Stockpile inventory for quarries and aggregates by drone

Stockpile inventory is the measurement a quarry gets judged on. It sets the value of stock on the books, it is the number the auditor tests, and when it is wrong by a few percent across a yard of twenty piles the write-off is real money. A drone makes the measurement fast and complete. Making it defensible is a matter of doing the same few things the same way each month, and writing them down.

Why drones took over the month-end count

The older methods each had a gap. Walking the piles with a GPS rover took a surveyor a day, measured a few hundred points per pile, and could not safely reach the top of anything steep. Counting truckloads (production less sales, by weighbridge) is the book figure itself, not a check on it. A drone flight of a 30 acre yard takes under half an hour, measures every pile to a few centimetres across millions of points, and leaves a picture of the yard on that date that can be re-measured years later. The flight is the easy part; the rest of this guide is the rest.

The monthly routine

1. Fly on the close date, after the last sale and before the first load of the next period. Same height and overlap every month, nadir camera, the yard still: no loaders working the piles. (Flight settings.)
2. Position it the same way every month. Ground control targets left in place around the yard, or RTK/PPK on the drone with three or four checkpoints. Month-on-month comparison needs the same datum each time more than it needs absolute accuracy. (What each method gives you.)
3. Process to a surface model. The DSM, not the terrain model: a terrain model has the piles filtered out. (The three files.)
4. Measure each pile against the right base, with the boundary on the ground around its toe. Reuse last month's boundaries where the pile has not moved; redraw where it has. (How the base is chosen, and why it matters most.)
5. Convert to tonnes with a density per product, from a measured source.
6. Reconcile to the book inventory and explain the variance, pile by pile.
7. File the evidence: the flight, the boundaries, the bases, the densities, the reconciliation, in one place that can be reopened.

The yard's particular problems

Piles that merge

Two products pushed together over a month become one heap with a ridge somewhere in it. The volume of the heap is fine; the split between products is a judgement. Draw the boundary along the ridge, note that it was a judgement, and ask the yard to keep a gap. Better still, measure what each product's pile gained and lost against last month's flight, which does not care where the ridge is.

Piles against walls, in bins, on old stock

A pile against a retaining wall has no toe on that side; the base there is the wall's foot, which the camera cannot see. Use a fixed level or corner levels for the bin floor from the drawings, and keep the wall out of the boundary. A pile that has sat on the same spot for years is sitting on compacted old stock, and the "ground" under it is a decision: agree it once, with the auditor if need be, and use the same base every month.

Density

Volume is measured; tonnage is volume times a number nobody measured this month. Crushed stone products range from about 1.4 to 1.7 t/m³ loose and rise when wet; sand and fines move most with moisture. A density table with one value per product, reviewed yearly against a weighed sample or a pile built from weighed loads, is the minimum. Record which value was used for each pile each month.

The material in motion

Trucks loaded but not yet across the weighbridge, product on the belt, the load in the loader bucket. Fly when the yard is still and the cut-off is the flight time, and say so.

What the auditor is looking for

A measurement that lives in a spreadsheet with a screenshot answers the first and last of these badly. One that lives with the flight it was taken from, with the base and the boundary attached, answers them all. Clarity keeps every volume with its flight, prints the base and the area measured beside it, takes a density per measurement and prints the tonnes, puts every month on one timeline so last month's pile is a click away, and exports the lot as CSV or Excel. (Stockpile volumes in Clarity.)

A worked month

A 25 acre aggregate yard, 18 piles, flown at 80 m with RTK at 7 a.m. on the last day of the month, 310 photos, 14 minutes. Checkpoints agree to 2 cm vertically. Every pile measured against a best-fit plane through its toe, except two in concrete bins measured from the bin floor level. Densities per product from last spring's weighed samples. Total 142,600 t against a book figure of 146,900 t: a variance of 2.9%, of which 2,100 t is traced to a merged pile of two gradings split by eye, 1,400 t to fines valued at their dry density after a wet week, and the balance unexplained and written down. Next month the two gradings have a gap between them.