Why count stockpile volume every month
Stockpile volume is not geology, it is accounting. For aggregate quarries (Lafarge, Cemex, Górażdże, Strabag) monthly inventory has three drivers. First - month-end close in ERP: the gap between extracted and sold material must reconcile with the physical stock on the yard, otherwise the Big Four auditor issues a qualified opinion. Second - working capital financing: banks lend against actual inventory and the quarterly reconciliation is run off a survey. Third - business sale: due diligence on a processing plant always requires independent stockpile measurements for the last 12 months.
In numbers, a 5 percent error on an 80,000 tonne pile of quartz sand at 42 PLN per tonne is a 168,000 PLN discrepancy in a single balance sheet line. One such gap and the financial controller demands recounts with tolerance no higher than 2 percent. That is where the total station era ends and the RTK-anchored drone survey begins.
Photogrammetry and LiDAR - what to pick and when
Two technologies solve the same task - building a digital surface model (DSM) of the pile - but with different accuracy, price and constraints. Photogrammetry uses a series of overlapping RGB images from a drone, processed in Agisoft Metashape, Pix4DMapper or DJI Terra. LiDAR is an active scanner sending laser pulses that sees the ground even through light vegetation.
| Parameter | Photogrammetry | LiDAR |
|---|---|---|
| Volume error | 1-3 percent | 0.5-1.5 percent |
| Vegetation on pile | Inflates volume | Filters out vegetation |
| Snow on pile | Cannot resolve surface | Penetrates upper layer |
| Coverage speed (20 ha) | 3-4 flight hours | 2-3 flight hours |
| Price per visit | 2,800-9,500 PLN | +50-80 percent over photogrammetry |
| Equipment | DJI Mavic 3E or Phantom 4 RTK | DJI Matrice 350 + Zenmuse L2 |
Practical choice: for aggregate, sand, gravel and crushed stone on an open yard - photogrammetry. The surface is high-contrast, no vegetation, the 1.5-2 percent error fits audit requirements. For coal, biomass, compost or road salt under a canopy or fringed with grass - LiDAR. LiDAR is also mandatory for winter inventory at quarries when the pile carries 5-20 cm of snow and photogrammetry over-reports volume.
RTK, GCPs and the certified surveyor's stamp
Accuracy of 0.5-1.5 percent does not come out of the drone box. A standard DJI Mavic 3E without RTK has a positioning error of 1.5-3 metres, which on an 8-metre pile turns into an 8-12 percent volume error. Serious inventory requires three components.
- RTK base on the quarry. A dedicated Trimble R12 or Leica GS18 base tied to the geodetic control network (PL-2000 datum) gives centimetre-grade drone position throughout the flight. Alternative - network RTK via ASG-EUPOS, but with latency and no guarantee of connectivity in a pit below ground level.
- GCPs (Ground Control Points). Six to twelve check points around the perimeter and at the centre of the area, marked with 60x60 cm aerial targets, whose coordinates are captured by total station or GNSS. Without GCPs an RTK flight gives planar accuracy but vertical drift of 5-10 cm per hectare.
- Certified surveyor (geodeta uprawniony) sign-off. For a report accepted by an auditor, a bank or a buyer of the business, the stamp of a geodeta uprawniony with the right scope (1 or 2) is needed. VisionAir works in tandem with a Warsaw surveying firm - our pilot flies, their certified surveyor signs the final operat with earthwork volume calculations.
Workflow of a single inventory day
- Day minus 2: alignment with the plant operations manager (kierownik ruchu zakładu). Receiving the plant map, pit layout, haul truck schedule. BVLOS notification to PAŻP if working above 120 m AGL or beyond VLOS - not required for most yards.
- Day minus 1: weather check. Rain, wind above 12 m/s, visibility below 5 km - reschedule. Photogrammetry fails on a wet pile with reflections and under flat overcast light. LiDAR is light-agnostic but not rain-agnostic - droplets distort the point cloud.
- Day 0, morning: GCP placement one hour before flight. Coordinates captured by total station, logged in the report.
- Day 0, flight: 60-80 m AGL for photogrammetry, 50-70 m for LiDAR. Frontal overlap 80 percent, side overlap 70 percent. Speed 6-8 m/s. For 20 ha - 3-5 hours with two battery swaps.
- Day 0, evening: image upload to cloud, preliminary DSM in DJI Terra Cloud for coverage QC.
- Day plus 1: final processing in Metashape (photogrammetry) or TerraScan (LiDAR). Filtering vegetation and equipment points, building DTM, computing each pile volume against a base plane or against the previous measurement.
- Day plus 2: operat with the certified surveyor's stamp, JSON export to client ERP, browser-based 3D viewer for the financial controller.
