1. Spot Heights from Contours

2. Why do you need to know As builders you may do this a few times but it is more likely your regular encounter with contour lines will be when you are given drawings to price

3. Spot Heights from Contours To determine ground levels from a contour drawing it is simply a reverse of what we did in the lesson above. From the contour drawing we recreate the grid

4. Step 1 On our contoured drawing were place an even grid. Grid size is IMPORTANT. You must use a scale ruler and select a dimension that is, Relevant- A grid size of 100 metres will be too large and no reliable quantities will be determined later. Ease – Select a grid size that will make your later calculations easier when you need to do the volume calculations e.g. uses a grid size of say 10m x 10m instead of 13m x 13m Draw the grid as described and this time lightly draw in the grid lines.

6. Step 2

7. Step 3 – Single Spot (B3) Height Between Contours Spot Height = Higher Contour – ((Higher Contour – Lower Contour)) x (Distance from Higher Contour to Grid ÷ Distance between Contours)) Spot Height = 100.800 – (100.800 – 100.600) x 18 ÷ 29 = 100.800 – 0.124 = 100.676 29 18

8. Step 4 – Multiple Spot Heights Between Contours C2 to F2 Distance between Contour 100.800 & 100.600 along Grid 2 = 125mm (Note that may change for you depending what scale the document is printed at) Rise/Fall per mm = (100.800 – 100.600) ÷ 125 = 0.0016m This means that for every 1mm measured the ground changes +/- 1.6mm.

9. Grid C2 Distance to From C2 to Highest Contour = 18mm measured from the 100.800 contour. Change in height = 18 x 0.0016 = 0.029m Rise/Fall = as we a heading away from the 100.800 contour towards the 100.600 contour it must be a fall. Spot Height = 100.800 – 0.029 = 100.771 18mm

10. Grid D2 Distance to From D2 to Highest Contour = 43mm measured from the 100.800 contour. Change in height = 45 x 0.0016 = 0.72m Rise/Fall = as we a heading away from the 100.800 contour towards the 100.600 contour it must be a fall. Spot Height = 100.800 – 0.072 = 100.728 45 mm x100.728

11. Grid E2 Distance to From D2 to Highest Contour = 43mm measured from the 100.800 contour. Change in height = 72 x 0.0016 = 0.115m Rise/Fall = as we a heading away from the 100.800 contour towards the 100.600 contour it must be a fall. Spot Height = 100.800 – 0.115 = 100.685 72 mm x100.728x100.685

12. Grid F2 Distance to From D2 to Highest Contour = 43mm measured from the 100.800 contour. Change in height = 98 x 0.0016 = 0.157m Rise/Fall = as we a heading away from the 100.800 contour towards the 100.600 contour it must be a fall. Spot Height = 100.800 – 0.157 = 100.643 98 mm x100.728x100.685x100.643

13. Heights Not Between Contours Grid Point B5 Interpolation of Contours assumes the ground level is in straight lines see Section A - A

14. Section A - A We can review this principle by determining the height of Grid Point B4. = 100.600 – ((100.600 – 100.400) x (12/30)) = 100.600 – 0.080 = 100.520

15. Distance between Contours = 30 Distance from Higher Contour to Grid Point = 48 We can determine the spot height for Grid B5 as follows = 100.600 – ((100.600 – 100.400) x (48/30)) = 100.600 – 0.320 = 100280

16. Grid F1 = 100.600 – ((100.600 – 100.400) x (-35/55)) = 100.600 – (-0.140) = 100.600 + 0.140 = 100.740 Spot Height = Higher Contour – ((Higher Contour – Lower Contour)) x (Distance from Higher Contour to Grid ÷ Distance between Contours))

17. 12.000 11.000 10.000 9.000 8.000 What is the Contour Interval ?

18. Garage - South West Corner

19. Garage – South East Corner

20. Garage – North East Corner

21. Can you see a problem !

22. Interpreting Contours The rise in elevation (0.750m) calculated is greater than the contour interval (0.500m) It would be unlikely that the survey was not conducted to the boundary line It is likely that the land plateaus and assume spot height of say 12.490

23. Garage – North West Corner