Compass Features

Compass Features


Know your way around your compass, and what to look for when buying one


Compass Key Features



Here we look at the key features of an outdoor compass and explain what they do.


      


Features Explained

Feature

Explanation

Base plate

Solid and flat plastic mounting on which the main compass housings is affixed

Housing or capsule

This is enclosure in which the needle resides, and is filled with liquid (believed to be light oil) which aids free movement of the needle

Needle

The magnetized part of the compass; the bit that points north

Bezel

Rotating housing of compass, which allows orienting lines to be matched to map, sighted bearing to be taken and magnetic variation added or subtracted

Direction of travel arrow(s)

Marked on the base plate, this arrow must always point in the direction in which you are, or intend to travel and can ensure that you don’t travel in exactly the opposite direction

Roamer scales

Marks on the base plate that are graduated for different scales of map.

Orienting arrow

It is inside the orienting arrow that you must position the red pointer (with the exception of back bearings, where you’ll use the white arrow)

Orienting lines

Drawn onto the base of the compass housing, these lines are the important interface between the compass reading and your map, and are aligned to vertical grid lines of your map

Degree marks

Shown on the outside of the bezel, these marks denote degrees and are usually shown for every other degree, i.e. even numbers

Index pointer/mark

This is the point from which you read the number of degrees from the compass bezel

Magnifying glass

A handy feature to enable closer inspection of complicated map features


Additional Features – not shown


Mirror

Some compasses have fold-down mirrors attached to the top of them, which allows the user to more accurately ‘sight’ on a distant object, whilst being able to place the needle inside the orienting arrow.



Differentiating Features

The following compass features are the ones that differentiate the various compass models when buying, or reviewing whether your compass is still up to the job.

Feature

Explanation

Base plate length

A long base plate is ideal when reading bearings to/from a map or where the map is a larger scale, i.e. 1:25,000. This is because vertical grid lines are twice as far apart as 1:50,000 and you always need to be able to align the orienting arrows to vertical grid lines. Short base plate compasses can make such alignment impossible or prone to inaccuracies creeping in

Roamer scales

Make sure that the roamer scales are graduated for the maps that you’ll be using, most likely 1:25,000 or 1:50,000 in the UK

Bezel

Must be easy to turn, else you’ll move the base plate at the same time, which will lead to inaccurate bearings if un-noticed. Picture yourself on the top of a cold windy mountain which is shrouded in cloud, and you are wearing gloves. You need to take an accurate bearing quickly, so a free moving bezel is important. If your compass is old and starting to stick, buy a new one.

Rubber feet

The compass must not move when you line-up the orienting lines. Some map cases are more slippery than others, so the rubber feet go some way to preventing slippage. Don’t under estimate this, it can be a real issue when you least need the problem. Consider buying a new compass if yours doesn't have rubber feet.

Luminous marks

There’s always the possibility that you will need to navigate in the dark, whether by intention or circumstances. Having luminous dots on the needle, index mark/pointer and at the edge of the orienting arrow will mean that you can follow a bearing without the need for a torch.


Video

The following video is a short punchy introduction to compasses, and introduced by a Search & Rescue chap from the USA.




This video produced by Ordnance Survey, which demonstrates some of the items listed above.



Note: At the end of the video the presenter speaks of 'declination', and he is incorrect to use this term as described in the Magnetic Variation & Declination section. He should be referring to variation, as he's in the UK. Other than the error in terminology, it's a great video.


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