Stockpile measurement: GNSS, drones and laser scanning compared
Key takeaways
- Stockpile volume equals the space between a measured top surface and a defined base surface.
- Point density drives accuracy: laser scanning and drones capture far more detail than a GNSS walking survey.
- The base surface and bulk density usually cause more error than the survey instrument.
- Indoor piles in sheds and domes suit terrestrial or handheld SLAM scanning; open yards suit drones and GNSS.
- Consistent methods and schedules make stockpile volume management reports comparable month to month.
What is stockpile measurement?
Stockpile measurement is the process of surveying a pile of bulk material to calculate its volume and, from that, its tonnage. Quarries, mines, ports, power stations, cement plants, recycling yards and agricultural stores all depend on it for inventory, production reporting and financial audits.
A stockpile survey, sometimes called a material stockpile survey, captures enough points on the pile surface to model its shape. Software then compares that surface with a base surface, the ground on which the pile sits, and reports the volume between them. The results feed stock reconciliation: comparing measured stock with what weighbridges and production records say should be there.
How is stockpile volume calculated?
Stockpile volume is calculated by building a 3D surface of the pile and subtracting a base surface. The two common methods are the TIN method and the grid method.
- TIN (triangulated irregular network) – measured points are joined into triangles, and the volume is summed prism by prism between the top and base surfaces.
- Grid method – both surfaces are sampled on a regular grid, and each cell's height difference is multiplied by its area.
Choosing the base surface
The base can be a pre-pile survey of the pad (most accurate), a plane fitted through points around the toe, or a surface interpolated from the toe line. On sloping ground, a flat base plane can cause large errors.
Volume to tonnes
Tonnage equals volume multiplied by bulk density. Density varies with moisture, grading and compaction, so it should be measured periodically rather than taken from a table.
Stockpile calculator: estimating stockpile volume by formula
For quick estimating of stockpile volume, simple geometric formulas give a rough figure. They are useful for planning but not for audit-grade reports.
| Pile shape | Formula | Notes |
|---|---|---|
| Cone | V = π × r² × h ÷ 3 | r = base radius, h = height |
| Cone from angle of repose | r = h ÷ tan(α) | α = material's angle of repose |
| Elongated (windrow) | V ≈ cross-section area × length + end cones | Assumes uniform cross-section |
Example: a conical pile 6 m high with a 10 m base radius gives V = 3.1416 × 100 × 6 ÷ 3 ≈ 628 m³. Real piles are rarely perfect cones, and loader activity, slumping and multiple peaks quickly break these assumptions. That is why stockpile quantity estimation for reporting uses surveyed surfaces.
Stockpile measurement methods compared
Five methods dominate stockpile volume measurement. The best choice depends on pile size, site access, safety rules, indoor or outdoor storage and how often you measure.
| Method | Point density | Typical volume accuracy | Strengths | Limits |
|---|---|---|---|---|
| GNSS RTK walking survey | Low (tens to hundreds of points) | Several percent, depends on spacing | Low cost, simple, georeferenced | Walking on piles can be unsafe; misses detail |
| Total station | Low to medium | Similar to GNSS | Works indoors and reflectorless | Slow on large piles |
| Drone photogrammetry | Very high | Typically 1–3 % | Covers whole yards quickly, no climbing | Airspace rules, weather, not for enclosed sheds |
| Terrestrial laser scanning | Very high | Typically around 1 % | Indoors or outdoors, high detail | Several setups needed for large or shadowed piles |
| Handheld SLAM scanning | High | Typically 1–2 % | Fast walk-around, good in sheds and domes | Needs features for tracking; check against control |
Accuracy figures are typical and depend on pile shape, base definition and method. For broad comparisons of these technologies see terrestrial laser scanning and RTK drone surveying.
How to do a stockpile survey with GNSS RTK
A GNSS RTK stockpile survey is a low-cost way to measure outdoor piles when they are safe to access. The method relies on capturing every break of slope.
- Check the rover on a known control point and confirm RTK fixed status.
