How is stockpile volume measured? LiDAR, drone photogrammetry and the base surface
A pile's volume depends less on its top than on the surface you cannot see beneath it.
Field takeaways
- Volume is the difference between the top surface and the base; how the base is defined moves the result most.
- On a pile 10 m in radius and 6 m high, getting the base 5 cm wrong changes the volume by about 2.5%.
- Photogrammetry works well on textured piles in the open; in covered storage and shadow, laser scanning comes first.
- The boundary between adjoining piles is defined before measuring and kept the same every time.
- Tonnage is computed with a measured density; any error in the density passes straight into the tonnage.
Volume is the difference between two surfaces
The volume of a stockpile is the volume between the pile's top surface and the base beneath it. The top surface comes from LiDAR, drone photogrammetry, terrestrial laser scanning or dense GNSS measurement; the base is invisible and has to be defined. The calculation method (grid, triangle network or sections) comes second; if both surfaces are right, the methods give similar results. That is why every volume report should state how the base was defined, as well as the date and method of the top survey.
How is the base defined?
There are several ways to define the base. The best is a base surface measured before the material was tipped, such as a concrete pad or a prepared platform. Without one, a surface is built from points measured along the toe of the pile: a horizontal plane, an inclined plane fitted to the toe, or a surface triangulated from the toe points. A horizontal plane through the lowest toe point systematically inflates the volume on sloping ground. Whichever way you choose, use the same base definition for successive surveys of the same pile; otherwise the difference comes from the definition, not the material.
What are 5 cm at the base worth?
The effect of a base error depends more on the base area than on the height of the pile. A cone-shaped pile 10 m in radius and 6 m high holds π × 10² × 6 ÷ 3 ≈ 628 m³, on a base of about 314 m². If the base is defined 5 cm wrong, the volume changes by 314 × 0.05 ≈ 15.7 m³, or about 2.5%. On a low, spread-out pile the same 5 cm error is even larger in proportion. Random errors on the top surface partly cancel out over a large area; a systematic error at the base adds straight to the volume.
LiDAR or photogrammetry?
On textured, well-lit piles in the open, drone photogrammetry is often enough; one flight measures many piles and also produces an orthophoto. Where it struggles is predictable: uniform fine material such as sand and cement, wet and shiny surfaces, deep shadow and covered storage. LiDAR carries its own light, so it is less affected by shadow and texture; in covered stores and dome silos, terrestrial or mobile laser scanning comes first. Whichever you choose, the survey is tied to coordinates through control points; in covered areas where GNSS does not work, that tie is made with a total station.
Adjoining piles and boundaries
Where piles touch, the imagery cannot show exactly where one material ends and the next begins. Define the dividing line and each pile's calculation boundary before measuring, use the same boundaries every time and keep them with the report. Remove machines, conveyors and vehicles standing on the pile from the calculation; if material is moving in or out during the survey, note the time. When a stock count carries financial value, even a one-metre shift in a boundary becomes a point of dispute.
From volume to tonnage
Most stock is managed in tonnes, while the survey gives cubic metres. Converting needs the material's density in the pile, which varies with the material, its moisture and its compaction. For example, with a density of 1.6 t/m³, 628 m³ is about 1,005 t; a 5% error in the density passes into the tonnage as the same 5%. Determine the density by measuring that site's material rather than from tables, and write the value used in the report. Choose how often to measure from the inventory cycle, and make every survey with the same method, the same base and the same boundaries.
Survey the pile, compute the volume from the surface.
MapLab Survey 3D produces point clouds from photos and opens LAS and LAZ files; it suggests a ground classification you can correct by hand, computes a terrain surface and cut-and-fill volume, and exports contours as DXF. Windows, on request.
Frequently asked questions
What if the base surface is not known?
Points are measured along the toe of the pile and a base surface is built from them. The method used is written in the report and applied again in later surveys.
Is drone photogrammetry good enough for stockpiles?
On textured piles in the open it often is. On uniform fine material, shiny surfaces, deep shadow and in covered storage, laser scanning gives more reliable results.
How do I convert volume to tonnes?
Multiply the volume by the material's density in the pile. Measure the density from that site's material; any error in it passes straight into the tonnage.
What does MapLab Survey 3D do for stockpile volume?
It produces point clouds from photos and opens LAS and LAZ files; it suggests a ground classification you can correct by hand, computes a terrain surface and cut-and-fill volume, and exports contours as DXF. It runs on Windows and is available on request.
Technical references
The field guidance in this article is aligned with the technical and official references below.
Related: Cut-and-fill calculation · Orthophotos and 3D models · Ground classification · MapLab Survey