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Thread: Declination

  1. #1
    Widdler
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    Hi Everyone

    i am after some advice on declination, i understand the grid to mag and mag to grid concepts but i am unsure as to how to adjust for Declination, as OS and Harvey's are very different any pointers would be welcomed

    Many Thanks

    Flip

  2. #2
    ▄bermensch
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    I'm not quite sure what it is that is puzzling you but here is a general explanation.

    True north is based on the north pole which forms the northern axis on which the earth spins. It is a fixed location and all OS map projections are based on it. Magnetic north is the the direction in which the compass needle points and is approximately 700 miles west of the north pole and moves about from year to year. Declination (also known as magnetic variation) is the difference, expressed in degrees, between true north and magnetic north.

    Declination varies according to where you are in the world. In Birmingham, which is approximately the centre of England, magnetic north is currently 2.5░ west of true north.

    Both the OS and Harveys use the same data for magnetic variation so there should be no difference between the information shown on maps of the same date. However, if you compare information about magnetic variation between current maps and those that are several years old, different figures will be quoted. This is because the magnetic pole us not in a fixed position but moves about as explained above.

    For accurate navigation, the magnetic compass has to be adjusted to take into account the difference between true north and magnetic north which is why the mnemonic 'add for mag; get rid for grid' is used.

    The more expensive orienteering compasses sometimes have an adjustable scale, known as a declinometer, which allows the navigator to dispense with making the simple mathematical calculation necessary when using simple compasses. The same result can be achieved by placing a strip of sticky paper on the underside of the compass at the appropriate declination (for Birmingham that would be at 358░ - which is about as accurate as you can get with an orienteering compass). Thus, when calcuating bearings etc you would work on 358░ rather than 360░

    There is another north shown on OS maps. Grid north is almost identical to true north and is indicated by the north-south grid lines on maps. Because a map is flat and the earth is an ellipsoid, there can be only one grid line on any individual map that is absolutely aligned to true north. However, the differences are so minute on maps used by walkers that they are ignored and won't affect your navigation skills.

    Hope that this helps!

  3. #3
    Widdler
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    thanks Hugh the fog has cleared :>

    Flip

  4. #4
    ▄bermensch
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    Flip:Part of my response was incolmplete.

    Both the OS and Harveys use the same data for magnetic variation so there should be no difference between the information shown on maps of the same date providing that they cover the same area. For example, the magnetic declination for Dover will not be the same as that for Land's End, even for maps of the same date. .

  5. #5
    ▄bermensch Mick w.'s Avatar
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    So, if we add/knock off 2.5 degrees, we'll be about right in the UK?

  6. #6
    ▄bermensch John Bailey's Avatar
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    However, the differences are so minute on maps used by walkers that they are ignored and won't affect your navigation skills.

    Hang on, Grid North varies from True North quite a bit. It's 3 degrees one way in Lowestoft and near to 2 degrees 40' the other way in Penzance, thats a total variance of 5 deg 40' across the country.

    The BGS report variation in declination (Grid vs Mag) of 8 degrees across the UK.

  7. #7
    ▄bermensch
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    John: I was referring to the difference between grid north and true north,

  8. #8
    ▄bermensch
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    Watch out when you are in other countries. In the Canadian Rockies the declination is over 25 degrees!

  9. #9
    Widdler
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    Some corrections to the above!

    True north is the pole. Grid north is the direction in which maps point - slightly different, and magnetic north is the direction in which compasses point.

    Magnetic declination is the difference between true north and magnetic north, with 'convergence' being the difference between true and grid north. Take convergence from declination and you have variation - the important one.

    The easy bit is that all OS maps have the important info at the top, and it's variation that's important to navigators.

  10. #10
    Super Moderator captain paranoia's Avatar
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    Not according to to OS, who say variation and declination are synonymous.

    Grid Magnetic Angle is the combination of declination and convergence, and is the value important to compass and Transverse Mercator projected map using navigators.

    http://www.ordnancesurvey.co.uk/oswe...rue-north.html

  11. #11
    Widdler
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    Terminology!

    There seems to be a little confusion over the correct terminology and how to use it. Normally, with general, basic navigation the exact terminology isn't that important; however, when progressing further into the subject you need to know what's what, so to speak and like most things if you learn it right first time it's done!