- Walk the toe around the pile, recording points every few metres and at every change in direction.
- Record breaklines such as crests and ridges with continuous or closely spaced points.
- Fill the surface with points on a regular pattern, tighter where the shape changes.
- Survey the base or use an existing pad survey.
- Process the points into TIN surfaces and compute volume.
A receiver with IMU tilt compensation, such as the E1 GNSS RTK receiver (tilt up to 60°), lets the operator record toe points without levelling the pole on loose material. Never climb an unstable pile; scanning from the ground is safer.
Laser scanning for stockpiles in sheds, domes and yards
Laser scanning measures millions of points from the ground, so it is ideal where piles are inaccessible, enclosed or irregular. Drones struggle indoors, and walking surveys are unsafe on loose material.
- Terrestrial scanners placed around a pile capture the full surface in a few setups. Registration joins them into one point cloud.
- Handheld SLAM scanners record continuously as the operator walks around the pile or through a storage shed, often in minutes.
- Control targets measured by GNSS or a total station tie scans to site coordinates and allow month-to-month comparison.
The Mars Geosystems handheld SLAM laser scanner (0.1–70 m range, ≤ 1 cm post-processed point-cloud thickness) suits sheds and yards, while the terrestrial 3D laser scanner covers large piles from fewer positions.
Cut and fill excavation volumes use the same principle
Cut and fill excavation volumes are calculated in the same way as stockpiles: by comparing two surfaces. Instead of a pile against its base, an existing ground surface is compared with a design or later surface.
Areas where the new surface is lower are cut; areas where it is higher are fill. Earthworks contractors use repeated surveys to measure progress, verify payment quantities and balance material across a site. The same tools, GNSS RTK, drones and laser scanners, serve both tasks, so one equipment investment often covers stockpiles and earthworks.
Sources of error and how to reduce them
Most volume errors come from definitions and density, not from the instrument. Address these before upgrading equipment.
- Base surface – survey pads before stockpiling and reuse that surface.
- Toe definition – mark where the pile ends, especially where piles merge.
- Sparse data – increase point density on irregular piles.
- Bulk density – measure regularly and record moisture.
- Timing – measure when loaders are idle and at consistent times each month.
To discuss equipment for your stockpile volume management programme, request a quote.
Frequently asked questions
How do you measure a stockpile?
Survey the pile's surface with GNSS RTK, a total station, a drone or a laser scanner, capturing the toe, crest and surface points. Create a 3D surface, define the base surface underneath, and calculate the volume between them in survey software. Multiply the volume by measured bulk density to get tonnes.
What is the most accurate method for stockpile measurement?
Dense methods, terrestrial laser scanning, handheld SLAM scanning and drone photogrammetry, usually give the most accurate volumes because they capture the full surface. Terrestrial scanning is often around 1 % under good conditions. However, a poorly defined base surface or wrong bulk density can cause larger errors than the choice of instrument.
How often should stockpiles be surveyed?
Many operations survey monthly to match financial reporting, with extra surveys at year end or before audits. High-throughput sites such as ports or plants may measure weekly. The key is consistency: use the same method, base surfaces and timing each period so changes in stock are real rather than differences in method.
Can drones measure stockpiles indoors?
Standard drones rely on satellite positioning and good lighting, so they are poorly suited to enclosed sheds, domes and silos. Indoor stockpiles are usually measured with terrestrial laser scanners placed around the pile or with handheld SLAM scanners carried by an operator, tied to control points for repeatable comparisons.
How do you convert stockpile volume to tonnes?
Multiply the measured volume in cubic metres by the material's bulk density in tonnes per cubic metre. Bulk density depends on material type, grading, moisture and compaction, so measure it on site by weighing a known volume of material rather than relying on generic tables, and update it when conditions change.
Is GNSS RTK good enough for stockpile surveys?
GNSS RTK works well for small to medium outdoor piles that are safe to access, provided points are captured densely along toes and breaklines. It is less suitable for very large, steep or unstable piles, where walking is unsafe and sparse data reduces accuracy. Scanning or drones are better there.
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