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I How to find true north not magnetic north?

  1. Jun 30, 2018 #21

    Dale

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    Hopefully sometime before tomorrow you can get the noaa calculator working :-)
     
  2. Jul 4, 2018 #22
    I finally figured out how to use noaa. These numbers are correct for GPS & Google Maps but not for noaa N35 52.802 W086 27.684

    N & W & 0 need to be dropped. The . needs to be moved 2 places to the left for noaa.

    N35 52.802 = 35.52802 = Longitude

    W086 27.684 = 86.27684 = Latitude

    DST has to be clicked

    Do not change time zone number -6

    Magnetic noon = 1:12 pm

    Solar noon = 1:18 50 pm

    Shadow movement angle from magnetic noon to solar noon is 6.8 degrees
     
  3. Jul 4, 2018 #23

    russ_watters

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    I get 12:46pm CDT by my previous calculation. The link tells me 12:50 CDT. As a cross-check, "the equation of time" of 4 minutes is the deviation, which matches the difference between the calculations (I didn't realize the deviation due to Earth's orbit was that much).

    So I think you are doing at least two things wrong:
    1. When separated by one space each, the position measurements are degrees and minutes. NOAA wants degrees, minutes and seconds. You have the degrees and minutes already, and just need to convert the seconds by multiplying the decimal of minutes by 60. Or just omit seconds because it makes very little difference.

    2. I don't know what the other error could be. The date maybe? Since the center of your time zone is 90 degrees, you are east of it, which means local noon is usually earlier, not later than what your clock tells you (DST adjusted).
     
  4. Jul 4, 2018 #24
    I should check magnet north again. I have learned a very good expensive compass will give a screw up reading in town it is so sensitive it picks up nails in the yard, metal car in the driveway 50 ft away, metal in the house, metal utility in the yard, chain link fence around the yard, etc. A cheap $1 toy compass is more accurate in town because near by metal things do not effect it.
     
  5. Jul 4, 2018 #25

    Vanadium 50

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    That's not possible. A compass measures a magnetic field. There is one magnetic field at each point in space, not multiple ones depending on whether nearby objects "count" or not.
     
  6. Jul 4, 2018 #26

    tech99

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    You can point the hour hand of a watch at the Sun, then split the angle between the hour hand and 12 o'clock, and that is True South.
     
  7. Jul 4, 2018 #27
    Metal attracts magnets. Compass needle is a magnet. Car is metal, chain link fence is metal, nails are metal. The cheapo compass is a magnet it does not point in the direction of metal. The expensive compass is a better magnet it will find everything metal in the yard & house you can not get a accurate reading unless your out in the wilderness where there is no metal. Buy a compass see for yourself.
     
    Last edited: Jul 4, 2018
  8. Jul 4, 2018 #28

    Vanadium 50

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    None of what you wrote is correct.
     
  9. Jul 5, 2018 #29
    Hour hand? All I see are numbers.

    Cheers
     
  10. Jul 5, 2018 #30

    phyzguy

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  11. Jul 6, 2018 #31

    sophiecentaur

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    Actually, it isn't unless you happen to be in the centre of a time zone. Your watch shows Universal time (plus or minus the appropriate hours offset) so there can be up to 180/24 = 7.5 degrees of error with that method. Near enough if you're lost out on the moors but not if you are trying to build a house or draw an accurate map.
     
  12. Jul 6, 2018 #32
    Prove it.
     
  13. Jul 6, 2018 #33

    Vanadium 50

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    You were the one who claimed that some compasses measure different magnetic fields than others. Apart from going against several centuries experience with magnetism, it is contrary to all good sense and logic. So it seems to me the burden is on you to prove your iconoclastic and heterodoxical beliefs, not mine. (However, before you go down that path, I recommend you review the PF Rules on personal theories. Wouldn't want to have it removed as crackpottery, would we?)

    A compass is a device to measure the direction of the local magnetic field. The local magnetic field has one value, with one direction. It does not have multiple values depending on the imputed presence or absence of various nearby objects. That would be like having two thermometers, one that tells you the temperature of the room with the air conditioning on and another with the air conditioning off. Instead, a thermometer measures the actual temperature in the actual room, and not the imagined temperature in some speculative room. A compass does the same thing - it measures the real magnetic field, and not some imaginary field for some counterfactual arrangement of nearby objects.
     
  14. Jul 6, 2018 #34

    berkeman

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    True, but a weak and very small one. Unless you get real close to some ferrous material, the magnetic attraction between the compass needle and the object will have basically no effect.

