Grids, coordinates and grid references

Imagine telling a friend where you dropped your drink bottle on a huge field. 'Over there' is no help. You need a starting point and two directions. That is exactly what a grid gives you.

The coordinate plane

A coordinate grid has two number lines that cross at right angles. The horizontal one is the x-axis. The vertical one is the y-axis. They cross at the origin, the point (0, 0).

Every point has an ordered pair, like (4, −2). The first number tells you how far to go across (right is positive, left is negative). The second tells you how far to go up or down (up is positive, down is negative). Order matters: (4, −2) and (−2, 4) are completely different places. A handy memory trick is 'along the corridor, then up the stairs'.

Four quadrants

The two axes split the grid into four quadrants:

Grid references on real maps

New Zealand's official topographic maps, called Topo50, have a grid of blue lines printed over them. Each square is 1 kilometre across in real life. Instead of positive and negative numbers, the lines are numbered from a starting point far to the south-west, so every number is positive. The numbers going across are called eastings and the numbers going up are called northings.

A grid reference works just like an ordered pair: eastings first, then northings. Trampers, search and rescue teams and helicopter pilots use grid references to describe exactly where someone is. A mix-up between the two numbers could send a rescue team to the wrong valley.

Why grids matter