Gone Outdoors

How to Convert a Grid to True North

by Sean Lancaster

Navigation is vital for backpackers, hikers and long distance travelers. Anyone that needs to know where they are or where they are going needs to understand how to read a map and in particular, which way is North. There are three different types of North readings with which to contend. True North is the geographic point upon which the Earth rotates. It is located in the Arctic. Compasses don’t point to this location when they point towards the N on the dial. The needle of a compass points to the magnetic North Pole. The magnetic North Pole is a point near the True North Pole but has an extremely high concentration of magnetic rocks. All compasses point towards this location. Cartographers, people that make maps, draw maps based on longitude and latitude lines. All lines of longitude meet at the True North Pole. Due to the scale of local maps, it is not possible to see the curvature of the lines of longitude. Cartographers therefore draw linear lines that point North and South. These lines define a grid on the map. The direction indicator that points North on the map defines Grid North.

1. Draw a line on the map using a marker that will not obscure or block you from reading any part of the map. The margin of the map is a good location to add notes concerning the map.

2. Calculate the convergence (CONV) for the map you are using by entering the information required by the following formula, CONV = (CM – LONG) X SIN (LAT). The CM represents the central meridian from the map you are using. As an example, assume that you are located in the Netherlands in UTM-5 (Universal Transverse Mercator). Your CM is 5 degrees, the longitude would be is 4 degrees 55 minutes and the latitude is 52 degrees 23 minutes. Express all angles in decimal degrees. Do this by dividing the minutes by 60 and adding to the number of degrees. CONV = (5 – 4 55/60) X SIN (52 23/60) = (5 – 4.92) X SIN (52.38) = 0.08 X 0.79 = 0.063.

3. Convert the correction into degrees by multiplying CONV by 60. 0.063 X 60 = 3.8 degrees.

4. Insert calculated value of CONV into the formula, TN (True North) = GN (Grid North) + CONV. TN = GN + 3.8 degrees.

5. Rotate the coordinate system 3.8 degrees West of the Grid North. The directions are now all referenced to True North.

Items you will need
  • Longitude location
  • Latitude location
  • Central meridian of grid map
  • Map

About the Author

Sean Lancaster has been a freelance writer since 2007. He has written for Writers Research Group, Alexis Writing and the Lebanon Chamber of Commerce. Lancaster holds a Doctor of Philosophy in chemistry from the University of Washington.

Photo Credits

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