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Non-magnetic compass? Is there such a thing?

Discussion in 'General Equipment Chat & Advice' started by Malcolm_Stewart, Jul 20, 2021.

  1. Malcolm_Stewart

    Malcolm_Stewart Well-Known Member

    I'm hoping that my off-beat request can be included under the "accessories" section of this page.

    I'm looking for an application, probably on a Smartphone, which can tell me which direction I'm looking at - even where the local ground magnetism is throwing a magnetic compass off. I thought I'd found it when I came across "OS Locate" but from my own experience and what I've now read, that is a magnetically based system. What I'm wondering about is a system which can triangulate to the nearest, say 3, of the mobile phone masts, and from them know in which direction the Smartphone is pointing. I've done a little searching on the web, but have found no in-depth info. Perhaps I'm looking in the wrong areas...

    [Background - photos taken on my GPS enabled Eos 7D MkII from an old iron-ore quarry, Burton Dassett near Banbury, showed errors of 20 degrees when shooting the Malvern Hills - a very distinctive feature about 40 miles away, and one that I'm unlikely to have wrongly recognised. The 7D MkII uses a magnetic compass for bearing info.]

    Thanks for any ideas ... and I do know about gyro compasses. Far too heavy for my application
     
    Last edited: Jul 20, 2021
  2. Andrew Flannigan

    Andrew Flannigan Well-Known Member

  3. GeoffR

    GeoffR Well-Known Member

    Yes, you can have a non-magnetic compass. Inertial navigation systems can determine True North without recourse to a magnetic sensor, aircraft then use Mag Var (Magnetic variation) tables to determine local magnetic north. GPS systems also derive True North and use Mag Var tables to get magnetic North.

    Directional Gyros, without access to a flux valve, will drift over time and accumulate huge errors unless reset manually, hardly convenient for hill walking. A Gyro compass could be more of a liability than an asset.

    I suspect part of the problem may be that the GPS is using tables that take into account the local variation and is thus little better than a magnetic compass. You could try switching the GPS to True North.
     
    Last edited: Jul 20, 2021
  4. GeoffR

    GeoffR Well-Known Member

  5. dream_police

    dream_police Well-Known Member

    Does your smartphone not have it's own compass app? Certainly the iPhone has one. I assume they are accurate.
     
  6. IvorETower

    IvorETower Little Buttercup

    I think that smartphone compass apps use a magnetic sensor, may be wrong.
    However doesn't something like Google Maps show you a symbol for north, which is based on dead-reckoning when you start to move around with the app open?
     
  7. MJB

    MJB Well-Known Member

    Most smartphones are magnetometer based. The compass on Google maps is GPS based.
     
  8. dream_police

    dream_police Well-Known Member

    Oh, OK. I assumed it was GPS.
     
  9. PeteRob

    PeteRob Well-Known Member

    Differential GPS will give you a heading. You need to walk 20 yds or so in a straight line for it to figure out which way it is going. I don’t know if a phone has it. Most Garmin type GPS navigation tools do and show you your position on a map.
     
  10. peterba

    peterba Well-Known Member

    Yes, that's right, Pete. Even the early Garmin eTrex GPS device that I had (in the early 2000s) was 'aware' of which way it was pointing, so I'd expect later models to be able to do likewise.
     
  11. PeteRob

    PeteRob Well-Known Member

    Only if you are moving in a straight line. When you are standing still it only knows its position. We do Dartmoor letterboxing which, in a pure way, involves finding “letterboxes” by triangulation using a sighting compass and a set of given bearings, e.g. pointed rock 150 paces on 130 degree. These days whole or partial GPS coordinates are sometimes given so we do use a GPS as well but walking a bearing with GPS over rough ground is quite difficult.
     
  12. Petrochemist

    Petrochemist Well-Known Member

    You could 'simply' use the position of the sun/stars which will never be affected by local magnetism. I suspect this would need a sextant as well as clear skies so is highly unlikely to be practical. To be honest I don't think my cheap sextant has the filters common on better models so would be hazardous to use for solar fixes...
     
  13. Malcolm_Stewart

    Malcolm_Stewart Well-Known Member

    Thanks everybody for your replies - I've got masses to think and learn* about.

    One item I do have is a sextant - in fact I have two. The first is made of plastic, and was bought, new, from a yacht chandlers in Porthmadog. The second made of metal, was found in an antiques shop in Machynlleth, and has some real history attached to it. A note in the case indicates that it was used on the run to Shanghai - before China got "modern" - and there's mention of a forwarding agency which I found on an old snap of Shanghai. But sadly, sextants need lots of practice and suitable sun/stars etc.

