Determining your location

Resection

Fix an unknown position from bearings to known landmarks

SelectedNone

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

Lesson 8 of 9

Determining Your Location – Resection

1Resection fixes your own position when you do not know it, using features you can both see on the ground and find on the map.
2Pick landmarks that are unmistakable: a summit, a tower, a group of buildings. A long ridge or the edge of a wood is a poor choice, because you cannot say which point of it you sighted.
3Sight the compass on a landmark and read the bearing at the index line. The compass reads magnetic unless the declination screw is set.
4Convert the reading to a grid bearing: grid = magnetic + GMA. If part of the angle is already set on the screw, add only what remains.
5Work out the back bearing: grid bearing plus or minus 180°. The bearing runs from you to the landmark, so the back bearing runs from the landmark to you.
6Plot the back bearing from the landmark. You are somewhere on that line: a line of position.
7Repeat with a second landmark. You are where the two lines cross. A third line checks the first two.
8Read the grid reference of the crossing, easting first, then confirm it against the ground around you.

Exercise

Fix an unknown position

Bearing error

Each bearing you take is out by a random amount up to this error. It applies from the next start or repeat.

Positional uncertainty

Why the lines rarely meet at a point

·A bearing is never exact. A hand compass is read to a degree or two, and 1° is about 17 m sideways for every kilometre to the landmark.
·So each line of position is really a narrow wedge that widens away from its landmark. The gold area on the map is where every wedge overlaps: your position is somewhere inside it.
·Two lines always cross somewhere, so two lines alone cannot show you a mistake. Three lines rarely meet at a point. They form a small triangle, and its size tells you how good the fix is.
·Lines that cross near a right angle give a small, compact area. Lines that cross at a shallow angle give a long, thin one: a small bearing error slides the crossing a long way.
·Choose landmarks well spread around you, 60° to 120° apart, and prefer near ones to far ones. Set a bearing error in the exercise above and compare a shallow pair with a well spread pair.

Worked example

A resection from three landmarks

1Bison Hill, at 052 496. The compass reads 269° magnetic. Grid = 269° + 13.4° = 282°. Back bearing = 102°. Plot 102° from Bison Hill.
2Relay tower, at 106 512. The compass reads 020° magnetic. Grid = 020° + 13.4° = 033°. Back bearing = 213°. Plot 213° from Relay tower.
3Hill 688, at 082 452. The compass reads 179° magnetic. Grid = 179° + 13.4° = 192°. Back bearing = 012°. Plot 012° from Hill 688.
4The three lines meet at 12U WD 0898 4876. The true position is 12U WD 0900 4875, 19 m away: the difference is what reading each bearing to a whole degree costs.

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

Resection

Which of these is the best landmark for a resection?

What is a back bearing?

You sight a landmark on a grid bearing of 050°. In which direction is the line plotted from the landmark?

Why take bearings to more than one landmark?

Three lines of position form a small triangle. Where are you most likely to be?

A bearing is 2° out and the landmark is 3 km away. About how far is that line of position from your true position?

Which set of landmarks gives the most reliable fix?

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.