Map Reading and Interpretation
Senior 5 · Geography
Map Reading and Interpretation
Lesson 1 of 4
25 min
Map A shows an invented stretch of highland country of the kind you meet on Rwanda's 1:50 000 topographic sheets: a river valley between two hills, a village at a road junction, a tea estate and a forest. Every exercise in this unit is worked from it, so study it now. Find the River Nyaga, the main road and its bridge, village K, Hill T with its trig station and Hill V with its forest.
The map has a grid of lines 1 km apart. The lines that run north to south are numbered along the top and bottom edges (20 to 30); the lines that run east to west are numbered along the sides (40 to 47). With these numbers any point on the map can be given an exact address.
Definition
Easting: a grid line running north to south, numbered along the top and bottom of the map; its number increases eastwards. Northing: a grid line running east to west, numbered along the sides; its number increases northwards.
Grid reference: the position of a point given by its easting first and its northing second. Example: the trig station on Hill T is at 224457.
A four-figure reference names a whole grid square by the easting and northing that meet at its south-west (bottom-left) corner. The tea factory lies in the square whose bottom-left corner is where easting 29 meets northing 44, so its four-figure reference is 2944. The church in village K is in square 2443. On a 1:50 000 map with 1 km squares, a four-figure reference locates a feature to within 1 km² only, which is good enough for large features such as a forest or a village but not for a single building.
For a point, divide each side of the square into tenths by eye (or with a romer, a small transparent scale). Read the easting first: the whole-number line to the west of the point, then the tenths eastwards from it. Then the northing: the line to the south, then the tenths northwards. A six-figure reference locates a point to within a square 100 m by 100 m.
Worked example 1: giving a six-figure reference
Problem. Give the six-figure grid reference of the trig station on Hill T.
Solution.
A memory aid: "along the corridor, then up the stairs".
Worked example 2: from a reference to a feature
Problem. What is found at grid reference 259422 on Map A?
Solution. Split it into 259 and 422. Go to easting 25 and move nine-tenths of the way east towards 26; go to northing 42 and move two-tenths of the way north towards 43. The symbol there is a square with S: a school, beside the murram road south-east of village K.
Definition
Bearing (whole-circle bearing): the angle between north and the direction of a second point, measured clockwise from north at the starting point, from 000° to 360°, always written with three figures. Example: the bearing of the trig station from the church is 321°.
Back bearing: the bearing of the starting point seen from the second point. It differs from the forward bearing by exactly 180°.
A compass direction such as "north-west" is quick but rough: each of the sixteen points covers a sector 22.5° wide. A bearing is precise to a degree. To measure one on a map:
Worked example 3: bearing and back bearing
Problem. Find the bearing of the trig station (224457) from the church (246430), and the back bearing.
Solution. From the church the trig station is 2.2 km west and 2.7 km north. The anticlockwise angle from north is
so the bearing is , which is close to north-west (315°). The back bearing, from the trig station to the church, is . On the map you would measure these with a protractor; the calculation is a check.
Which north? Bearings on a map are measured from grid north, the direction of the eastings. True north points to the geographic North Pole, and magnetic north is where a compass needle points. Near the equator the three differ by only a few degrees, and the margin of a topographic sheet states the difference. A surveyor walking on a compass bearing must correct for it; for map exercises, use grid north.
| Back-bearing rule | Example |
|---|---|
| Forward bearing less than 180°: add 180° | 050° gives 230° |
| Forward bearing 180° or more: subtract 180° | 321° gives 141° |
Warning
Common mistakes
Tip
Exam technique. Before you measure, estimate. If the second point is up and to the left, the bearing must be between 270° and 360°. A measured answer outside the estimated quadrant means you read the protractor's wrong scale.
Geographer Online: "Map Skills – Calculating Bearings in a Geography Examination". Watch how the north line is drawn at the starting point before the protractor is placed, the step most often missed in examinations.
Practice
Practice (use Map A)
Summary
Sources and credits
Written by Ganzaa. Map A and its features are invented for teaching and drawn by Ganzaa; every reference, bearing and distance in this lesson was calculated from the map's own coordinates. Figures 1 and 2 were drawn by Ganzaa.
Video: "Map Skills - Calculating Bearings in a Geography Examination", Geographer Online, embedded from YouTube.