    *One thing I have learnt about before posting here, is how to remove the effects of magnetism from iron or steel objects on a ship, but I've yet to find much about the effects of iron stone, iron ore and their magnetism on normal terestial compasses.
     
  14. SqueamishOssifrage

    SqueamishOssifrage Well-Known Member

    Point one - a marine sextant won't help you, as there is no horizon of known relative elevation. You would need a bubble sextant, although the truly skilled can make do with a pseudo horizon from a bowl of water. Bubble sextants used to be easily obtainable as military surplus, but now they are about as rare as the Lancasters they used to fly in. (Actually they were bubble octants, but whose quibbling). My estimate is that at least 1,000 sextants sights are needed to become acceptable, and at least 2,000 to become proficient, allied with constant practice.

    Point two - you have everything you need on your mobile 'phone and in the sky. You have a precise chronometer, a GPS unit, and a compass for which you need to calculate the error to within about three degrees:-

    First, get a set of Sunrise/Sunset tables for the area you are in, or find a web-based program which will enable you to input your GPS location to obtain the precise time of local noon, or, better still, tabulated values for the sun's elevation and azimuth from your GPS position. Then take a compass reading of the azimuth of the sun, best done by finding a tall thin object and standing at the end of it's shadow to take a bearing of the object, and compare it with the tabulated value, This will give you the compass error. Alternatively, you could use Alpha Ursae Minoris (Polaris) but bear in mind that it is currently about 0.45° away from true north at present.

    As a final alternative, post here the GPS position, the precise time of the observation (UTC) and the observed Solar azimuth, and I will tell you the error, as I have my own home-grown astro-nav program for Windows and Android, the bone of much contention in the local yacht club as I steadfastly refuse to transplant it to Apple iAnything.
     
  15. Petrochemist

    Petrochemist Well-Known Member

    The horizon is only needed if your trying to measure the elevation of the star that's not required when your position is already known. To find north you would instead be using it on it's side to determine the angle from the star (of known position). Not the easiest way to hold a sextant I admit, but in the UK the lack of a visible celestial object is frequently going to be a bigger problem.
     
  16. SqueamishOssifrage

    SqueamishOssifrage Well-Known Member

    A complicated way of doing it! You need your position, a second position, and then calculate the azimuth of the second position, in which case you have solved the problem anyway. Just compare the calculated azimuth with the compass bearing, and you're done. If you have a decent map, however, you will just need your position, probably in National Grid coordinates, a clearly identifiable landmark at some distance, and just a protractor and/or scale rule. No need for any astro-nav measurements at all. Actually, Google Maps coordinates are almost certainly good enough if a sufficiently distant second point is taken. Then it's just a quick bit of trig as I don't think Google Maps give bearings, and using a protractor on a 'phone screen is problematic, to say the least.

    Edited footnote: I have taught several yachtsmen the art of astro-nav over the years, and if ever the issue of measuring horizontal angles arises, I make the point that on the deck of yacht at sea it is sufficiently difficult and therefore unreliable that a decent fluxgate or hand bearing compass will give a much more dependable result.
     
    Last edited: Jul 21, 2021
  17. GeoffR

    GeoffR Well-Known Member

    Then I shall, partially, enlighten you. Deposits of Iron ore and other magnetic materials are what cause magnetic variation. Mappings the variation across an area is what allows map makers to put the magnetic variation on their maps. Global mapping of magnetic variation leads to Mag Var tables for aircraft navigation and allows the use of inertial compassing with no magnetic detection onboard*

    The fact that large metal objects, usually those make of steel, cause local variation and can be detected by a Magnetic Anomaly Detector. The long tail boom on the Nimrod for example. The piles that support the taxiway at London City airport have an undesirable effect too but inertially derived headings using MagVar tables can be unaffected. Moving objects that can affect a magnetic compass aren’t accounted for in the tables (obviously) so flying over a super tanker doesn’t affect the compass.

    *There is still a standby magnetic compass.
     
  18. Andrew Flannigan

    Andrew Flannigan Well-Known Member

  19. PeteRob

    PeteRob Well-Known Member

    Actually, what does it matter ? Anyone standing there and wanting to know the bearing (??) would suffer the same error.
     
    Terrywoodenpic likes this.
  20. EightBitTony

    EightBitTony Well-Known Member

    Also, the GPS in the 7d2 is (IME) terrible and I wouldn't trust it very much.
     

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