    Magnetic Variation and Magnetic Declination are exactly the same; however, to be pedantic the term Magnetic Variation is used on bathymetric maps (charts) and the term Magnetic Declination should be used on topographical maps. The reason for this is that the word 'Declination' has another meaning when using astro navigational methods, usually by mariners. Both are a measure of the angular displacement between True North and Magnetic North.

    True North is the ‘Geographical North Pole’ where all the Meridians of Longitude intersect at the top of the geoid (the Earth is slightly flattened at the poles and not a true Sphere, neither is it a Spheroid or Ellipsoid). It is the geoid which is defined as the best 'model' for our planet if we discount all elevations above sea level. In simple terms, if our planet was completely covered with water the resultant 'shape' would be the geoid. To be more specific, the three dimensional surface of a geoid has equal gravitational potential wherever it's measured. OS construct their maps on a Transverse Mercator Projection with an OSGB 1936 grid overlay in conjunction with an Airy Ellipsoid (they term it as a Spheroid on most maps) giving the best accuracy taking all other things into consideration.

    Spheroid and Ellipsoid. When the lengths of the two axis of an Ellipsoid are equal, it's termed a Spheroid; when the two axis of a Spheroid are unequal it's an Ellipsoid (Oblate or Prolate depending on which axis is the longer). The shape that may well represent the form of the Earth is an Oblate Spheroid (slightly flattened), that is now termed an Ellipsoid!

    Grid North/South is the direction of the vertical grid lines on our OS maps. Not all topographical maps have grid lines. On OS maps Grid North is exactly the same as True North at 002 degrees West of the prime meridian. At this point the vertical grid line at 002 degrees West aligns perfectly with a Meridian of Longitude that joins both geographic poles.

    Grid Convergence Angle is a measure of the angular displacement that forms between the vertical grid lines on the map and the Meridian of Longitude at a given point. At 002 degrees West GCA is 000 degrees.

    Grid Magnetic Angle is the bit that most of the navigation books including the Mountain Leader books (unless they've altered it!) refer to as Magnetic Variation. It's the GMA that you add or subtract when you're working with map and compass, not Magnetic Variation or Magnetic Declination to use the correct term. The GMA will be the same as the Magnetic Declination at any given point on the vertical grid line at 002 West.

    Magnetic Deviation is the amount the compass is being pulled-off by objects that have a magnetic influence, thus causing an error.

    Magnetic Inclination, sometimes called 'Dip Angle' is the angle at which the magnetic flux enters the Earth. This is the reason that you should buy a compass that suits your geographical zone. A needle that is balanced for another zone may tilt or dip, catching on the bottom of the liquid filled housing, rendering it almost useless. Quite a few compasses are now available with globally balanced needles that allow worldwide use.

    As for whether it's necessary to make the GMA adjustment depends on its magnitude, the weather conditions and the distance between your waypoints. As a general rule perhaps only about 5% of navigation is by compass, that said, if you do need to use the compass it's best to make your navigation as accurate as possible. Personally, I always make the allowance when using the compass (Silva Type 54). I feel that the old saying 'Perfection is the road, not the destination,' holds true here.

    We haven't touched on map datums and co-ordinate systems yet and how they fit in with GPS and WGS 1984!

    Hope all this helps and doesn't sound too much like a sermon. Navigation is an extremely interesting subject and by getting the basics right allows the rest to fall into place much more easily.
    Last edited by Overland Professional; 22-12-2016 at 03:31 PM.

  12. #12
    ▄bermensch Trevor DC Gamble's Avatar
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    Cheers for this info, as is always nice to see a nice proper bit of OM Forum zombie thread reanimation, in the proper tradition, you understand!
    Trevor DC Gamble

  13. #13
    ▄bermensch Trevor DC Gamble's Avatar
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    Cheers for this info, as I feel it is always nice to see a nice proper bit of OM Forum zombie thread reanimation, in the proper tradition, you understand!
    http://forums.outdoorsmagic.com/show...X5ldrdIbMls.97

    And now linked into the navigational/map reading aids thread for good meaasue here on OM Forum too as well, many thanks!
    Trevor DC Gamble

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