Use comparable sections.
The method averages adjacent cross-sectional areas and multiplies by their spacing. The areas need the same basis, units, chainage logic, and cut/fill interpretation.
Keep planning allowance separate.
An allowance can help a preliminary estimate, but it is not a universal swell, shrinkage, or compaction factor. Those adjustments depend on the material and the measurement basis.
What still needs judgement
- Station spacing and sections through transitions or irregular terrain.
- Survey control, end areas, side slopes, unsuitable material, and over-excavation.
- Bulking, shrinkage, compaction, moisture, haul, disposal, and stockpile definitions.