Map Work: Scale, Distance and Area

Why map work?

Map reading is a core skill in the Senior School Geography pathway. Kenya's Survey of Kenya topographical sheets (such as 1:50,000) are used in planning roads, settlements, irrigation and county development.

Map scale

Scale is the ratio between a distance on the map and the same distance on the ground.

1. Statement scale: "1 cm represents 0.5 km."
2. Representative fraction (R.F.): 1:50,000.
3. Linear scale: a ruled line marked in kilometres.

Converting R.F. to statement scale: 1:50,000 means 1 cm = 50,000 cm = 500 m = 0.5 km.

Measuring distance

• Straight line: measure with a ruler and multiply by scale.
• Curved features (rivers, roads): lay a thread or the edge of paper along the curve, mark it, then measure against the ruler or linear scale.

Worked example 1: A road measures 9.4 cm on a 1:50,000 map.

• Ground distance = 9.4 x 0.5 = 4.7 km.

Grid references

The map is covered by a grid of eastings (vertical lines, numbered left to right) and northings (horizontal lines, numbered bottom to top).

• Four-figure reference: identifies a square, e.g. 3456 (easting 34, northing 56).
• Six-figure reference: identifies a point; divide each square into tenths. Read eastings first, then northings: "Go along the corridor before up the stairs."

Calculating area

Area = length x width, taking scale into account.

• Each grid square on a 1:50,000 map is normally 2 cm by 2 cm = 1 km x 1 km = 1 km2.
• For irregular areas, count full squares and add half squares: whole squares + (half-squares / 2).

Worked example 2: A lake covers 14 full squares and 6 partial squares counted as half.

• Area = 14 + 6/2 = 17 km2.

Direction and bearing

• Compass points: N, NE, E, SE, S, SW, W, NW.
• Bearing is measured clockwise from north in degrees using a protractor. A school at 090 degrees from a market is directly east.

Height and relief

• Contours join points of equal height; the contour interval is the vertical difference between adjacent lines.
• Close contours: steep slope. Wide apart: gentle slope. Concentric circles: hill or mountain.
• Spot heights and trigonometrical stations give exact heights.

Gradient = vertical interval / horizontal distance. From 1,800 m to 1,900 m over 2,000 m: gradient = 100 / 2,000 = 1 in 20.

Map interpretation

Describe relief, drainage (rivers), vegetation, settlement pattern (nucleated, linear, dispersed), transport and economic activities such as tea farming or tourism.

Try it

• Convert the R.F. 1:25,000 into a statement scale.
• A river is 11 cm on a 1:50,000 map. Find its length in km.
• Give a six-figure reference for a point in your own atlas map.
• Calculate the gradient between points 1,500 m and 1,650 m that are 3 km apart.