01The challenge
The operator's finance team closed stock each month from a mixture of production estimates and occasional walked GNSS surveys of the largest piles. Discrepancies between book stock and physical stock were only discovered at year-end, when a full survey was commissioned for the auditors, and write-downs were difficult to explain.
- Walking a GNSS pole over stockpiles put surveyors on unstable slopes close to operating plant.
- Piles were irregular, frequently reshaped by loading shovels, and some sat against bay walls or on uneven ground.
- Month-end figures were needed within two working days for the management accounts.
02Our approach
We established six permanent ground control targets on concrete bases around the stocking area, coordinated to OSGB36/ODN, plus four checkpoints re-observed every visit. Each month, on the last working day, a DJI Matrice 4E captured nadir and oblique imagery during a planned pause in loading, coordinated with the quarry manager under the site's permit-to-work system.
| Item | Specification |
|---|---|
| Aircraft | DJI Matrice 4E with RTK |
| Flight height | 80 m AGL, 2.2 cm GSD, 80/75% overlap plus oblique pass for steep faces |
| Control | 6 permanent GCPs, 4 checkpoints re-observed monthly |
| Typical checkpoint RMSE | 12–16 mm XY, 18–24 mm Z |
| Base surfaces | Pre-construction ground model for stocking pads; triangulated toe for piles on hardstanding |
| Tonnage conversion | Client-supplied bulk densities per product, reviewed quarterly |
Each pile was given a persistent ID and boundary so that month-on-month comparisons were like-for-like. Where a pile was stored in a walled bay, the bay floor and walls were surveyed once and used as the base surface. Volumes were computed from the photogrammetric surface model against the agreed base, and a short method statement accompanied every report so auditors could follow how figures were derived.
03The outcome
Over the first twelve months, measured stock reconciled to within ±1–2% of the figure derived from opening stock, production and weighbridge sales for all major products. Larger variances were traced to specific causes, such as an incorrect bulk density for one scalpings product and material moved between piles without being recorded.
- Year-end stock figures were accepted by the external auditors using the monthly survey reports as supporting evidence.
- Nobody now needs to climb stockpiles to measure them.
- Time on site dropped from two days of walked survey to around two hours of flying, with loading paused for less than an hour.
- The monthly orthomosaic doubles as a record of haul road condition, water management and restoration progress.
04Deliverables
- Monthly stockpile report with volume and tonnage per pile (PDF, XLSX)
- Month-on-month change summary per product
- Orthomosaic of the stocking area (GeoTIFF)
- Surface model and pile boundaries (LandXML, DXF, SHP)
- Annual audit pack with method statement and control records
