We compute volume three different ways:
- The simplest is the difference between the elevation models of two surveys.
- A slightly more complex calculation is "Lowest Point," where we take the lowest point on a given boundary and calculate the volume relative to a level plane at that height.
- The most complex is "Best Fit," where we create an estimate of the original surface given the elevation along the boundary.
Survey to Survey Comparison
Computing volume relative to a prior (baseline) survey is conceptually simple. An aerial survey gives us an elevation model containing several hundred data points per square yard of ground surface. We subtract the elevation from the baseline survey from the subject survey at each of these points, add up all the differences, and multiply by the area of each point. While the computation may be simple, preparing the data such that the survey elevation models align both vertically and horizontally is crucial, and we have developed tools to make this relatively easy. Surveys compared in this way must be staked to monuments with elevation data. This method is useful for examining change over time.
Volume Based on Lowest Point
Computing volume based on the lowest point of the boundary is similar to the case above, but instead of a prior survey, we take the baseline elevation of each point as the elevation of the lowest point on the boundary. This method can be applied to surveys that have not been staked or otherwise corrected. This method is useful for stockpile inventory in a flat yard.
Volume Based on Best Fit
Computing volume based on the estimated original surface – "Best Fit" is similar to both of the above. It is similar to a baseline comparison in that each point has a different elevation, and it is similar to lowest point in that the elevation at each point is computed from the elevation on the boundary and that it does not require the survey to be staked. We estimate the elevation of the original surface as a weighted average of the elevation of the points on the boundary. This method is useful for estimating cuts and fills in uneven terrain when a baseline survey is not available.
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