COMPASS READING: Your GPS has died and you are lost in the wild!
So our GPS batteries have died and we need desperately to know where we are. Fortunately we are careful hikers and we have packed a 1:20 000 map, a pencil and a standard hiking Polaris compass in our kit. We do know the names of two distant peaks (Call them Peak 1 and Peak 2. How will we go about “triangulating” our position?
We haul out our Map, pencil and compass. Find a relatively flat surface for our map.
Study the compass above and note the names of the components
- Point our compass at the first peak (Peak 1). When we do this we make sure that we aim the Direction of Travel Arrow directly at Peak 1.
- We turn the “N” on our Rotating Housing (with the Degree Dial) until the Orienting Arrow is exactly below the Red painted end of the Magnetic Needle. (ie. The magnetic needle fits perfectly into the Orienting Arrow).
- We can now stop pointing the compass at the peak because the angle is fixed on our compass.
- Read the angle between the “N” on the Rotating Housing and the Direction of Travel Arrow. (e.g 109°) This is the angle between the red Magnetic Needle and the Direction of Travel Arrow. (Note: Degrees are always read clockwise starting at “N” if no direction is indicated (e.g. East or West).
- Now adjust the angle to take into account the MAGNETIC DECLINATION of our location.
- NB: IMPORTANT NOTE ON DECLINATION: Remember that the compass needle does not point at the “True” North Pole. It points at the Magnetic North Pole which slowly migrates across the globe in the vicinity of the North Pole. As of May 2018 it is situated off the North Pole at 86°N/173°W). In Cape Town the Declination is 25° West (-25°). In Mumbai, India, the Declination is 0°, i.e. True North = Magnetic North!!. In Sydney Australia the Declination is 12° East (+12°).
- In our actual example we are have an angle of 109° between Magnetic North and Peak 1. To determine “True North” we must subtract 25° from 109°. This gives us an angle of 84° between True North and Peak 1.
- Now rotate the Rotating Housing to the right so that the angle between True North and the Direction Housing is 84° (East of North).
- Now we can place our compass on our Map and line up the meridian lines with the Grid on the Map. Thus True North is now lined up with the Grid lines on the map and the Orienting Arrow is pointing at 84° East. Once we have done this, we carefully slide the compass on the map until the one edge of the compass is exactly on Peak 1. Draw a pencil line towards where we are located.
- REPEAT for Peak 2 – e.g. Let us say that Peak 2 is located at 73° East of us. Now we must ADD 25° because Peak 2 is West of us. Thus 73° + 25° = 98° East,
- Place the Polaris compass on the Map again and line up the meridian lines with the Grid on the Map. Carefully shift the compass until the one edge of the compass is exactly on Peak 2. Draw a pencil line towards where we are located.
- Where these two lines intersect is our location (within a few feet / meters).
- Any mistakes are my own – Let’s hope there are none!