Maps and Grid Reference System
Senior 4 · Geography
Maps and Grid Reference System
Lesson 3 of 3
25 min
On a map sheet covering hundreds of square kilometres, "the bridge" or "the school" is not enough: there may be several. Map 1 shows how a map gives every place an exact address. Two families of lines cross it: lines running north–south, numbered 31 to 39 from west to east, and lines running east–west, numbered 52 to 58 from south to north. On this 1:50 000 map they are 2 cm apart, so every square is 1 km by 1 km on the ground.
Map 1: an imaginary area drawn for practice. Contours are at 40 m intervals; the swamp is in square 3352 and the forest in square 3756.
Definition
Grid: a network of numbered lines drawn on a map at equal intervals, used to give the position of any point.
Eastings: the vertical (north–south) grid lines; their numbers increase towards the east. Northings: the horizontal (east–west) grid lines; their numbers increase towards the north.
Grid reference: the numbers that locate a point or a square, always written eastings first, then northings. Example: the swamp on Map 1 is in square 3352.
A grid square is named by the easting along its west side and the northing along its south side, that is, by the lines that meet at its bottom-left corner. Read along the bottom first, then up the side: "along the corridor, then up the stairs". The square whose bottom-left corner is where easting 33 meets northing 52 is square 3352, and the swamp lies in it.
Warning
The two most common mistakes. (1) Giving the northing first: 5233 is a different square, far away. (2) Using the line on the east or north side of the square: the forest lies between eastings 37 and 38 and northings 56 and 57, so it is in 3756, not 3857.
A square is 1 km wide, which is too coarse for a single building. To locate a point, imagine each side of its square divided into ten equal parts and estimate the tenths, as in Figure 4.
Figure 4: grid square 3355 of Map 1, enlarged and divided into tenths in the mind's eye.
Worked example 1: giving and reading references on Map 1
Definition
Six-figure grid reference: three digits for the easting (the line number plus tenths) followed by three digits for the northing (the line number plus tenths). On a 1:50 000 map with 1 km squares, a tenth is 100 m, so a six-figure reference locates a point to within 100 m.
Worked example 2: a distance from two grid references
Problem. How far is it in a straight line from the school (334556) to the market (386527)?
Solution. On Map 1 each grid unit is 1 km. The differences are
By Pythagoras,
Check by measurement: at 1:50 000 this is cm on the map.
Worked example 3: an area by counting squares
Problem. On a 1:50 000 map a forest fills 9 whole grid squares and about half of 6 more. Estimate its area.
Solution. Each square is 1 km², so
Count a square as whole if more than half of it is covered and ignore it if less than half is covered, or count part-squares as halves, as here. State which rule you used.
Tip
Grid references and GPS. A phone's GPS gives latitude and longitude in degrees. A topographic map's grid is a separate system measured in metres, and its numbers are printed in the margin; the full reference of a grid line can be several digits long, and the map shows only its last two. Inside one map sheet, the short four- and six-figure references are enough.
Atomi: "Y11-12 Geography: Grid References". Four- and six-figure references worked step by step. Before each answer is shown, pause and give the reference yourself.
Practice
Practice (use Map 1)
Key points
Further reading
Sources and credits
Written by Ganzaa. Map 1, Figure 4, the worked examples and the exercises were drawn and written by Ganzaa; Map 1 shows an imaginary area.
Video: "Y11-12 Geography: Grid References", Atomi, embedded from YouTube.
Quick check
