Using a compass with a map

These basic instructions are meant for use with a good quality map and a Baseplate compass, otherwise known as an Orienteering compass.

The Baseplate compass consists of a rectangular base made of a transparent material, usually plastic or perspex. Marked on the Baseplate is a direction of travel arrow and possibly other markings, such as a ruler or Romer scales.

Mounted on top of this is a rotating dial, marked with degrees, containing a magnetic needle suspended in a transparent fluid, like oil or alcohol. The fluid dampens the movement of the needle so that it settles quickly and makes it easier to take a reading. The dial also has marked on it, underneath the needle, an orienteering arrow which rotates with the dial. There is also a bearing line, which does not move with the dial, but remains in line with the direction of travel arrow.

Travelling along a given bearing

With the direction of travel arrow facing away from you, set the dial so that the correct number of degrees is on the bearing line. Make the correction from magnetic North to grid North using the legend on a map of the area you are in.

Now rotate the baseplate, or better hold the baseplate still and turn yourself so that the magnetic needle is aligned with the orienteering arrow on the dial. The direction of travel arrow now points along the correct bearing.

Travelling between two points on a map

Assuming that you know where your position is and also where your destination is on your map:

Lay the compass on the map with the direction of travel arrow pointing in a direction parallel to that which you wish to take. Use one of the long sides of the baseplate along your actual intended direction to facilitate this.

Now rotate the dial so that the orienteering arrow is in line with the grid lines on the map and pointing North on the map. Check the map legend and adjust the dial so that the orienteering arrow points to magnetic North.

The dial is now set to the bearing along which you need to travel, so, remove the map and turn the baseplate so that the magnetic needle is aligned with the orienteering arrow. The best way to do this is to hold the baseplate against your chest with the direction of travel arrow pointing away from you and turning yourself until the needle is aligned with the orienteering arrow. The direction of travel arrow now points in the direction of travel.

It's a good idea to practice this a couple of times with clearly visible features which are also marked on your map such as church spires, bridges or tv masts or something.

Determining your position on a map

First turn the dial so that the orienteering arrow is pointing the same way as the direction of travel arrow, (so that 360 degrees or"N" is on the bearing line). Place the compass on your map so that the grid lines of the map are aligned with the orienteering arrow on the dial. Now carefully turn the map with the compass so that the magnetic needle is aligned with the orienteering arrow. (Difficult on a windy day with no decent place to lay your map, but the key is to understand the principle.) The grid lines on your map are now aligned with magnetic north. (Make sure the map is not pointing South!) Keep the map in this position and pick up your compass.

Choose a visible landmark which you can identify on your map, like a mountain or a headland. Point the direction of travel arrow towards your chosen landmark, hold it steady and turn the dial so that the 360 degree point or the "N" is aligned with the red point of the magnetic needle. You can relax now, but don't turn the dial.

The dial is now set to the bearing of the landmark, you need to correct this from magnetic North to grid North using the legend on the map.

Place one of the top corners of the baseplate exactly on the position of the landmark and turn the baseplate so that the orienteering arrow is aligned with the grid lines on the map. Draw a line along the side of the baseplate, you are somewhere on this line.

Don't move the map yet, repeat the process for another landmark and the point where the lines intersect is your poistion. Try to choose a landmark about 90 degrees away from the first one, this will improve the accuracy of the procedure.