Volumes: against a datum and between two surfaces
Exact prismatic volumes, with the method and the area stated so the number can be checked.
6 min read
Against a level datum
- Surfaces → Volumes.
- Choose the surface and type the datum elevation.
- Measure. You get cut, fill and the plan area it was measured over.
This is the stockpile answer: every triangle is a prism between the surface and the plane, summed exactly.
Between two surfaces
- Choose the base surface and the design surface, two explicit dropdowns, so there is no doubt which way round it is.
- Measure.
- Read cut, fill, net, and the area the two surfaces actually overlap.
Exact, not sampled
The two triangulations are overlaid and the volume between them is computed on the overlaid geometry. There is no cell size to second-guess and no sampling error to argue about, and the result names the method it used.
Find changes: every pile and cut, found and measured on its own
Two flights of the same site a month apart, and the question is not one number but many: which piles grew, which pits opened, and by how much each. Find changes answers that from two surfaces without anyone drawing a boundary first.
- Surfaces → Find changes between two surfaces. Choose the earlier surface and the later one.
- Set the change to ignore, the noise between two flights is usually 0.05–0.15 m, and the smallest region worth reporting.
- Find changes. Every connected region that rose is reported as fill and every one that dropped as cut, each with its area, cut, fill, net, deepest change and centre.
- Leave the outlines box ticked and each region is drawn on layer CHANGE REGIONS as a polygon labelled with its volume, and with its name, once you have named it on any run of the same comparison, ready to check, edit, plot or use as the boundary of an ordinary volume.
How it decides
Both surfaces are sampled on one grid over the ground they both cover, and cells that changed by more than your threshold are grouped into touching regions. A region's outline is where its change falls back under the threshold, the toe of new material or the crest of a cut. A small region's volume is the exact overlay of the two triangulations inside its outline; a large or intricate one is measured on a quarter-metre grid instead, and says so, at that cell the difference is edge cells along the outline, far below what two flights can resolve of each other. Nothing is learned or guessed, and the reproducibility id is recorded like any other volume.
It measures height change, not material
Trees that leafed out, water that rose, and trucks parked on the later flight all change the surface and will be reported. Build both surfaces from ground-classified points, and read the regions before you bill from them. Every run also states the difference summary, the later surface minus the earlier over all common ground, before any threshold. A median far from zero means the whole site shifted: a vertical datum question or vegetation, not earthworks, and the tool warns when it is as large as your threshold. Ground under the threshold at a region's edge is not counted, so regions total a little less than a whole-site volume between the same two surfaces.
End-area volumes
For linear work, sample lines across an alignment give cross sections, and the end-area method gives volumes between stations, the way a road job is measured and paid.
Read next
- Building a surfaceTriangulate points into ground you can contour, measure and stake from.
- Alignments, profiles and cross sectionsCentrelines with stationing, existing ground and design profiles, and sections across them.
- Construction layout and cut sheetsStakeout points and a cut sheet the crew can work from.
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