Construction Site Drone Surveys: Tracking Earthworks Progress
On an earthworks job the question every week is the same: how much has moved, and how much is left. A drone survey answers it for the whole site in one flight, where a ground crew samples a few hundred points. But a flight only produces a surface. The progress figures come from comparing that surface with two others — the design, and the previous survey — and those two comparisons answer different questions. This guide covers both, how to read a cut/fill map, and what has to be true of the surveys for the numbers to hold.
Three surfaces
Earthworks monitoring runs on three surfaces: the original ground before work started, the design the job is being built to, and the current survey. Every progress figure is a difference between two of them.
| Compare | Against | Answers |
|---|---|---|
| Current survey | Design surface | Remaining cut and fill: how much is left to move, and where |
| Current survey | Previous survey | Progress in the period: what was moved since the last flight |
| Current survey | Original ground | Total to date: what the job has moved so far |
| Design surface | Original ground | The planned quantity the job was priced on |
Against the design: what is left
Cut and fill between the current survey and the design surface gives the remaining earthwork. Where the ground is above design, it still has to be cut; where it is below, it still has to be filled. The totals are the outstanding quantities, and the map shows where they are.
This is the comparison that catches over-excavation early. An area that has gone below design shows as fill required, on a part of the site that was meant to be in cut — far cheaper to see on a map than to discover at final trim.
Against the last survey: what moved
The difference between two dated surveys is the progress in that period: volume removed, volume placed, and the net. It is the figure a monthly valuation or a production report is built on, and it is independent of the design.
Small differences need care. Every survey has noise, and subtracting two noisy surfaces produces change everywhere — a few centimetres of apparent cut and fill across ground nobody touched. A level of detection sets a threshold below which a difference is treated as noise, so the reported volume is only the change that is larger than the surveys can explain by themselves.
Reading a cut/fill map
A cut/fill map colours each cell by the height difference between two surfaces: one colour family where the upper surface is higher, another where it is lower, and usually a neutral band around zero. The intensity is depth, not volume — a deep narrow trench and a shallow wide strip can hold the same quantity and look nothing alike.
Check which way round the map is before reading it. Cut and fill swap when the two surfaces are swapped, and conventions differ between organisations. The legend, not the colour, says which is which.
What the surveys have to share
Two surveys can only be differenced if they are in the same coordinate system and the same vertical datum, tied to the same control. A constant vertical offset between flights — a few centimetres from different ground control or a different base — multiplies by the whole site area and becomes a large, entirely fictitious volume.
Fly the same extent at a similar resolution, check both surveys on stable ground that has not been worked — a road, a slab, natural ground outside the limit — and correct any systematic shift before computing change. Standing water, parked plant and stockpiled material on the day all end up in the surface, so note them.
Earthworks progress in STREAM
STREAM works on the outputs of the flight — the DEM, the orthophoto, the point cloud — and keeps each survey as its own scenario, so an earlier surface is never overwritten. It does not plan flights or process photographs.
Cut and fill is computed against a flat design level, a surface through the area's own corners, the area's lowest point or another scenario, and drawn as a cut/fill heat map with the totals; Volume vs Model measures between a design TIN and the surveyed ground. Change detection compares two scenarios and reports deposition, erosion and net volume with an uncertainty band, keeps only change above the level of detection you set, and can remove a systematic vertical shift between the two surveys first. Compare Mode splits the view with a draggable curtain, one survey on each side, for a visual check before measuring.
Design surfaces come in as TIN surfaces, including from LandXML. Results go out as a PDF report, the change map as a raster, and drawings as DXF. Everything runs on the machine; no survey is uploaded.
Frequently asked questions
How are drones used to monitor earthworks on a construction site?
The site is flown at intervals and each flight is processed into a surface. Comparing the current surface with the design gives the remaining cut and fill; comparing it with the previous survey gives the volume moved in the period.
How do you measure construction progress from two drone surveys?
Subtract the earlier surface from the later one inside the work area. Cells that went down are material removed, cells that went up are material placed, and the totals are the progress for the period. Both surveys must share the same coordinate system, vertical datum and control.
What is a cut/fill map?
A map that colours each cell by the height difference between two surfaces — one colour family for cut, another for fill. Colour intensity shows depth, not volume, and the legend says which surface is being compared with which.
Why do two surveys show change on ground that was not touched?
Survey noise, or a systematic offset between flights. Subtracting two surfaces turns centimetres of error into apparent cut and fill everywhere. A level-of-detection threshold removes change smaller than the noise, and a vertical shift between the surveys should be corrected before volumes are computed.
How often should a construction site be flown?
As often as a decision depends on it: commonly at each valuation or pay period, and before and after any stage that buries the previous surface, such as placing fill over a formation. There is no fixed interval.
Does STREAM process the drone photos?
No. STREAM works on the outputs of photogrammetry or LiDAR — DEM, orthophoto, point cloud — and computes cut and fill against design, change between surveys and reports from them.
Sources
Compare two surveys and measure the change in STREAM
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