Surely North is North? Why do we need to complicate it? Hopefully,the following explanation will clear things up a little. On many large scale maps with grid overlays you will notice that there are three types of North listed; let’s take a look at what they are and why we need to know.
If you travel in the direction of ‘True North’ from a position anywhere on the planet you will eventually arrive at the Geographical North Pole; this is where all the meridians of longitude intersect at the top of the Earth. When dealing with Latitude and Longitude ‘True North’ is used in conjunction with ‘Magnetic North’; when dealing with grids ‘Grid North’ is used in conjunction with ‘Magnetic North’.
If you are working on a map that has a grid overlay things get a little more complicated. A ‘grid’ is a way of easily locating a position with a series of numbers and/or letters relating to uniform squares; it is 2-dimensional and this is where the problems begin. If you can imagine looking at the Earth through an enormous grid you will see that because the grid is not wrapped around the Earth that only one of the vertical lines can go through both poles; on large scale maps we can take these anomalies into account with the information supplied in the maps legend.
This difference between ‘Grid North’ and ‘True North’ is called the Grid Convergence Angle and is normally expressed as a quantity at the sheet corners. If you travel in the direction of ‘Grid North’ you may go directly to the Geographical North Pole or you may not. Let me explain. When any North/South grid line aligns perfectly with a meridian of longitude True North and Grid North are the same but only on this grid line; the grid lines to either side will not take you to the Geographical North Pole, one will take you to the East and the other to the West. Maps that have a Latitude/Longitude system cannot strictly be described as a ‘Grid System’ in the true sense as the lines of Latitude/Longitude follow the Earth’s surface and as such they depict a 3-dimensional curved surface on our 2-dimensional map. This makes the shapes of the ‘Grid’ irregular and difficult to work with using a base-plate compass.
Magnetic North is the direction that the needle inside our compass swings to. Its location is in the North Eastern part of Canada and its position changes by a small amount each year; it can be calculated by using the information contained within the map’s legend. When we navigate with a magnetic compass we have to adjust the readings to coincide with our map.
Orientating Your Map Using the Compass
To orientate your map using the compass first set the compass to the G.M.A. (Grid Magnetic Angle); this is the difference between Grid North and Magnetic North. Now align the edge of the base-plate so that it runs along any North/South grid line with the ‘Direction of Travel’ arrow on the base-platepointing to the top of the map. Now, turn the map and compass together until the red end of the compass needle lines up with the ‘Magnetic North’ marker within the compass module. The map is now orientated.
Taking a Grid Bearing from the Map and Changing it to a Magnetic Bearing to Walk On
Firstly, there is no need to orientate the map when taking a grid bearing from a map. Place the compass on the map so that one of the long edges of the base-plate (or one of the parallel lines) passes through your present position and your destination with the ‘Direction of Travel’ arrow pointing in the direction of your destination. Now turn the compass module so that the lines are running parallel with the North/South grid lines on the map and the ‘Magnetic North’ marker within the compass module is pointing to the top of the map (Grid North). You now need to adjust the grid bearing by an amount equal to the G.M.A.; this will turn the grid bearing into a magnetic bearing to walk on. Hold the compass level and at waist height so that the ‘Direction of Travel’ arrow is directly in front of you, turn your body and compass until the red end of the compass needle aligns with the ‘Magnetic North’ marker within the compass module. The ‘Direction of Travel’ arrow now points to your destination.
Finding Your Position Using Your Map & Compass
If you have lost your way and are unsure of your location, providing you have a reasonable amount of visibility you can find your position by a method called a ‘Resection’. First, find a prominent landmark that you can see but is not far enough away as to be off your map; say a large lake, mountain or telecommunications tower. point the ‘Direction of Travel’ arrow at your landmark and turn the compass module until the ‘Magnetic North’ marker is under the red end of the compass needle.
Before you can transfer your magnetic bearing to the map it will need to be adjusted allowing for the G.M.A. When this has been done the next step is to find your chosen landmark on the map, then place the corner of the base-plate over the landmark with the ‘Direction of Travel’ arrow also pointing towards it. Now, slowly pivot the whole compass around the landmark until the lines in the base of the compass module align with the North/South grid lines on the map and the ‘Magnetic North’ marker within the compass module is pointing to ‘Grid North’. Draw a line from the center of your landmark in the opposite direction to the ‘Direction of Travel’ arrow; you are somewhere along this line! You will need to complete this three times in all at approximately 120 degrees to each other. The three lines that you will produce should intersect each other and give your position; however, this rarely happens and the lines will normally end up forming a small triangle. This triangle is referred to as a ‘Cocked Hat’; it is regarded by many that you will be inside this cocked hat but in actual fact the calculations don’t show this to be the case. Nevertheless, it will give you a good enough position to be able to navigate to a specific point to use as an accurate reference to continue. Remember to keep your compass away from anything magnetic that may affect its precision.
Below are some terms that you will come across when navigating.
Grid Magnetic Angle = The angle between Grid North and Magnetic North.
Magnetic Declination = The angle between True North and Magnetic North (referred to as Magnetic Variation).
Grid Convergence Angle = The angle between Grid North and True North.
Magnetic Inclination= The angle that the magnetic flux enters the Earth’s surface.
Magnetic Deviation = The angle that the compass needle deviates from true position due to magnetic influences other than that of the Earth’s.
You now have enough technical information to assist you in getting lost in more remote and interesting places!