    But it's true that ferrous objects do deflect the Earth's magnetic field nearby. That's why it's hard to find a good place to mount a compass in a typical car made of steel.

    https://i2.wp.com/mumetal.co.uk/wp-content/uploads/2013/10/Picture1.jpg
    Picture1.jpg
     
  15. Jul 6, 2018 #35

    Nugatory

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    This isn't quite right. A compass points in the direction of the magnetic field at wherever the compass is located. Nearby metal objects do affect the direction that the compass points, but not because they attract the needle; they change the direction the magnetic field in their vicinity points.

    The difference in readings between a cheap compass and an expensive one has nothing to do with one being better than the other at finding nearby metal. They're both reading the exact same magnetic field because there's only one magnetic field at each point in space; the expensive one is just doing it better (more accurately and more repeatably).

    And all of this reminds me... we just replaced some stuff in the console of the boat, and the added metal is affecting the local magnetic field differently than what had been there.... so in the next few days I have to slightly adjust the compass housing so that it reads zero degrees when I'm going due north. Because it's the 21st century I'm not going to use Polaris to determine north, I'm going to use the GPS - head out to open water, drive due north according to the GPS, adjust the compass so it agrees. And why am I messing with a compass at all when I have GPS? Long before I had a GPS I would get through the fog by following a compass heading from buoy to buoy and I want the backup in case the GPS dies on me.
     
  16. Jul 6, 2018 #36
    I am not claiming a compass can measure a magnetic field. A claim a cheap toy compass does not have a good strong pointer needle magnet like a much better quality more expensive compass. Directions that come with a compass says, keep it away from metal objects to get an accurate reading. Sometimes wind moves my TV antenna then I have to reset it. I have noticed the toy compass works best to aim the TV antenna in the correct direction because the toy compass is not attracted to the metal antenna tower from 5 ft away. I have to walk 20 ft away from the antenna using the good quality compass. The good quality compass is effected by all the metal objected in the yard, patio furniture, cars in the driveway, garden tools leaning up against the tree, tiller in the garden, lawn mower in the shed, chain link fence around the yard. I have noticed the good quality compass can be used to find nails in the sheet rock wall once I find a nail I know there are other studs in the wall all 16" apart this helps me hang pictures on the wall for my wife. The toy compass is a pain in the butt trying to find nails in the wall but when either of the compass get close to a nail the needle points right at the metal nail.

    Do this experiment. Buy a stack of magnets here. https://www.ebay.com/itm/Super-Stro...rth-New/173384504513?var=&hash=item285e8594c1 put the stack on a float in a plastic tray of water in the yard away form all metal. The float boat will rotate to North south direction. Now move a piece of metal near the plastic tray the float boat will move toward the metal. Compass needle does the same thing.
     
    Last edited: Jul 6, 2018
  17. Jul 6, 2018 #37

    sophiecentaur

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    I'm not sure what you referred to here because you didn't actually quote one of ~Gary's posts. I am not sure that he has been putting across what he really means. It could be muddled rather than plain wrong. - could be a language thing? (@gary350 ?)

    But it is true to say that finding magnetic North is affected by nearby objects and their actual orientation. Ships (even humble boats. sometimes) have various arrangements round a compass binnacle which attempt to correct for the iron in the vessel. There are permanent magnetic effects, varying electromagnetic effects and there are effects of the susceptance of the ship parts on the Earth's field. See this link.
    On top of that, doing it properly involves using a 'Deviation Table' which is a set of deviations from the ship's apparent magnetic N and the magnetic N on the chart. You 'swing the ship' over 360° and take readings every few degrees. and comparing with a known geographical bearing. If you want a good magnetic bearing, you correct with the deviation table and then you apply the Variation, which is on every chart. That tells you the variation on a certain date with a yearly rate figure to correct further.
    Something I have not understood is how a Fluxgate compass is reckoned to be better behaved than a magnetic needle. Also, why do they tell you to mount a fluxgate sensor low down in the ship? That could be very near an iron keel.
     
  18. Jul 6, 2018 #38

    Vanadium 50

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    Back in Days Of Old, the ship's compass was placed in something called a binnacle. It had two nearby iron correcting spheres to compensate for all the ferrous material on the boat. Add (or remove) ferrous material and "simply" readjust.
     
  19. Jul 6, 2018 #39

    Vanadium 50

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    Sure. But it's not true that a cheap compass is not affected by the nearby objects and only the earth's magnetic field. (The claim "A cheap $1 toy compass is more accurate in town because near by metal things do not effect it.") As mentioned, the magnetic field is what it is, and nearby objects can change it, but change it they do. A cheap compass does not ignore these changes.
     
  20. Jul 6, 2018 #40

    Nugatory

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    Yes, and your scare-quotes around the word "simply" are well-taken - it's trickier than it sounds.
     
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