1. The sheet and its grid
- A topographical sheet is a large-scale map of a small area showing relief, drainage, settlements, communications and land use.
- The ICSE paper uses a real Survey of India sheet. These notes use an imagined practice sheet (not a real place, not a Survey of India sheet).
- Eastings are the vertical grid lines, numbered west to east along the top and bottom (here 40 to 50).
- Northings are the horizontal grid lines, numbered south to north along the sides (here 70 to 80).
- Each grid square is 1 km by 1 km (1 sq km); at 1 cm to 0.5 km it measures 2 cm by 2 cm on the printed sheet.
| Item | Value on the imagined sheet |
|---|---|
| Eastings | 40 to 50 |
| Northings | 70 to 80 |
| Square | 1 km by 1 km |
Key terms
- Easting
A vertical grid line; its number increases towards the east.
- Northing
A horizontal grid line; its number increases towards the north.
2. Four-figure and six-figure grid references
- Four-figure reference (a square): easting first, then northing, taken from the lines at the south-west corner of the square. The temple is in square 4674.
- Six-figure reference (a point): easting (2 digits) + tenths eastwards, northing (2 digits) + tenths northwards. The temple is 463746, the post office 468738 and the dispensary 431726.
- To find the feature at a reference, split it into the easting half and the northing half, go to that point and read the symbol with the key.
| Feature | Four-figure | Six-figure |
|---|---|---|
| Temple | 4674 | 463746 |
| Post office | 4673 | 468738 |
| Dispensary | 4372 | 431726 |
Key terms
- Four-figure grid reference
Two digits for the easting then two for the northing; names a whole 1 km square.
- Six-figure grid reference
Adds a tenth digit to each half; names a point to the nearest 0.1 km.
3. Contours and the contour interval
- A contour is a line joining places of equal height above mean sea level (brown on the sheet).
- The contour interval is the constant height difference between neighbouring contours: 20 m on the imagined sheet.
- Index contours (every 100 m here) are thicker and labelled.
- Finding the interval: height difference between two labelled contours divided by the number of gaps (lines between + 1).
- Heights between contours: a point between the 160 m and 180 m contours has a height between 160 m and 180 m.
| Index contours | Lines between | Interval |
|---|---|---|
| 200 m and 300 m | 4 | 100 ÷ 5 = 20 m |
Key terms
- Contour interval
The constant difference in height between two neighbouring contours.
- Index contour
Every fifth contour, drawn thicker and labelled with its height.
4. Slope from contour spacing
- Close contours: steep slope. Wide spacing: gentle slope. Even spacing: uniform slope.
- Average gradient = height difference ÷ horizontal distance.
- On the imagined sheet the escarpment rises 108 m in about 1.3 km (about 83 m per km); the southern plain changes about 14.8 m per km.
- A cross-section drawn from the heights along a line shows the slope as a profile.
| Spacing | Slope |
|---|---|
| Close | Steep |
| Wide | Gentle |
| Even | Uniform |
Key terms
- Gradient
Height difference divided by horizontal distance, for example in metres per km.
- Cross-section
A profile drawn from the heights along a chosen line.
5. Landforms from contours
- Conical hill: concentric closed contours, steep on all sides, small summit (478 m here).
- Ridge: long narrow closed contours with a crest (398 m here), steep on both long sides.
- Escarpment: contours bunched along a line (the steep face) with wide spacing on one side and a flat top on the other.
- Steep slope and gentle slope follow the spacing; a valley shows V-shaped bends pointing to higher ground.
- Sketch each by drawing the contours and labelling the highest value.
| Landform | Contour pattern |
|---|---|
| Conical hill | Concentric rings |
| Ridge | Elongated loops |
| Escarpment | Bunched line, then wide spacing |
Key terms
- Conical hill
A hill with steep slopes on all sides shown by concentric closed contours.
- Escarpment
A long steep face between a higher flat area and lower ground.
- Ridge
A long narrow upland with a crest and slopes falling on both sides.
6. Triangulated heights, spot heights and relative height
- Triangulated height: black triangle with a figure, fixed by survey triangulation on a summit or prominent point (478 m on the hill, 398 m on the ridge).
- Spot height: black dot with a figure, measured at ordinary ground to fill gaps between contours.
- Both are in metres above mean sea level.
- Relative height = higher height − lower height (for example 262 m − 154 m = 108 m for the escarpment). Relative depth is the same difference measured downwards.
- Never add the two heights.
| Symbol | Meaning |
|---|---|
| Black triangle and figure | Triangulated height |
| Black dot and figure | Spot height |
Key terms
- Triangulated height
A height fixed by a survey network and marked by a black triangle on a summit or prominent point.
- Spot height
A height of an ordinary point, marked by a black dot and a figure.
- Relative height
The difference in height between two points.
Exam tips
Common mistakes
- Writing the northing before the easting.
- Reading tenths from the top right instead of the south-west corner.
- Dividing by the number of lines instead of the number of gaps when finding the contour interval.
- Adding two heights instead of subtracting for relative height.
- Calling a ridge a hill, or forgetting units.
How marks are usually awarded
- Grid reference (1 mark each): correct digits in the right order.
- Contour interval or height (1 to 2 marks): value with the unit and the working.
- Landform (2 marks): the name and the contour evidence.