Navigational error

Error and relocation

Walk a route with a hidden error, notice it, and find yourself again

SelectedNone

Tap any location for its grid reference · tap a contour for its elevation · map turned 0°

Lesson 9 of 9

Navigational Error and Relocation

1Every leg you walk has an error in it. The skill is noticing when the ground stops agreeing with the map, and stopping before the error grows.
2A bearing error grows with distance: the sideways offset is distance × sin(error). One degree is about 17 m in every kilometre, 6° is over 100 m.
3A distance error leaves you short of, or past, the point you think you reached, along the line of the leg.
4Errors accumulate. Each leg starts from where you actually are, not from where the route card says, so the next leg carries the last one's error and adds its own.
5Run the scenarios below. You are told where you believe you are and what the ground around you looks like. Your true position stays hidden until you have fixed it yourself.

Scenario · Drill

Off the bearing

Four legs from Camp Harrow to the relay tower. Somewhere a bearing is walked several degrees off.

How errors happen

Common navigation errors

·Bearing error. The compass is misread, or the bearing is not held while walking. You end up to one side of the checkpoint.
·Distance error. Paces are miscounted or slope and rough ground are not allowed for. You stop short or overshoot.
·Map not oriented. Directions taken by eye from a map that is not set to the ground are wrong by the amount the map is turned.
·Compass not adjusted. A grid bearing walked as a magnetic bearing is out by the whole grid-magnetic angle, on every leg.
·Misidentified feature. One hill is taken for another, and everything fixed from it is wrong.
·Terrain not allowed for. Going round an obstacle or contouring a slope without correcting pulls you steadily off the line.
·Losing track of position. No checkpoint is confirmed for a long time, so there is no recent known point to work from.
·Wrong route. The wrong line of the route card, or the wrong track at a junction, is followed with confidence.
·Accumulated error. Several small errors, none noticeable alone, add up across the legs.

Procedure

Relocation

1Stop and reassess as soon as the ground stops matching the map.
2Do not carry on along a route you are unsure of: every step adds to the error.
3Recall your last known position, the last place you positively confirmed.
4Check the last confirmed leg: its bearing, its distance and how long you have walked.
5Look around and pick out features you can recognise.
6Compare the ground with the map: height, the direction of slope, water, roads.
7Decide which landmarks on the map those features could be.
8Take compass bearings to them and resect.
9Estimate your position from the lines.
10Confirm it with something independent: the slope, a creek, a road, a third bearing.
11Decide: resume the route from a confirmed position, or retrace to a known point.

Practice

How far off?

Terrain association

Match the ground to the map

Open the 3D terrain to compare the shape of the ground with its contours. Tap the 3D ground and the same spot is selected on the map, with its grid reference. Use it to confirm a fix: the height, the slope and the features around it must agree with what you can see.

Knowledge check

Error and relocation

The ground around you does not match the map at the checkpoint you believe you have reached. What do you do first?

You hold a bearing 5° off for two legs of 2 km each. Roughly how far off line are you?

Every leg has taken you to the right of your checkpoints by a similar angle. What is the most likely cause?

When is retracing to your last known point the better choice?

Compass

Bezel 000° · declination 0°

Tap any component of the compass to inspect it.

Bezel at the index line: 000°
Declination set on the screw: 0°
On the map at: Not on the map

On the map: drag the compass to move it, drag its rear end to turn it, and drag the ring to rotate the bezel. Its settings stay as you leave them in every lesson.

Grid reference

Tap the map

Precision: Names a 1 m square

Precision is not accuracy. More digits name a smaller square; they do not make the map, or your plotting, any more exact. On a 1:50 000 sheet a careful reader is good to about 50 m.