Understanding Ordnance Survey Maps and Slope Gradients

1. MAR-2-119R7 Map Information Part 2 First Class Training

2. Objectives • Describe the main land features found on Ordnance Survey maps when planning routes • Use contour lines on Ordnance Survey maps to plan routes • Explain why land features shown on Ordnance Survey maps are important for planning routes • Assess the slope gradient when planning routes • Analyse sections of land profile by projection from map contour lines to get accurately from one point to the next • Plan accurate routes using the features of an Ordnance Survey map

3. Relief • Features such as hills and valleys • Includes both height and shape • Hard to represent on a two-dimensional map • Shown by: • Spot heights and trig points • Contour lines

4. Height • It is important to understand the units used • OS maps are measured in metres

5. Shape • Shape can be shown by: • Hachures

6. UPHILL

7. Shape • Shape can be shown by: • Hachures • Thickness and closeness shows gradient • Mostly used to show cuttings and embankments

10. Shape • Shape can be shown by: • Hachures • Shading – not used on OS maps • Contour lines

11. 65 UPHILL 60 55 50 45

12. Shape • Shape can be shown by: • Hachures • Shading – not used on OS maps • Contour lines • Link points of equal height • Numbers always point up-hill • Thicker line at 50m intervals • Intervals • 1:25,000 – 5m • 1:50,000 – 10m

13. 68

14. 50 68 65 60 55 50

15. Understanding Slopes • The closer together contour lines are, the steeper the slope • Remember to work out which way the slope goes! • Use the numbers • Slopes go uphill from small to larger numbers • Numbers have their tops uphill from the bottoms • Features give clues as well – eg lakes will generally form at the base of slopes rather than the top

16. Shallower slope 65 65 60 60 55 55 Steeper slope 50 50 45 45

17. Understanding Slopes • A numerical figure can be calculated for how steep a slope is – the gradient • Expressed as a ratio: The smaller the second number, the steeper the slope

18. Understanding Slopes • 1:10 • For every metre climbed (vertical distance), the slope covers 10 metres horizontal distance. 1 10

19. Understanding Slopes • A numerical figure can be calculated for how steep a slope is – the gradient • Expressed as a ratio: The smaller the second number, the steeper the slope • To keep ratios understandable, the first number should always be a 1. This can be done by reducing the ratio (just like reducing a fraction).

20. Understanding Slopes • 2:10 • For every 2 metres of vertical distance the slope covers 10 metres horizontal distance. • 1:5 • For every metre of vertical distance the slope covers 5 metres horizontal distance. 2 1 10 5

21. Understanding Slopes • Calculating the gradient: • Count the gaps between the contour lines

22. Understanding Slopes 1 2 3 4 65 60 55 50 45

23. Understanding Slopes • Calculating the gradient: • Count the gaps between the contour lines • Measure the horizontal distance

24. Understanding Slopes 65 60 4mm 55 50 45

25. Understanding Slopes • Calculating the gradient: • Count the gaps between the contour lines • Measure the horizontal distance • Convert both to metres (using the scale) • 4 gaps x 5 metres = 20 m vertical • 4mm x 25,000 • 4m x 25m = 100 m horizontal • This gives 20:100

26. Understanding Slopes 20 : 100 2 : 10 1 : 5 10 2 = 5

27. Understanding Slopes • If the gradient doesn’t easily reduce, then reduce the second figure to the nearest integer (whole number)

28. Understanding Slopes 23 : 128 Remainder is more than half of 23, so round up 1 : 6 128 23 = 5 r15

29. Understanding Slopes Or…

30. Understanding Slopes 23 : 128 23 ≈ 20 128 ≈ 120 20 : 120 2 : 12 1 : 6

31. Understanding Slopes • If the gradient doesn’t easily reduce, then reduce the second figure to the nearest integer (whole number) • In some cases, the second number is 1 and the first number is greater than one – this means that the slope is steeper than 45 degrees

32. Understanding Slopes • 2:1 • For every metre of horizontal distance, the slope rises 2 metres vertical distance. 2 1

33. Understanding Slopes • If the gradient doesn’t easily reduce, then reduce the second figure to the nearest integer (whole number) • In some cases, the second number is 1 and the first number is greater than one – this means that the slope is steeper than 45 degrees • This can also be expressed as a ratio with a 1 followed by a fraction, eg the 2 : 1 slope can also be expressed as 1 : 0.5.

34. Understanding Slopes • 1 : 0.5 • For every metre of vertical distance, the slope rises half a metre of vertical distance. 1 0.5

35. Understanding Slopes • Convex – steeper at the bottom than the top • Concave – steeper at the top than at the bottom • Some parts of a convex slope may not be visible from others • The entirety of a concave slope can be seen from any part of the slope Concave slope Convex slope

36. Convex slope 50 45 55 60 65 70 75 80

37. Concave slope 50 45 55 60 65 70 75 80

38. Undulation • Only features larger than the contour interval will show on contours – but the land between contours could undulate significantly • This is the reason why hachures are used for notable features too small to show with contours 15 15 10 15 10

39. Land Features - Ridge • A long narrow stretch of elevated ground

40. 823

41. 780 785 790 795 800 805 810 RIDGE 815 820 820 823 815 810 805 800 795 790 785 780 775 770 765

42. Land Features - Ridge • A long narrow stretch of elevated ground • If between two peaks, it is known as a col or a saddle

43. 847

44. COL (SADDLE) 847

45. Land Features - Valley • A valley is the inverse of a ridge

47. v VALLEY

48. Land Features – Spur • A spur can be thought of as a ridge running perpendicular to a slope. • It is a bit like half a col: SPUR COL

49. 847

50. 847 SPUR