [0] - https://www.usgs.gov/products/maps/topo-maps https://www.usgs.gov/products/maps/topo-maps

emmelaich 8y ago
For naviagtion, charts are updated via [0] Notices to Mariners . It is of course the responsibility of the mariner to mark/attach the notice to the chart. [0] https://en.wikipedia.org/wiki/Notice_to_mariners https://en.wikipedia.org/wiki/Notice_to_mariners