A compass does one thing: it points at magnetic north. Everything useful we get from it, a direction to walk, a way to identify a peak, a line to hold in fog, comes from combining that single fact with a map and a little discipline. This article walks through the full procedure for a baseplate compass, from map to ground and back, and then untangles declination, the small angle that has sent more than a few careful walkers up the wrong valley.
The parts of a baseplate compass
The transparent baseplate carries a direction-of-travel arrow and usually a ruler or romer scales for measuring map distance. On it sits a rotating bezel graduated in degrees, 0 to 360, with an orienting arrow and a set of parallel orienting lines printed on its floor. Inside the housing the magnetic needle floats, red end north. Better models add a mirror for sighting and an adjustable declination screw; neither is essential, but both reduce error.
Two habits protect the instrument. Keep it away from phones, watches, metal poles and car bodies when reading it, because any of them can pull the needle by tens of degrees. And check it occasionally against a known bearing at home, since a compass stored next to a magnet can be reversed and will then point confidently at south.
Taking a bearing from the map
Suppose we are at a stream junction and want to reach a col that we cannot see. Lay the map flat. Place one long edge of the baseplate along the line from where we are to where we want to go, making sure the direction-of-travel arrow points towards the destination and not back at us. Ignore the needle entirely for this step. Now rotate the bezel until its orienting lines run parallel to the map’s north-south grid lines, with the orienting arrow pointing to the top of the map. Read the number on the bezel at the index mark. That figure is the grid bearing, the direction from A to B measured relative to grid north.
The most common mistake at this stage is setting the orienting arrow to the bottom of the map, which gives a bearing 180 degrees out. The second is letting the baseplate slide while turning the bezel. Pressing the map and baseplate down with one thumb while the other hand turns the ring solves both.
Walking on the bearing
Lift the compass off the map and hold it flat at chest height, direction-of-travel arrow pointing straight away from the body. Turn the whole body, not the compass, until the red end of the needle sits inside the orienting arrow; the phrase “red in the shed” is the memory aid. The direction-of-travel arrow now points along the bearing. Look up, pick an object on that line as far away as visibility allows, a boulder, a lone tree, a patch of pale grass, and walk to it without looking at the compass. Arrive, repeat.
Walking to intermediate objects is what makes a bearing accurate. Staring at the needle while moving produces a wandering line, because the needle swings with every step. In thick fog where nothing stands out, a companion can walk ahead to the limit of sight and be waved left or right onto the line, becoming the object. Either way, we expect a real error of around 3 to 5 degrees over a leg, which works out to roughly 50 to 90 m of sideways drift per kilometre (about 260 to 460 ft per mile). That number decides how far we can sensibly walk on one bearing before we need a feature to correct against.
Back bearings and identifying what we see
The same tools work in reverse. To identify a summit on the horizon, point the direction-of-travel arrow at it, rotate the bezel until red is in the shed, and read the magnetic bearing. Apply declination, lay the compass on the map with an edge through our known position, and rotate the map and compass together until the orienting lines match the grid; the edge now passes through the peak. Two such lines from two identifiable features cross at our position, a resection that is still the best way to confirm a location without electronics.
A back bearing is simply the bearing plus or minus 180 degrees. Checking it against a feature behind us reveals drift before it becomes expensive.
Declination, explained once and properly
There are three norths. True north is the geographic pole. Grid north is the direction of the vertical lines printed on the map, which differ very slightly from true north because a flat grid cannot wrap a sphere. Magnetic north is where the needle points, and it wanders over the years as the planet’s core shifts. Declination, called magnetic variation on some maps, is the angle between grid north and magnetic north at a given place and date. It is printed in the legend, along with the annual rate of change.
The size of the angle is the whole problem. In much of western Europe it is currently within a couple of degrees and can almost be ignored. In the northeastern United States and Atlantic Canada it exceeds 15 degrees west; in the Pacific Northwest and Alaska it is 15 to 20 degrees east; near the Great Lakes it passes through zero. A walker who ignores a 15 degree error drifts about 260 m per kilometre (roughly 1,400 ft per mile), which is enough to miss a hut in the dark.
The rule is easier than the folk rhymes suggest. Declination east means the needle points east of grid north, so a magnetic bearing is smaller than the corresponding grid bearing: going from map to compass we subtract, from compass to map we add. Declination west is the reverse: map to compass we add, compass to map we subtract. Written out for a place with 15 degrees east variation, a grid bearing of 90 becomes a magnetic bearing of 75. A compass with an adjustable declination screw does this arithmetic permanently, provided we remember to reset it when we travel. Note too that declination changes with time; a twenty-year-old map in an area shifting a fifth of a degree a year is already four degrees wrong.
A working sequence for the hill
Under pressure, procedure beats memory. The sequence we use every time is:
- Fix the current position on the map and mark the target.
- Edge of baseplate from A to B, travel arrow towards B, bezel lines to grid north, read the grid bearing.
- Apply declination to get the magnetic bearing, or trust the preset screw.
- Measure the leg distance and convert it to time or paces before setting off.
- Red in the shed, pick an object, walk, repeat; check a back bearing at least once per leg.
None of this is fast the first few times. Practised on a clear day with the destination in plain sight, it becomes a two-minute routine we can perform with cold hands at a col in cloud, which is precisely when the compass and the small angle between two norths earn their place in the pack.