Map Reading, Scale and Practical Cartography
This domain tests the practical skills of reading topographical sheets and converting map measurements into ground reality.
Scale
Scale may be a statement (1 cm = 5 km), a representative fraction (1:50,000) or a graphical bar. A large-scale map (small denominator, e.g. 1:25,000) shows more detail over a small area; a small-scale map (e.g. 1:1,000,000) covers a large area with little detail. To convert an RF: 1 cm on a 1:50,000 sheet equals 50,000 cm, i.e. 0.5 km on the ground.
Grid references and direction
Grid references give the easting first, then the northing; a four-figure reference locates a grid square, a six-figure reference a point within it. Bearings are measured clockwise from grid north.
Contours
Contours join points of equal height above mean sea level, drawn at a fixed contour interval. Closely spaced contours mean a steep slope, widely spaced contours a gentle one. A V pointing upstream marks a valley; concentric closed rings mark a hill; hachures mark a depression.
Conventional signs and sections
Conventional symbols show settlements, roads, railways, streams and crops. Cross-sections are drawn by transferring contour heights along a line at a chosen vertical exaggeration.
Worked example
On a 1:50,000 sheet, two villages are 4 cm apart. Ground distance = 4 × 50,000 cm = 2,00,000 cm = 2,000 m = 2 km. If contours cross at 20 m intervals and six contours rise between them, the height gain is 100 m.
Traps
- Forgetting to convert centimetres to kilometres (divide by 1,00,000).
- Reading northing before easting.
- Assuming a fixed contour interval on every map rather than checking the sheet.