Geography · Lesson 17

Practical geography 2: contours, landforms, gradient, cross-sections and intervisibility

JAMBWAECNECO

What the syllabus asks

JAMB: map reading and interpretation, gradients, cross profiles, intervisibility. Builds on gg8 using the Oke-Ofa extract.

WAEC: identification of physical features (spurs, valleys, etc.), measurement of gradients, drawing of cross profiles, intervisibility, description of relief

NECO: identification of physical features (spurs, valleys, etc.), measurement of gradients, drawing of cross profiles, intervisibility, description of relief

What you will learn

Find heights by reading and interpolating contours.
Recognise slope shapes from contour spacing.
Identify relief landforms and give their grid references.
Calculate gradients as ratios, percentages and angles.
Measure the gradient of a river along its course.
Draw and label a cross-section with a stated vertical exaggeration.
Determine intervisibility and name the obstruction.
Describe relief and link it to human activity.

Key points

Contours join points of equal height; spacing shows steepness.
Valley Vs point to higher values; spur Vs to lower values.
Convex slopes hide their foot; concave slopes do not.
Gradient = VI ÷ HE in the same units, 1 in n.
River gradients are measured along the course.
Section base line = map line length; join points smoothly.
State title, end references, scales and VE on every section.
VE = horizontal denominator ÷ vertical denominator.
Intervisibility: rule the line of sight on the section.
Describe relief with heights, references and named landforms.

Start here: Concept summary: reading the shape of the land

Relief means the shape of the land surface: its heights, slopes and landforms. On a topographical map relief is shown mainly by contours, supported by spot heights, trigonometrical stations and, on some maps, hill shading or layer colouring. Almost every WAEC map question includes at least one relief task: identify landforms, calculate a gradient, draw a cross-section or decide whether two places are intervisible, and then describe the relief of the area. JAMB asks the same things as short objective items.

This unit uses the Oke-Ofa extract from unit gp1 again (see Figure 2 there). Its contour interval is 20 m and index contours are drawn every 100 m. The land rises from about 85 m in the south-west, where the River Ofa leaves the extract, to 455 m at the trig station on the forested north-western hill.

Three ideas carry you through the whole unit:

Spacing tells steepness. Contours close together mean a steep slope; contours far apart mean a gentle slope; equally spaced contours mean a uniform slope.
Shape tells landform. Closed rings are hills; Vs pointing to higher ground are valleys; Vs or Us pointing to lower ground are spurs; a gap between two sets of rings is a col.
Numbers tell direction. Always find which way the contour values increase. A pattern that looks like a valley may be a spur if you misread which way is uphill.

Every height, gradient and section in this unit was computed directly from the model surface used to draw the extract and checked by program, so the answers you see are the answers a perfect measurement would give. Your own measurements on a printed map should agree within the tolerances stated.

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Practice questions

1. Contours are lines joining places of equal

height
rainfall
temperature
population

Show answer

A: height
Above sea level.

2. Contours close together indicate a

gentle slope
flat plain
river
steep slope

Show answer

D: steep slope
Rapid change of height.

3. Contours V-shaped pointing uphill indicate a

ridge
plateau
valley
spur

Show answer

C: valley
V points upstream.

4. Contours V-shaped pointing downhill indicate a

gorge
spur
valley
col

Show answer

B: spur
Spur projects from high land.

5. A low point between two hills is a

col (saddle)
spur
knoll
escarpment

Show answer

A: col (saddle)
Contours form an hourglass shape.

Key words

Relief: The shape and height of the land surface.
Contour: Line joining points of equal height above sea level.
Contour interval: Vertical difference between neighbouring contours.
Spot height: A point with its exact height printed beside it.
Index contour: Every fifth contour, drawn thicker and labelled.
Col: A low point or saddle between two hills.
Spur: Tongue of high ground projecting into lower ground.
Knoll: A small isolated hill.
Gradient: VI ÷ HE, the average steepness between two points.
Vertical exaggeration: How many times the vertical scale exceeds the horizontal.
Cross-section: Side view of the land along a line.
Intervisibility: Whether two points can be seen from each other.

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