Map Reading, Scale and Interpretation

What you need to know

• Scale is expressed in three ways: a statement (1 cm represents 0.5 km), a representative fraction or RF (1:50,000), and a linear or line scale drawn on the map. The linear scale is the only one that remains correct after a map is enlarged or reduced, because it grows or shrinks along with the map.
• To turn an RF into a statement in kilometres, divide the denominator by 100,000, since 1 km = 100,000 cm. So 1:50,000 means 1 cm represents 0.5 km, and 1:100,000 means 1 cm represents 1 km. Most Nigerian topographical sheets you will meet in the exam are 1:50,000.
• Measure a straight distance with a ruler. Measure a winding feature such as a road or river by laying a thread along it, or by pivoting the straight edge of a sheet of paper round each bend and marking off each segment, then straightening the thread or paper against the ruler.
• Grid references are read eastings first, then northings: along the corridor, then up the stairs. A four-figure reference such as 3847 names a whole grid square; a six-figure reference such as 384476 fixes a point to the nearest 100 m on a 1:50,000 sheet because each grid square is divided mentally into tenths.
• Bearings are measured clockwise from north and always written in three figures: 045 degrees, 135 degrees, 270 degrees. Place the protractor centre on the starting point with zero pointing to grid north, then read round clockwise to the target.
• A back bearing is the bearing of the first point from the second. Add 180 degrees if the forward bearing is less than 180 degrees, and subtract 180 degrees if it is more. The bearing of B from A as 060 degrees gives the bearing of A from B as 240 degrees.
• Contours are lines joining places of equal height above sea level, and the fixed difference between successive contours is the contour interval, usually 50 ft or 15 m on older Nigerian sheets. Contours packed close together mean a steep slope; contours far apart mean a gentle slope; evenly spaced contours mean a uniform slope.
• Learn the signature of each relief feature: a conical hill is a set of small closed concentric rings; a ridge is a long narrow set of elongated contours; a spur shows contours bending outward towards the lower land; a valley shows contours bending as a V that points upstream, that is towards the higher land.
• An escarpment or cuesta has contours crowded on one face and widely spaced on the other, a plateau is a broad flat top ringed by crowded contours, a col or saddle is the low gap between two hills, and a depression is marked by contours with short ticks pointing inward.
• Gradient is vertical interval divided by horizontal equivalent, with both quantities converted to the same unit, and it is quoted as 1 in n. A gradient of 1 in 20 means the land rises 1 m for every 20 m travelled horizontally.
• To draw a cross-section, lay the straight edge of a sheet of paper along the section line, tick off every point where a contour, river or road crosses it and write its height, then transfer the ticks to a base line, choose a vertical scale, plot the heights and join them with a smooth curve rather than straight jagged lines.
• Vertical exaggeration is the vertical scale divided by the horizontal scale, and it is the reason your drawn profile looks more mountainous than the real land. Always state it when the question asks for it.
• Intervisibility is tested by drawing the cross-section between the two points and ruling a straight line of sight from one to the other; if any land rises above that line, the two points are not intervisible.
• Area of an irregular feature is found by counting complete grid squares, then adding half the number of incomplete squares. On a 1:50,000 sheet each grid square measures 1 km by 1 km, so each square counts as 1 square kilometre.