Map reading & topographic interpretation

Map reading & topographic interpretation

1. Read the map before answering

A topographic map combines relief with features such as settlements, roads, streams and vegetation. Start with its title, date, legend, north arrow, scale and contour interval. A symbol shows the mapped feature, not necessarily its present condition. A footpath on an old map is not proof that it is open today.

Use these skills together: locate → measure → describe → explain → evaluate. Locate a settlement accurately; measure its distance from a road; describe its valley-floor site; explain possible access advantages; then consider flood exposure. Keep direct map evidence separate from a geographical explanation or a prediction.

Map examples in these notes and the practice paper are invented teaching examples. They are not extracts of a real Hong Kong survey. Read the printed scale or labelled coordinates; resizing a screen image changes centimetres measured with a ruler.

2. Scale, distance and area

A representative fraction of \(1:20{,}000\) means one unit on the map represents \(20{,}000\) of the same unit on the ground. Thus \(1\,\mathrm{cm}\) represents \(200\,\mathrm{m}\). At \(1:5{,}000\), \(1\,\mathrm{cm}\) represents \(50\,\mathrm{m}\): this is a larger scale and normally shows more detail over a smaller area.

Worked example: a straight route measures \(6.5\,\mathrm{cm}\) on a correctly printed \(1:20{,}000\) map. \(\text{Ground distance}\) \(=6.5\times20{,}000\) \(=130{,}000\,\mathrm{cm}\) \(=1{,}300\,\mathrm{m}\) \(=1.3\,\mathrm{km}\). Write the conversion before rounding. For a winding route, follow its centre line with a thread or short straight segments, add the lengths, then convert. A straight-line distance is usually shorter than the walking route.

A graphic scale enlarges with the map, so it remains useful after proportional resizing. Compare a route with the bar in the same image rather than assuming the original printed fraction still applies to screen centimetres.

Area uses the squared conversion. At \(1:20{,}000\), \(1\,\mathrm{cm}^{2}\) represents \(200\,\mathrm{m} \times 200\,\mathrm{m} \) \(= 40{,}000\,\mathrm{m}^{2} \) \(= 0.04\,\mathrm{km}^{2}\). A \(3\,\mathrm{cm}\) by \(2\,\mathrm{cm}\) rectangle represents \(0.24\,\mathrm{km}^{2}\). For an irregular area, count complete grid squares and estimate fractions; state that the result is approximate.

3. Grid references and bearings

Read eastings first, then northings: move right, then up. A four-figure reference identifies a square using its south-west corner. If a point lies between eastings \(23\) and \(24\) and northings \(47\) and \(48\), its square is \(2347\). Keep leading zeros.

For six figures, divide each square mentally into tenths. A point \(0.4\) of a square east of line \(23\) and \(0.7\) north of line \(47\) is \(234477\): easting \(234\), northing \(477\). It is not \(237447\). On a grid whose lines are \(1\,\mathrm{km}\) apart, a tenth is \(100\,\mathrm{m}\). A six-figure reference does not imply survey-grade accuracy.

Bearings are measured clockwise from the stated north at the starting point and written with three digits: north \(000{}^{\circ}\), east \(090{}^{\circ}\), south \(180{}^{\circ}\), west \(270{}^{\circ}\). Draw the north line through the origin before using a protractor. Grid north, true north and magnetic north may differ; use the one required by the question.

Worked example: B is \(1\,\mathrm{km}\) east and \(1\,\mathrm{km}\) north of A, so the grid bearing of B from A is \(045{}^{\circ}\). The reverse bearing is \(225{}^{\circ}\). Add or subtract \(180{}^{\circ}\) to find a reverse bearing, keeping the answer between \(000{}^{\circ}\) and \(359{}^{\circ}\). “B from A” starts at A.

4. Contours reveal relief

A contour joins places of equal elevation above the stated datum. The contour interval is the vertical difference between neighbouring contour values, not their horizontal spacing. With a \(20\,\mathrm{m}\) interval, five successive lines might be \(20\), \(40\), \(60\), \(80\) and \(100\,\mathrm{m}\). Count four intervals from \(20\) to \(100\), not five.

Closely spaced contours indicate a steep slope; wide spacing indicates a gentle slope, provided the map scale and interval are the same. Uniform spacing suggests an even slope. A convex slope steepens towards its foot; a concave slope becomes gentler towards its foot. Compare several intervals rather than a single gap.

Closed contours with values increasing towards the centre normally show a hill. A spot height gives a measured height at one point; the highest enclosing contour is not the summit height. Contours crossing a stream often form a V pointing upstream towards higher land. Water flows downhill in the opposite direction. A ridge projects from high land; verify contour values and drainage before naming a shape.

Do not infer a cliff from any two close lines without checking scale. A map shows relief indirectly: use elevation values, stream directions and neighbouring landforms together. The coast is often near sea level but the datum and tidal information still matter.

