Every part of a compass identified

It is very difficult to find out what all the various parts of a compass are for, so I thought it would be useful to list them all here.

There are many, many different types of compasses, each designed for a specific purpose.

For example

  1. Canoe/kayak compass
  2. Yacht seafaring compass
  3. Military sighting compass
  4. Pocket compass
  5. Orienteering compass
  6. Diver’s underwater compass

 

On our courses we use what is known as a Base Plate Compass – as everything is attached to, printed on or stamped into a clear acrylic plastic plate.

The compass shown is the Expedition 4 made by the Silva Company.  We use them as we feel they are an ideal training tool.

How does a compass work?

In the centre dial of a compass is a small magnetised metallic needle which rotates on a low friction bearing.

As you know the Earth has a magnetic field with North and South Poles. 

You will also know that opposite poles attract, so one end of the needle will be attracted the Earth’s North Pole and the other to the South Pole. 

The end of the needle, which points towards the North Pole of the Earth, will normally be coloured red and/or marked N, to signify it points North.

If the compass is turned the needle will rotate until it again points toward the Magnetic North Pole.

Parts of a compass.

Not all compasses will have all of these features.

Do you “really” need all these features?

The simple answer is no, as it’s very easy to navigate just using the information on a map. 

So no, you don’t absolutely need all the features, but they make navigating much simpler.

1 - Direction arrow.

Once the compass bearing has been set the arrow points in the direction of travel.

2 - Luminous pointer.

This is used during night navigation instead of the direction arrow.

3 - Romer scales (1:25k, 1:50k, 1:40k).

These are used to assist in taking accurate grid references and to calculate distances from maps.

4 - Rotating dial with degrees marked.

When the dial is rotated the Compass Bearing is the number (of degrees) at the top of the dial.

5 - Declination scale.

The declination is the difference between Map North and Magnetic North and is shown on all OS maps.  The scales are used to adjust the bearing either from a Map or compass.  Depending on your location the angle of declination (the angle formed by the lines pointing at Map North and Magnetic North) will change.  For some areas, such as in the UK during 2019/20, the angle of declination will be so small it can normally be ignored.  In other locations the angle may be large and so an adjustment (using the Declination Scales) is vital.

6 - Orienting lines.

he are used primarily to align the dial to the Northings (the blue vertical lines on a map) when setting a Map Bearing on to the compass.

7 - Ruler (imperial inches).

We're not really sure how this would be useful with modern maps (which are mostly metric) but it's there if needed.

8 - Risers.

In this case these are the three small round items placed around the dial (they may be in different positions on other compasses), they play no part in the operation of the compass. Built in to the underside of the Base Plate they prevent the plastic being scratched when placed on to a flat surface.

9 - Lanyard attaching point.

This provides a strong fixing point to attach a lanyard in a place which will not impede the use of the compass.

10 - Ruler (metric CM).

Useful for measuring longer distances from a map.

11 - Magnetic needle.

One end is (normally) painted red and points at the Magnetic North.  If the whole compass is rotated the needle will move until it is again pointing at Magnetic North.  The needle rotates around an (almost) frictionless bearing.  Note that to allow the needle to rotate freely the compass should be held reasonably level.  The needle is housed inside the dial, which is filled (in good quality compasses) with an antistatic fluid.

12 - Orienting arrow.

This is marked on the base of the inside of the dial.  As it is inside the dial, it will rotate as the dial is rotated. It is used to orientate the dial to the map and for following a Magnetic Compass Bearing when the Magnetic Needle is directly over the Orientating Arrow.

13 - Index Mark / Line

A stationary line placed under the rotating section of the dial, this is an extension  of the Direction Arrow. It marks the point at which a compass bearing is read.

14 - Hole with crosshairs.

Some compasses have circular or triangular holes, or both. The hole can be used to draw around (with a pen or pencil) a location on a map.  For example if you were planning a walk you could draw a circle around the start and end points and then add your intended route. The crosshairs simply ensure the circle is accurately placed.

15 - Magnifier.

Most maps have many "very" small details printed on them, the magnifier make reading these much easier.  It is also very useful when map reading at night.

16 - Parallel Line(s).

Most (good quality) compasses have these lines stamped (not just printed) onto the bottom of the base plate running parallel to the edge of the compass. As they are on the bottom of the base plate they can be placed directly on to map features. Their lower position means that they are not viewed at an angle (between the line and a map feature) so a map reader can be in any position and still take an accurate bearing. Note - normally the edge of the compass is used to take bearings, instead of the lines.

17 - Label holder.

Some (a very few) professional, expedition and military compasses use a decaying isotope, such as Tritium, to illuminate the dials, numbers or luminous pointer.  In this case  a Radioactive warning sticker will be placed here.

 Notes if you are considering buying a compass.

The base plate.

  • In this case bigger is better and easier to use.  Plus you can take map bearings from points further apart.
  • Ensure it is very clear and not frosted so you can see your map through it.


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Look at the numbers (degrees) on the dial.

  • They should be stamped, NOT printed (like on many cheap compasses), as they will rub off very quickly.


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Look inside the dial.

  • Does it have Orienting Lines?  Lots of cheap compasses don’t.
  • Are the Orienting Lines clear, straight and regular.


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Look at the bottom of the base plate

  • Are the parallel lines stamped into the plastic or are they simply printed, in which case they will rub off very quickly.


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If buying on EBay or other online sites, be careful.

Most people will want a normal map reading compass so CHECK that the compass dial uses (civilian) degrees and not (military) mils.  This is an all too prevalent ruse and, in some cases, it can be difficult to get a refund.