VisionAir 2026 inventory pricing
| Site type | Area | Price per visit | Cadence |
|---|---|---|---|
| Small aggregate yard | up to 5 ha | 2,800-4,500 PLN | One-off or 1x/quarter |
| Medium processing plant | 5-20 ha | 4,500-7,500 PLN | 1x/month |
| Large quarry | 20-60 ha | 6,500-12,000 PLN/visit | Monthly subscription |
| LiDAR add-on | Any | +50-80 percent | On demand |
| Certified surveyor sign-off | Any | Included in package | Every report |
A monthly subscription cuts 15-25 percent off the one-off visit price. For KGHM, Lafarge or Cemex, running 6-12 yards in parallel, we quote a separate framework agreement with a fixed hectare rate (from 380 PLN/ha for photogrammetry, from 620 PLN/ha for LiDAR).
Case study: processing plant outside Warsaw, 28 ha, 11-month subscription
- Client: mid-size crushed aggregate production plant, Wyszków area. 28 ha of yards, 9 active piles, daily turnover 4,200 tonnes.
- Before VisionAir: surveying crew once a quarter, 4-day measurement, one quarter of the pit halted during survey. Quarterly cost 18,000 PLN plus 23,000 PLN of lost revenue from downtime.
- With VisionAir since February 2025: drone visit once a month, 6,800 PLN per visit, no pit halt. LiDAR visit once a quarter (for piles fringed with vegetation) - additional 4,100 PLN.
- Result over 11 months: the inventory gap fell from 4.1 percent (total station) to 1.3 percent (photogrammetry with GCPs), which let the bank raise the credit line by 2.4 million PLN against actual stock.
- Side effect: change detection between monthly measurements exposed a systemic under-feed of haul truck output onto pile 7 (-2.8 percent from plan), caused by a faulty weighbridge at the chute, fixed in 3 days.
Before the drone my month-end was a standing compromise with reality. A total-station survey once a quarter, and in between - bookkeeping by haul truck plan. The auditor asked for higher accuracy every year, but the cost of a quarterly survey would not let us run it more often. The RTK drone with LiDAR closed that gap. Now I have stockpile data in JSON on the 5th of every month, 1.3 percent gap, the auditor sleeps. Subscription payback - 2.5 months.
Frequently asked questions
- What is the acceptable error for an accounting stockpile inventory?
- The Big Four standard (PwC, KPMG, EY, Deloitte) for audits of quarries and processing plants - no higher than 2 percent of pile volume for material below 50,000 tonnes and no higher than 1.5 percent above. Photogrammetry with RTK and 6-12 GCPs delivers 1-2 percent steadily, LiDAR - 0.5-1.5 percent, both with margin. A total station gets 0.3-0.8 percent on a single pile but, sampling only 8-15 points per pile, accumulates 3-5 percent integrated error across a yard with 10+ piles.
- Can you scan a pile under a canopy or inside a closed warehouse?
- No, the drone needs line of sight and a GPS signal, neither of which is available under a roof. Alternatives for covered storage: terrestrial laser scanner (Faro Focus, Leica RTC360) from 4-8 stations - 0.8-1.2 percent accuracy, cost comparable to LiDAR drone, but 2-3 times slower. If a pile is partly covered (e.g. a silo with a half-open roof) we scan the open part with the drone and the covered part on the ground, then merge into a single report.
- How long does a 20 ha yard take and when do I get the report?
- Flight - 3-5 hours with one pilot and a battery swap every 35-40 minutes. GCP placement - an additional 60-90 minutes before flight. Image processing in Metashape on an RTX 4090 workstation - 12-18 hours for photogrammetry, 6-10 hours for LiDAR. Final report signed by the certified surveyor - on the second working day after the survey. A 24-hour rush is possible, +35 percent surcharge.
- Does VisionAir sign off as a certified surveyor?
- Not on our own - our pilots hold UAVO A1/A3 and A2, NSTS-01/05 and BVLOS, but not geodetic licences. We work in a fixed partnership with a Warsaw surveying firm, and their geodeta uprawniony with scope 1 (geodetic control) and 2 (boundary work) signs the final operat. The signature is included in the package, no extra fee. If you have your own certified surveyor we hand over the point cloud and DSM in LAS/GeoTIFF and they build the report themselves.
- Can you survey in winter with snow on the piles?
- Photogrammetry does not work in winter: snow removes contrast, the algorithm fails to resolve the surface and over-reports volume by 5-12 percent. LiDAR does work - the laser pulse penetrates 8-15 cm of upper snow layer and returns points from the pile itself. For winter quarry inventory we switch fully to LiDAR (Zenmuse L2 on DJI Matrice 350), the price goes up but the error stays inside the 1-2 percent band. The alternative - rescheduling to a thaw - is rarely an option for a December close.
