Spot Heights from Contours

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

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

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.

Step 2

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 = – ( – ) x 18 ÷ 29 = – =

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

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

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

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

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

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

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

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

Grid F1 = – (( – ) x (-35/55)) = – (-0.140) = = Spot Height = Higher Contour – ((Higher Contour – Lower Contour)) x (Distance from Higher Contour to Grid ÷ Distance between Contours))

What is the Contour Interval ?

Garage - South West Corner

Garage – South East Corner

Garage – North East Corner

Can you see a problem !

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

Garage – North West Corner