Does a stockpile survey give tonnage?

It gives volume. Tonnage is derived from that volume using a density figure, and that density has to come from somewhere else — sampling, laboratory work or an established operational value. The survey measures the shape of the pile. It does not measure what the material weighs.

Why does the volume change when nothing was moved?

Most often because the base surface changed. Volume is the space between two surfaces, so altering the assumed base alters the answer even when the pile itself is untouched. The other common cause is a change in capture method between measurements.

Which figure is correct when survey and production records disagree?

Neither is automatically correct, because they are not measuring the same thing. The useful question is where the difference comes from: the survey geometry, the density conversion, material moved between the survey and the reporting cut-off, or moisture. Working through those in order usually locates it.

Can the same method be used every month?

It should be, where the numbers matter commercially. Changing capture method between measurements introduces differences that are not real material movement, and those differences can be larger than the change being measured.

Survey Volume and Declared Tonnage: Why They Disagree

Survey volume and tonnage are different measurements, and treating them as interchangeable is the most common cause of stockpile reconciliation disputes. A survey measures geometry. A tonnage figure applies an assumption to that geometry. When the two disagree, the disagreement is usually in the assumption rather than in the measurement.

This matters because the numbers are used commercially. Month-end stockpile figures inform reporting, inventory and sometimes payment. When they are questioned, the survey is usually questioned first — often incorrectly.

What a volumetric survey actually measures

A volumetric survey measures the space between two surfaces.

The upper surface is the pile as it stands. That can be captured by GNSS, total station, laser scanner or aerial photogrammetry, depending on size, access and required accuracy.

The lower surface is the ground beneath the pile. This is the part that causes trouble, because during the survey it is not visible. It has to come from an earlier yard survey, a design surface, or an agreed assumption.

The volume is then computed between those two surfaces, within a defined boundary.

Three things are being decided rather than measured: where the pile ends, what sits beneath it, and what is excluded. Each is an input. Each can be changed after the fact, and changing any of them changes the answer without a single grain of material moving.

Where tonnage comes from

Tonnage is not measured by survey. It is derived.

The conversion needs a density figure for the material in its current condition. That figure comes from sampling, from laboratory determination, or from an established value the operation already uses.

Density is not a constant. It varies with the material itself, with how compacted the pile is, with particle size distribution, and with moisture. A pile after heavy rain does not have the same bulk density as the same pile in dry conditions.

So a tonnage figure carries two sources of uncertainty: the geometric measurement, and the conversion applied to it. The second is frequently the larger of the two, and it is usually the one nobody has checked.

Why the two figures disagree

When measured volume and declared tonnage do not reconcile, the difference usually sits in one of four places.

The base surface. If the assumed ground beneath the pile is wrong, or has changed since it was surveyed, the volume is wrong by that amount across the whole footprint. On a large pad, a small error in the base is a large error in the total.

The density conversion. An operational density value that was established years ago, on different material, in different conditions, will not hold. It is rarely revisited because it is rarely visible.

Material movement. Survey happens at a moment. Production reporting covers a period. If material moved between the survey and the reporting cut-off, the two describe different piles.

Capture method. Comparing a surface captured by drone against one captured by GNSS introduces differences that belong to the methods, not to the material. Point density, coverage of steep faces and edge definition all differ.

What to do about it

The practical answer is to record the inputs alongside the output.

A volume figure on its own is not defensible. A volume figure with the survey date, the pile boundary, the base surface used, the capture method and the exclusions applied can be checked by anyone, including in a year's time.

The same applies to tonnage. If a density value has been used, the report should say what it was and where it came from. A derived figure presented as a measured one is how disputes start.

For recurring measurement, keep the method, the boundaries and the pile identifiers consistent between periods. Consistency matters more than absolute accuracy when the purpose is to track change.

Where survey cannot help

Survey measures the pile that exists at the time of survey.

It cannot tell you what the material weighs. It cannot recover a base surface that was never captured. It cannot resolve a disagreement caused by material moving after the observation.

What it can do is measure the geometry accurately, state the assumptions clearly, and make the difference between the measured and the derived visible. In most reconciliation disputes, that is enough to locate where the discrepancy actually is.

If you need an independent view on figures that are being questioned, Survey House is regularly appointed to verify survey performed by others.