Full width home advertisement





Post Page Advertisement [Top]

After Map Reading, using a compass is the next most important skill in navigation. This series of articles explains the essential techniques for land navigation with a compass, from the basics of setting the map to advanced relocation techniques in poor visibility.

If you are unsure which compass to buy or confused about any of the features on your compass, please see our accompanying article on choosing the right compass for you, which explains the features of the compass in detail. A compass is not something you should economise on, a cheap compass will almost certainly let you down.

This introductory article covers the basic functions of the compass, how to handle and store the compass correctly and introduces the following key compass skills for beginners: Setting the Map and Determining Direction of Travel.

Figure 1 - The parts of the compass

Functions of the Compass

The compass has 3 basic functions for the land navigator:

1. To show the direction of Magnetic North. From this constant you can determine other directions.

2. As a Protractor, enabling you to calculate directions relative to Magnetic North. These directions, or Bearings, are expressed in degrees clockwise from Magnetic North. East is 90° clockwise from Magnetic North, South is 180° and West 270°.

3. To measure distances on the map. The baseplate of your compass should include a Metric Ruler (CM, MM) and a selection of Romer Scales, corresponding to 1:25k, 1:50k and 1:40k map scales. As well as measuring distances, romers also assist with calculating Grid References.

Ensuring Performance and Safe Storage

How you handle and store your compass affect its performance. The compass needle rests on a pivot in a liquid-filled gasket, enabling it to move smoothly, freely, and provided there is no interference, accurately. The needle rotates in the horizontal plane so although the pivot can overcome a slight tilt, be sure to hold your compass flat when taking a reading. Furthermore, the Earth's magnetic field is relatively weak, approximately 400 times weaker than a fridge magnet, so any nearby metallic or magnetic objects will attract the compass needle and cause inaccurate readings. Common causes of interference include mobile phones, jewellery, magnetic rucksack clips and metal gates. Storing your compass in a metal box or in a pocket with your mobile phone can cause permanent inaccuracies.

Magnetic North Vs Grid North: Magnetic Variation

Magnetic North (the North your compass needle points to) is slightly different to Grid North (North on your map). This is due to the need to produce consistent mapping despite the inconsistency of the Earth’s Magnetic Field, which causes the magnetic pole to move around. As a navigator, it’s important therefore to know and account for the degree of difference, the Magnetic Variation. Magnetic Variation at time of publishing is shown on all maps and The British Geological Survey maintain an online service which provides the magnetic variation at any given location (http://www.geomag.bgs.ac.uk/data_service/models_compass/gma_calc.html).

Magnetic Variation is expressed in degrees East or West of Grid North. At the time of writing, magnetic variation at our Peak District base, The Adventure Hub (located at National Grid Reference SK 205 826) is 1 degree West of Grid North. To account for magnetic variation we therefore must correct 1° East by adding 1° to any bearing we take from the map. If Magnetic Variation was 1° East of Grid we would correct 1° West, subtracting 1°.

For the purposes of Setting the Map and Determining Direction we can ignore Magnetic Variation until we cover Compass Bearings in article 2.

Setting the Map

Setting, or Orienting, the map means to have north on the map aligned with north on the compass. To do this, hold your compass flat in your hand, ensuring it’s away from any metallic or magnetic objects and let the red end of the needle settle on North. North is at the top of the map, you read most of the text on the map from West to East. Turn your map until North on the map aligns with the compass needle.

The map is now aligned with the landscape and you can begin to identify things you can see and anticipate the things you cannot see. In the example below, the navigator can see on the map the courses of the rivers obscured in reality.

Figure 2 - A Set Map. Features perfectly aligned with their representations on the map.

Determining Direction

Having a Set Map makes route-finding and decision-making significantly easier in the field. Checking your direction of travel against your compass you can determine your direction of travel at any time. But which way are you going to turn at the next junction? You have a clear path to follow, so you don’t need a compass bearing, but you want to be certain that you continue along the correct route. In this circumstance a Quick Bearing is your best option and does not require you to adjust the compass in any way.

Taking a Quick Bearing

1. Place your compass on your map and align the edge of your compass with the route you wish to follow, ensuring that the Direction of Travel Arrow on the compass baseplate matches the direction you intend to travel.

2. Now, keeping the map and compass as flat as you can, turn around on the spot until North on the compass aligns with North on the map. The Direction of Travel Arrow now points the way on.

Figure 3 - A Quick Bearing: Travelling North along the Bridleway West of Crook Hill, the quick bearing confirms the direction you need for the Bridleway to Crookhill Farm.

Setting the map with the compass is a building block of sound navigation; a quick bearing will help you travel fluidly and decisively. Take the time to familiarise yourself with the compass and practice these techniques, then look out for next month’s article covering Compass Bearings and Intermediate Strategies.

The Author

This article was written by Pure Outdoor Staff Member - Stephen

An experienced mountain skills coach, road cycling guide, caver, climber and fell runner, Stephen instructs navigation and mountain skills year round and has led countless mountain challenges and expeditions throughout the UK and Europe. Always cheerful in the face of adversity, and with a taste for the epic, Stephen achieved a long-held ambition by completing the Bob Graham Round in June 2015 - loving every minute of it!

No comments:

Post a Comment

Bottom Ad [Post Page]