Navigation Skills 7 The Anatomy of the Compass
By Dave Roberts
on October 8, 2014 5/5 (1)
Navigation Skills 7 The Anatomy of the Compass
What Compass? You’ll now need to buy a compass. We recommend that you get yourself kitted out this week ready for the next sections.You’ll also need 5 toggles, a map case and if you can’t store it in the map case then get a compass case. The compass does remain in place better on a map case than on the surface of a waterproof map as they’ll have little rubber feet that stick to the surface, and it’s easier to navigate and handle the map when it’s in a case.
While there are a number of different types of compass, we’ll be concentrating on the Baseplate, Orienteering or Protractor variety usually seen in the UK hills. These have a magnetised needle housed within a liquid filled compact housing in which the needle moves. The liquid serves to dampen the movement, and makes it more accurate. The other types you may come across are your marching / prismatic and sighting compass, each with their own particular application but we’ll be sticking firmly to the protractor type.
If you take a look around your outdoor store you’ll spot that there are two main varieties of protractor compass (the hybrid sighting variety not withstanding) – the long and the short variety, with some that fall in between. The difference that comes with this sizing is that you can take a longer bearing (distance wise) on the map with the longer baseplate. For serious nav, go for the longer baseplate, but if it’s a choice between a known brand short baseplate and a cheap brand long baseplate, always go for the brand name to ensure that the housing is as reliable as possible. Want to see the real difference between a cheap and ‘proper’ brand compass? Read our cheap v expensive compass article.
If you intend on using the compass globally, ensure that the depth of the housing is adequate as deeper housing allows more room for the needle to tilt in global compasses. Also ensure that it’s in degrees and not military mils (unless that’s what you’re used to of course!)
Your compass should now look something like the one below:
- Compass Housing and Needle -The north finding end is usually red and may also be luminous. The needle is a magnetised bit of metal, so any strong magnets will cause it to fail. Don’t store them on speakers!
- Rotating Bezel – With graduated scale- usually accurate to 2 degrees and with an annoying propensity for the numbers to wear off (why compass cases are essential). The example is in Degrees, but you can also get them in mils – though you should probably get the former if you’re not already proficient in using one in mils.
- Declination Scale – for noting the magnetic declination or variation (See Different sort of norths later)
- Index Line / Pointer – point at which you actually read the bearing. This compass is luminous so you can even read the value in the dark.
- Direction of Travel (DOT) Arrow – basically the direction of travel,
- Orienting lines – useful to set the map and the compass to align with the map grid.
- Orienting Arrow – get the needle sat in here (the north seeking end!) Will point at the Index Line and the Direction of Travel (DOT) Arroe
- Baseplate – often contains the following:
- Magnifying glass – for determining those tiny features or starting fires.
- Romer Scales – help determine the grid reference
- Scale / Ruler – for measuring distance
- Lanyard – Sometimes mistakenly called a neck cord, though like map cases, you shouldn’t be walking about with it around your neck.
- Toggles – for keeping track of pacing (later) and attached firmly to your lanyard.
Naming them’s one thing – we’ll show you how to make practical use of them in the coming weeks.
The Different sort of Norths
While the needle points north, just to complicate things, there are three norths. True north is the direction to the North Pole, while Magnetic pole points to the Magnetic Pole which moves about and is currently just north of Ellesmere Island. The grid lines on your map, conveniently, correspond to neither and is known as Grid North. Thankfully, forget I mentioned True North as you’ll only need to be aware of Magnetic and Grid and how this affects your compass use as it’s how you relate these two to each other that’s important.
This difference between grid and magnetic north is known as magnetic variation, and just to confuse us even further changes not only from one location in the UK to another, but keeps on moving from one year to the next. The difference between the Grid North and Magnetic North is known as Magnetic Declination or Variation.
This magnetic variation is however noted on your map. So you’ll need to do some maths along the lines of: Magnetic North 2° East in 2010, moving east at 1° a year – so in 2014 it would be 7° east. Better still, the British Geological Survey allow you to determine it for your map at their Grid Magnetic Angle Calculator.
Using this – for Pen y Gwryd it’s currently :
Magnetic north is estimated to be 1º 4 ‘ west of grid north (British National Grid) at this location in July 2014
This means that a bearing of 11° on your compass (magnetic north) you need to take away the magnetic variation – so 10° in this instance to get to grid north
If you’ve been given a bearing to follow of 11° then you’d need to go from Grid to Magnetic and add it – so you’d set your compass for 12°.
Basically that means you can ignore it for all intents and purposes in North Snowdonia at present, though you can try to adjust your compass by 1° – good luck with that when the compass is only accurate to 2º.
For Grid to Mag – ADD, For Mag to Grid – Get RID
This rule of thumb is true so long as Magnetic North is to the WEST which currently holds true for most of us, but SW of Penzance it has now moved to the east for the first time in centuries with the rest of the UK due to follow suit in the next 10 years or so as it moves northwards at a rate of 60km a year (magnetic north on the move again). Realistically, it’ll be a few years after that before the variation becomes enough for you to be able to practically adjust your bezel anyway.
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siDave Roberts founded Walk Eryri in 2004, with the aim of providing routes that are off the beaten track. Walk Eryri is now part of Mud and Routes which continues to provide more off beat routes and walks in Snowdonia and beyond. Dave has been exploring the hills of Eryri for over thirty years, and is a qualified Mountain Leader. Dave also established Walk up Snowdon, Walk up Scafell Pike and Walk up Ben Nevis just to mention a few.