Stockpile volume measurement is only useful month to month if each survey is taken on the same footing as the last. A stockpile survey that changes its method without saying so produces a jump that looks like material movement, and nobody can tell the two apart afterwards.
Quick Answer
Two stockpile volumes can only be compared directly when both surveys used the same extent of the pile and the same base surface treatment. Hold the capture method constant too, and record whether the pile was being worked. When a monthly figure jumps, check those four things before concluding that material moved. If all four match, the change is more likely to be real.
The question usually arrives as a surprise: the monthly figure moved, and nothing on site seems to explain it. This article is about keeping repeat surveys comparable. Why a survey gives volume rather than tonnage is covered in our article on survey volume and tonnage.
What keeps stockpile volume measurement comparable
Four things decide whether two stockpile volumes can be read against each other: the extent captured, the base surface, the capture method and the state of the pile.
| Hold constant | Why it matters |
|---|---|
| The extent of the pile captured | A larger or smaller footprint changes the volume on its own |
| The base surface treatment | A different base changes the volume with no material moving |
| The capture method | Different methods and point densities can disagree on the same surface |
| A note of the pile's state | A pile being worked during capture is not the pile at rest |
Extent
A stockpile volume depends on how much of the pile was captured. If this month's survey covered an edge that last month's did not, the figures are not measuring the same thing. Record the footprint each time, and use the same one.
Base surface
The volume is measured against a base surface. If that base changes between surveys, the volume changes with it, even if no material moved. Choose the base once, record it, and keep it unless there is a documented reason to change. Where the original ground was never surveyed before the pile was placed, agree how the base will be modelled and use that same model every month.
Method
Surfaces captured by different methods, or at different point densities, can differ in ways neither surface shows on its own. Where volumes matter commercially, keep the method consistent between surveys, whether that is aerial capture, GNSS or another method.
State of the pile
Note whether the pile was being built, reclaimed or reshaped while it was captured. A pile surveyed during loading is a moving target, and the note explains a figure that otherwise looks odd.
Key point
Hold extent, base surface and method constant, and record the state of the pile. Then a change in the figure can be read as a change in the material.
Monthly figures that will not settle?
Send us two months of survey reports. We will check whether they are comparable.
Ask us to compare the surveysWhen the change is real
Material can be added to, removed from, or reshaped on a stockpile between two survey dates, and that movement can explain part or all of a change in volume.
Once the measurement has been checked, look at what happened to the pile. Was material delivered or reclaimed? Was the pile pushed up or reshaped? Were loaders working on it during the survey? A change explained by movement is not a survey problem, and it should not be treated as one. Ask the people who work the pile; they often know about a delivery or a reclaim that never reached the records.
Checking movement does not replace checking the measurement. Both questions need an answer. If the survey basis matched and the pile was confirmed static, a remaining difference needs a closer look. That is the point where our stockpile reconciliation procedure starts.
A documented change is also legitimate. An extended footprint, a corrected base surface, or an area that became accessible can all explain a difference. What matters is that the change is written down with the survey that introduced it, so the next reader does not mistake it for movement.
A simple monthly record makes all of this routine. For each pile, keep the date, the footprint, the base surface, the method and the state of the pile. Stockpile volume measurement then becomes a comparison of like with like, and an unusual month stands out for the right reason. Without the record, every jump starts a new argument about whether the survey changed.
Key point
Stockpile volume measurement answers two questions in order: did the measurement change, and did the material move? Answer the first before the second.
Setting up a monthly survey routine?
Send us the piles and the schedule. We will propose a method you can repeat.
Get a stockpile survey quoteWhere survey cannot help you
Consistent stockpile volume measurement makes month-to-month figures comparable, but some questions sit outside what any volume survey can answer.
A survey cannot record movement nobody noted, and it cannot confirm a base surface that was never surveyed. It measures volume, not mass, so a tonnage depends on a density that comes from sampling, not from the survey. On larger operations, the same discipline applies to opencast work and to wider volumetric surveys. The Southern African Institute of Mining and Metallurgy is a useful source for the wider mineral resource practice that stockpile figures feed.
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Stockpile surveys repeated on the same footing each month, so a change in the figure means a change in the pile.
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