Three norths
How to take a bearing from a map
Short answer
Line the compass edge from where you are to where you are going, rotate the housing until its grid lines match the map's north lines, read the bearing, then add your local declination to walk it.
This is the one procedure on the site where all three norths appear at once, and where confusing them has real consequences. A map has grid north, the world has true north, and your compass has magnetic north, and they are three different directions.
The good news is that the conversions are simple addition. The bad news is that getting the sign wrong doubles the error rather than removing it, which is how people end up twice as far off as if they had not corrected at all.
Method
Step by step
In this order. Each step assumes the one before it is done.
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01
Measure the grid bearing
Put the long edge of the baseplate along your intended line of travel, with the direction-of-travel arrow pointing where you want to go. Ignore the needle completely at this stage. Rotate the compass housing until the orienting lines inside it are parallel with the map's north-south grid lines, north to north. Read the number at the index. That is your grid bearing.
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02
Convert grid to true, if the map is gridded
On a national grid the difference between grid north and true north is grid convergence, and it is usually under two degrees. Most walkers ignore it, correctly. It matters if you are working at long range or near the edge of a grid zone.
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03
Convert true to magnetic
Add west declination, subtract east declination, to go from a true bearing to the magnetic bearing you will actually steer. The old mnemonic is "east is least, west is best". Get the current figure for your area, because declination changes measurably over a decade and a map printed twenty years ago is out of date.
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04
Walk it without drifting
Hold the compass level in front of you, turn your whole body until the needle sits in the orienting arrow, and look up. Pick a feature on that line, a tree or a rock, walk to it, and only then take the next sighting. Watching the needle while you walk is how people describe long shallow curves and arrive somewhere else.
How wrong can this be?
Accuracy
A careful baseplate measurement is good to about two degrees, and walking on features rather than on the needle keeps that. Two degrees over a kilometre is about thirty-five metres of lateral error, which is worth knowing before you rely on hitting something small.
Related
Other guides
The basic one
Which way is north?
If you are on a phone, press Start compass and turn until the bearing reads 000. If you are not, and the sun is up, stand with the sun on your right in the morning and you are facing roughly north.
No instrument
Without a compass
Put a stick upright in the ground, mark the tip of its shadow, wait fifteen minutes and mark it again. The line from the first mark to the second points roughly east.
Fixed installation
Aim a solar panel
In the northern hemisphere, face a fixed array due true south, 180 degrees. In the southern hemisphere, due true north, 000. Tilt it at roughly your latitude.