— by Steve G —
That is a good question and one that is relatively easy to answer if you have access to LIDAR data within a reasonably-featured GIS (Geographic Information System) e.g. QGIS. Let’s start by finding a typical pit…
This particular one (circled in the LIDAR image) has a nice oval shape and is about 20m across the widest dimension. It is one of a cluster located in Burtt’s Harley, an ancient woodland a couple of kilometres North of Gussage All Saints. (The better preserved pits are often located in woodland since they will have escaped modern day ploughing.)
First of all the contours are extracted from the DTM LIDAR layer – in this case by specifying a contour interval of 25cm. The resultant contour map allows us to see the boundaries of the pit much more clearly and it is then simply a case of drawing a polygon around the pit extent using the area calculation tool built into the GIS.
In this example the area of the polygon (pit) is 155 square meters.
That is pretty large – more than twice the area of standard badminton court! So how do we calculate the volume?
Well in the diagram you can see that the area within the boundary of the pit has been coloured based upon the depth of the pit – with the deeper parts showing as warmer colours. The dimension of each square of colour is actually 1m x 1m since the visualisation has been created using LIDAR data produced by the Environment Agency which usually has a resolution of 1m.
Therefore if we calculate the volume beneath each of the squares and add them all up we should get a reasonable estimate of the volume of the pit as a whole.
pit volume = sum of (volume of each square i.e. 1 x 1 x depth in m below ground level)
Fortunately this can also be calculated by a function built into the GIS. You simply have to specify the contour line that is closest to the rim of the pit (in this example 101.75m) and specify the extent of the pit (the polygon with dashed lines).
And the answer is…. 96 cubic meters.
Or put another way about a thousand wheelbarrow loads. That’s a lot of digging!