5. Gradient and cross-sections

Gradient compares vertical change with horizontal distance. Use the same units: \(\frac{\text{rise}}{\text{run}}\) \(=\frac{40\,\mathrm{m}}{800\,\mathrm{m}}\) \(=\frac{1}{20}\), written \(1:20\). \(\text{Percentage gradient}\) \(=\frac{40}{800}\times100\) \(=5\%\). A \(1:10\) slope is steeper than \(1:20\). Neither ratio means an angle of \(20{}^{\circ}\).

For a profile, place a strip of paper along the section line. Mark each contour crossing, spot height and river. Transfer positions to a horizontal distance axis, plot heights on the vertical axis and join them with a plausible continuous ground line. Label endpoints, units and both scales. Do not connect the contour labels in numerical order: follow their actual order along the section.

Example profile rows: distance from A (m): \(0\) | \(200\) | \(400\) | \(600\) | \(800\); elevation (m): \(20\) | \(40\) | \(80\) | \(60\) | \(20\). The highest sampled point is at \(400\,\mathrm{m}\). The average gradient from \(0\) to \(400\,\mathrm{m}\) is \(60/400 \) \(= 0.15 \) \(= 15\%\), about \(1:6.7\). The average endpoint gradient from \(0\) to \(800\,\mathrm{m}\) is zero despite the hill between them.

If \(1\,\mathrm{cm}\) represents \(200\,\mathrm{m}\) horizontally but \(20\,\mathrm{m}\) vertically, vertical exaggeration is \(200/20 = 10\) times. It helps show relief but makes slopes look steeper. For intervisibility, join the two observation heights on the profile: ground crossing above that sight line blocks the view. Account for eye height if supplied; do not judge visibility only from endpoint heights.

6. Explain patterns using evidence

Describe a distribution with location, pattern and an exception: “Most buildings form a line along the road on the valley floor; two isolated buildings lie on the western hillside.” Quote a grid reference or distance when it helps. “There are many buildings” gives little spatial information.

Explain with a chain: relatively flat land → easier construction and transport → settlement concentration. Add a trade-off: low land beside a stream may flood. A road following contours may avoid steep gradients but be longer; a direct climb is shorter but harder to build. The map supports a reasoned inference, not certainty about household motives.

For site selection, compare options against the stated purpose. A clinic needs access and a reasonably level site; an exposed summit might suit a lookout but not an accessible clinic. Cite two different mapped features and acknowledge an unmapped factor such as land ownership or current hazard information.

Exam routine: underline the command word; identify the starting point and units; show calculation steps; use map evidence in every explanation. Describe means state what is shown. Explain adds reasons. Evaluate weighs benefits against limits and ends with a supported judgement. Common traps are reversing coordinates, measuring bearings from the destination, mixing metres and kilometres, and treating screen centimetres as fixed map distances.

Quick check: on \(1:5{,}000\), what does \(4\,\mathrm{cm}\) represent? Answer: \(200\,\mathrm{m}\). A rise of \(30\,\mathrm{m}\) over \(600\,\mathrm{m}\) gives \(1:20\) or \(5\%\). A bearing of \(310{}^{\circ}\) reverses to \(130{}^{\circ}\). Work these independently before reading the answers.

7. An integrated route-planning example

Synthetic source rows: route R follows a contouring road for \(1.6\,\mathrm{km}\) and rises from \(20\,\mathrm{m}\) to \(60\,\mathrm{m}\); route S takes a direct path for \(0.8\,\mathrm{km}\) horizontally and rises from \(20\,\mathrm{m}\) to \(100\,\mathrm{m}\). Both begin at the same settlement, but end at different destinations. R passes a bridge; S crosses a steep hillside. Assume distances stated for gradient calculations are horizontal distances.

Calculation rows: R — height change \(40\,\mathrm{m}\); horizontal run \(1{,}600\,\mathrm{m}\); gradient \(2.5\%\) or \(1:40\). S — height change \(80\,\mathrm{m}\); horizontal run \(800\,\mathrm{m}\); gradient \(10\%\) or \(1:10\). S is half as long in horizontal distance but four times as steep on average. Because destinations differ, this is a comparison of route characteristics, not proof that S is a shortcut to the same place.

For a group with limited mobility, R’s gentler average is an advantage, but inspect whether a bridge has steps and whether any short section is much steeper than the average. A map can suggest a route; current path condition, crossing safety and opening status need checking. Explain both the numerical evidence and the practical implication.

Recording template: feature → grid reference → mapped evidence → inference → uncertainty. Example: hillside path → stated grid square → close contours → difficult ascent → surface condition unknown. A field sketch can supplement this record: draw the main skyline, slope and stream, label viewpoint and direction, then add short process annotations. It should select relevant evidence, not reproduce every visible object.

Use the practice figures to rehearse a second method: convert grid differences to ground distances, then check against the scale bar. Agreement is a useful check; disagreement usually signals reversed axes, a unit conversion or use of a resized image with an unchanged numerical scale.