Standard loci

• Points at a fixed distance from a point: a circle.
• Points equidistant from two points: the perpendicular bisector of the line joining them.
• Points equidistant from two intersecting lines: the angle bisector.
• Points at a fixed distance from a line: two parallel lines (with semicircular ends).

Bearings

• A bearing is measured clockwise from North, with three figures (e.g. $045^\circ$, $270^\circ$).
• Back bearing $=$ bearing $\pm 180^\circ$.

Scale

$$\text{Linear scale} = \frac{\text{map length}}{\text{actual length}} \qquad \text{Area scale} = (\text{linear scale})^2$$ For a scale $1 : n$, $1\ \text{cm}$ on the map represents $n\ \text{cm}$ on the ground.