Surveying & Mapping

Volumetric Survey Drone Pilots

A volumetric survey measures stockpiles, excavations and voids by building a model of the surface from overlapping aerial images. The output is a volume figure with the survey behind it, used for stock reporting, earthworks and reconciliation. Turning a volume into a weight depends on a density figure the survey does not supply.

What this service involves

A volumetric survey measures how much material is in a pile, a void or an excavation. The aircraft photographs it on a planned pattern, the imagery is processed into a surface, and the volume is calculated as the space between that surface and a base surface underneath it. Stockpiles at quarries, aggregate yards, waste transfer stations and ports are the recurring case. Cut and fill on an earthworks site, and the remaining airspace in a landfill cell, are the same measurement in a different setting.

The base surface is the part worth understanding before commissioning anything, because it is where a volume figure is decided rather than measured. The top of a pile is photographed. The ground underneath it is not, because the pile is on it. So the base has to be established some other way, either from a survey of the ground taken before the material arrived, or by fitting a surface to the toe of the pile where it meets the ground. That is a judgement, it changes the answer, and a volume quoted without saying which base was used cannot be checked or repeated.

A volume is also not a tonnage. Converting one into the other requires a bulk density for the material, and bulk density is a measured property with a published test method of its own, BS EN 1097-3 in the case of loose aggregate. A tonnage quoted without stating the density applied, and where that density came from, cannot be verified separately from the survey, which means an error in the conversion is indistinguishable from an error in the flying.

Who hires drone pilots for this, and why

Quarry and aggregate operators reconciling stock, commonly at month end or year end, where the resulting figure goes into an account somebody signs.
Waste and recycling operators, who have a commercial reason and usually a regulatory one to know what is held on site.
Earthworks and civil contractors measuring cut and fill against what was priced, where the difference between the two is the commercial position.
Ports, terminals and bulk handlers storing material that belongs to somebody else.
Auditors, lenders and purchasers, for whom the value is in the measurement being independent of the operator rather than in the number itself.

What to expect on the day

The site normally keeps working around the flight, with one exception that matters. A pile being loaded or tipped while it is being photographed will not reconstruct correctly, because the method depends on finding the same point in several photographs taken moments apart. Expect to be asked to leave the piles being measured undisturbed for their part of the visit, and to have the toe of each pile visible, which means material not pushed hard against a wall where the base cannot be seen at all.

Accuracy is a stated tolerance rather than a property of the flight, and there is a published expectation to hold it against. Work published in the International Archives of the Photogrammetry, Remote Sensing and Spatial Information Sciences records that legislation covering mine engineering often requires volumetric calculations to be within three percent of the whole amount. That is what that literature reports about the regulatory expectation in mining, and it is a reasonable thing to ask a surveyor to state a position on rather than a figure to assume.

Deliverables are normally a volume for each pile with the base surface method stated alongside it, the date of the survey, the surface and point cloud the figures came from, and a plan identifying which pile is which. Where a tonnage is wanted, expect the density figure and its source to be printed next to it, so the conversion can be checked separately from the measurement.

Volumetric Survey: frequently asked questions

Why does the answer depend on the base surface?

Because a volume is the space between two surfaces and only one of them was photographed. The ground under a pile cannot be seen while the pile is on it, so the base is either taken from an earlier survey of the bare ground or fitted to the toe of the pile. Different choices give different answers from identical imagery, which is why the method used belongs on the report.

Is the figure in cubic metres or in tonnes?

Cubic metres is what is measured. Tonnes is derived from it using a bulk density for the material, and bulk density is itself a measured property with a published test method. Ask which unit is being quoted and, if it is tonnes, which density was applied and where it came from.

Can you measure a pile that is under cover?

Not from the air, because the camera has to see the surface. Covered stores, sheds and enclosed silos are measured by other means, commonly laser scanning from the ground, and that is a separate instruction rather than a variation on this one.

How often should stock be measured?

It depends on what the figure is for. Monthly suits a reconciliation that feeds an account, annually suits an audited position, and ad hoc suits a discrepancy that has already appeared. Repeat surveys flown the same way compare more reliably against each other than any single survey compares against a book figure.

Does the site have to stop working?

The site does not, but the piles being measured do. Anything moving while it is being photographed will not reconstruct properly, so the usual arrangement is that loading moves away from the piles being flown for the length of that part of the visit.

Volumetric Survey on Terasor

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