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.
- Statement scale: "1 cm represents 0.5 km."
- Representative fraction (R.F.): 1:50,000.
- 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.