This guide is designed for non-military personnel, as the basics of land navigation should be familiar to anyone in uniform (though if you have a commission, perhaps not so much).
Imagine the following scenario – you’re treating a casualty in a remote rainforest – the initial patient management has been attended to, and you’re moving into a prolonged field care phase. You want to evacuate the casualty before nightfall. You’re about to use your mountain radio so that a winch retrieval can be organised, but you quickly realise the helicopter will have a hard time finding you with such a thick canopy overhead. You know what valley you’re in, but not your exact grid reference.
In today's day and age we often rely on GPS, or simply activate a PLB, but in the absence of a (working) device it's useful to know how to manually calculate and communicate where you are - the old school way. Remember - mission-critical items can get forgotten, devices and batteries can fail, and belongings can get lost off the edge of a cliff, buried in snow, or float down the river.
Well - you have a 1:50,000 map, and you think you're somewhere here:
The problem is, even this rough knowledge of where you are only narrows the search down to about three square kilometres - every millimetre on this map represents 50m.
This is where your ability to perform a resection comes in.
Step 1. Identify 2-3 prominent features visible to you on that ground that you can identify on the map.
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You can see the shape of Mt Madeline’s snow capped peak in one direction, Prospector Peak of the Bryneira Range on the opposite side of the valley, and in front of you is an unnamed hill - spot height 400m. I've marked each of these with a red dot on the map above.
Step 2. Using a compass take the magnetic bearing (MB) of each (you may see this referred to elsewhere as an "azimuth" - though these terms do mean slightly different things). Also, depending on your compass and whether you're wearing camouflage pants or not this bearing will either be in degrees (°) or milliradians (mils) - the latter is more accurate but is often only found on military compasses. We will use degrees.
Taking a bearing is a skill in and of itself. Holding your compass flat, 1) point the direction of travel arrow directly at the landmark you're wanting to take a bearing to, 2) rotate the bezel until the "N" for North on the bezel meets up with the tip of the magnetised red compass needle, and 3) the MB is the number on the bezel that lines up with the index line. Repeat this for all three landmarks and note down the MB for each. In the example below (sourced from Google Images with no permission whatsoever) the magnetic bearing is 40°.
You shoot your magnetic bearings to the three landmarks and find that:
The MB to Mt Madeline is 219°.
The MB to Prospector Peak is 85°.
The MB to Spot Height 400 is 325°.
Step 3. Convert your magnetic bearing (MB) to a grid bearing (GB) by adjusting for magnetic variation / declination.
At the bottom left corner of your topographic 1:50,000 map is the following symbol:
This tells you two pieces of information - one, what the magnetic variation was at the time the map was made, but because the magnetic pole is slowly moving, it also allows you to calculate what it will be right now.
The magnetic variation (MV, also known as "declination") for this region was 20° in 2017 and is expected to increase by 0.5° over 14 years - this yearly addition is negligible, unless you're using a very old map or working in milliradians, where these numbers can add up.
What you do with your MV will depend on whether the MV is east (a positive number) or west (a negative number). This generates confusion, because converting a MB to a GB always involves ADDING the MV, but sometimes the MV is a negative number - and adding a negative number is ... subtraction. For this map, we add a positive MV =
GB = MB + MV
The MB to Mt Madeline was 219° so the GB will be 239°.
The MB to Prospector Peak was 85° so the GB will be 105°.
The MB to Spot Height 400 was 325° so the GB will be 345°.
Step 4. Convert your grid bearing (GB) into a grid back-bearing (GBB) - this will be 180° off.
If the GB is <180° then add 180°, and if >180° then subtract 180°. In our example:
The GB to Mt Madeline was 239° so the GBB will be 59°.
The GB to Prospector Peak was 105° so the GBB will be 285°.
The GB to Spot Height 400 was 345° so the GBB will be 165°.
Step 5. Using the protractor you keep in your navigation kit (!) (or, the decidedly second best option, using a phone app that you previously downloaded), draw each GBB as a line from each of the prominent features identified in Step 1. If done correctly, these three lines should intersect to form a triangle, or better yet a single point. The smaller the triangle, the more accurate you will be. You have now triangulated your grid reference (GR).
In this case your six-figure grid reference is GR 117507. When done correctly and carefully, this is accurate to within 100m, markedly reducing the time it will take to be identified under the canopy by SAR, and improve the chance of successfully evacuating your casualty by nightfall.