A base surface is the reference surface a stockpile volume is measured down to. A drone survey measures the top of the pile; the volume is the space between that surface and the base. Change the base and the volume changes, even though the flight, the imagery and the processing were identical.
This is the single most misunderstood input in stockpile measurement. Accuracy arguments usually focus on the survey, but the base surface routinely moves a volume by more than any realistic survey error does.
The four ways a base is usually defined
- Surveyed pad. The pad was measured while it was empty, so the base is the real ground under the pile. The most defensible option, and the one to establish before material goes down.
- Triangulated or interpolated base. The base is built from the visible ground around the toe of the pile and interpolated underneath. Fine on a flat, clean pad; progressively less reliable where the ground beneath is uneven, sloped or itself buried.
- Flat plane at a fixed elevation. Simple and consistent, and correct only if the ground truly is flat at that elevation.
- Previous survey surface. Used for period-over-period movement rather than absolute stock — it answers how much was added or removed, not how much is there.
Why it has to be stated
A volume without a stated base surface is not a comparable number. Two surveys of the same pile can report different volumes purely because one interpolated the base and the other used a surveyed pad, and nothing in either report would show why.
For inventory that feeds reporting or a payment milestone, the base surface should be named in the deliverable and held constant between periods. If it has to change — the pad was re-graded, a proper base survey was finally captured — that change belongs in the report, because it will otherwise appear as production or shrinkage that never happened.
Practical points
- Capture the pad while it is empty whenever you can. A base survey done once removes the largest source of doubt for every future flight over that pile.
- Material that has consolidated into the pad, or ground that has settled beneath the pile, sits below any interpolated base and will not be counted.
- Toe definition matters: where the pile ends and the pad begins determines the boundary the base is interpolated across.
- Tonnage inherits every base surface assumption, then multiplies it by a density factor. State both, and state where the density figure came from.
Related
- GSD (Ground Sample Distance)
- Photogrammetry
- RTK vs PPK
- Drone stockpile measurement buyer's guide
- Drone stockpile measurement services
- All glossary terms