Navigation: How to Maintain an Accurate Direction Using a Compass

person holding compass facing towards green pine trees

Generally yes, but as is so often the case, other forces are at work to scupper your best efforts. There are a number of things that may affect accuracy when you navigate from one point to another using a compass; it is important to say at this point that using a compass is only one of many methods used to navigate and is rarely used on its own. However, on this occasion you will need to look at the compass as if it were our only method of navigation in order to fine tune the level of accuracy you would like to achieve.

Taking a Grid Bearing

Many people use the edge of their compass baseplate to join their own position with their next position (these positions are often referred to as rendezvous or R.V’s) to obtain a grid bearing. This needs to be done accurately as any error will be transferred when the required adjustment to a magnetic bearing is made.

To improve accuracy, try placing the transparent baseplate over the two points and use one of the parallel baseplate lines to join the two points; this will remove the possible error induced by thick transparent baseplates where the bottom edge is often difficult to see.

Making the Grid Magnetic Angle (G.M.A.) Adjustment

When making the adjustment that will change our grid bearing to a magnetic bearing, be as accurate as you can, bezel marking limitations accepted. Changes in the G.M.A. with time are normally calculated from the map on a yearly basis and are likely to be different when in other areas; be certain to work it out accurately for the time and area that you will be in.

This figure can be obtained by calculation from information normally contained in the legend before you go. If you are working with a map that has no grid overlay the adjustment will be from true north to magnetic north and is called magnetic declination. The information required for the calculation is likely to be in the same place.

The Position So Far

You have now adjusted your compass as accurately as is practical. Your next task is to ensure that you stay on your set bearing as accurately as possible and I’m sure it will come as no surprise to find that there are a number of ways accuracy can be diluted when using the compass.

Knocking the Bezel

Sometimes you can move the bezel inadvertently when handling the compass, this will obviously result in a navigational error. It’s a good idea to make a mental note of the bearing on the bezel and check this every time you pull out the compass.

Reading the Compass

Due to something called parallax error, reading the compass accurately involves sticking to a few basic rules. When you use a compass to indicate your desired direction, you align the needle so that it is inside the rotating module markers and then walk in the direction indicated by the direction of travel arrow on the baseplate. A problem arises because the needle is at a different height to the markings, this may make the needle look inside the markers when in fact it isn’t if viewed from any position other than directly overhead. This problem is easily rectified by always viewing the compass from directly above the centre of the module.

Magnetic Deviation

By viewing the compass module from directly overhead you bring the compass closer to your body. If you have items that can exert a magnetic influence in your pockets or on your belt it may pull the compass needle from its true position of magnetic north, resulting in a directional error; this is called magnetic deviation. Always keep magnetic items away from the compass when navigating.

Following the Direction of Travel Arrow

Trying to follow the direction of travel arrow when using a compass can be more difficult than many people appreciate. If you walk along looking at the compass all the time you will either fall over or incur large crosstrack errors.

The most accurate method is to stand still and view the compass in the correct way; look at something in the distance that is in line with the direction of travel arrow (a large mound, a rock, a tree or similar) and then walk to the object.

You can repeat this as many times as required, making allowances for changing visibility. It is difficult getting an accurate line from the direction of travel arrow to an object in the distance, that is why a sighting baseplate compass such as the Silva type 54 gives more accurate results. Visibility less than 50 meters requires a different approach that is not covered here.

Magnetic Inclination

This is also known as dip angle. A compass needle is usually balanced for the area of the world where it is to be used; this is determined by the angle that the magnetic flux enters the Earth. If you use a compass that is well out of its zone the needle may dip down and drag on the bottom of the module, this will of course restrict the movement of the needle and result in a navigational error. Some manufacturers produce compasses that work in all zones, so if you travel throughout the world go for one of these.

Navigation is such an involved subject that it is difficult to separate things like this away from connected items, often giving a distorted picture; look on this as no more than some useful tips that may help you in your quest to improve your navigational skills.

Leave a Reply

Your email address will not be published. Required fields are marked *