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MEASURING THE AREA

MEASURING THE AREA. Measuring the Area - Considerations. All eastings are 2 cm (1 km) apart All northings are 2 cm (1 km) apart Each grid square measures 2 cm X 2 cm OR 1 km X 1 km = 1 sq. km. How do we measure the area?. Count the number of grid squares (n)

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MEASURING THE AREA

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  1. MEASURING THE AREA

  2. Measuring the Area - Considerations • All eastings are 2 cm (1 km) apart • All northings are 2 cm (1 km) apart • Each grid square measures • 2 cm X 2 cm • OR • 1 km X 1 km • = • 1 sq. km

  3. How do we measure the area? • Count the number of grid squares (n) • Area = n sq. km

  4. Example 1 Calculate the area enclosed within eastings 26 and 29 and northings 58 and 62. Solution • Eastings difference (p) = 29 – 26 = 3 • Northings difference (q) = 62 – 58 = 4 • Area enclosed = p X q • = 3 X 4 • = 12 sq km

  5. Visual method

  6. Visual method

  7. Example 2 Calculate the extent of cultivated area enclosed within eastings 43 and 49 and northings 85 and 89. CWrong Solution • Eastings difference (p) = 49 – 43 = 6 • Northings difference (q) = 89 – 85 = 4 • Area enclosed = p X q • = 6 X 4 • = 24 sq km

  8. So, what is the correct solution?

  9. Visual method

  10. Correct solution • Area enclosed by full grid squares (f) • Area enclosed = f X 1 • Area enclosed by half grid squares (h) • Area enclosed = h X ½ • Area enclosed by more than half grid squares (m) • Area enclosed = m X 2/3 • Area enclosed by less than half grid squares (l) • Area enclosed = l X 1/3

  11. TOTAL AREA • Total area • f X 1 • + • h X ½ • + • m X 2/3 • + • l X 1/3

  12. Correct solution - Visual method

  13. TOTAL AREA • Finding the total area • f X 1 = 10 X 1 = 10 sq. km + • h X ½ = 4 X ½ = 2 sq. km + • m X 2/3 = 7 X 2/3 = 4.67 sq. km + • l X 1/3 = 5 X 1/3 = 1.67 sq. km = • Total Area = 18.34 sq. km

  14. REPRESENTING HEIGHTS ON TOPOGRAPHICAL MAPS

  15. How are heights measured? • Start from mean sea level • Determine the heights using theodolite (principles of trigonometry) • Use these heights as bench marks to determine further heights

  16. Types of heights Triangulated Height Spot Height Relative Height

  17. Triangulated Height • Determined using principles of trigonometry • Accurate • Expressed on maps using a ∆ • For example, ∆ 224

  18. Prominent Surveyed Tree • Triangulated height written on tree bark • Tree is shown in black colour- • For example

  19. Bench Mark • Triangulated height written on nearby rock or wall • Shown using BM • For example, BM 403

  20. .544 560 540 Spot Height • Height estimated using the value of adjacent contours • Shown with a dot • For example, .544

  21. Relative Height • Height (depth) of a feature relative to surroundings • Shown using r • For example, 20r

  22. Example of relative heights 7r 11r 14r

  23. End of Presentation

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