Two methods, one output
Aerial photogrammetry. An unmanned aircraft flies a planned flight line with ground control points. It covers wide slopes in a short time and produces an orthophoto, a dense point cloud and a digital surface model.
Terrestrial scanning and close-range photogrammetry. Used on vertical rock faces, overhanging sections and pockets invisible from the air; it completes the surfaces the aerial survey leaves in shadow.
The two are combined in one coordinate system. The delivery file states which part was measured by which method.
What the measurement produces
| Product | What it is used for |
|---|---|
| Point cloud and surface model | The ground of the whole design; volumes, areas and distances are read from it |
| Orthophoto | Scaled aerial image; cracks, outcrops and vegetation limits are marked on it |
| Contours and sections | Section along the fall line; barrier line, anchor plan and bill of quantities |
| Block-size inventory | Mass of the design block; the energy class starts from this number — see How to choose the energy class of a rockfall barrier |
| Discontinuity orientations | Input for planar, toppling and wedge analysis |
| Difference between two flights | Deformation monitoring, scaling quantities and payment certificates |
The output goes straight into the design: the terrain model of the trajectory analysis, the slope geometry of the anchor calculation and the position of the barrier line all come from the same file.
How accurate
Accuracy is not one number. It depends on the number and distribution of ground control points, the flight height, the texture of the surface and the imaging geometry. On every job the achieved accuracy is reported together with the control-point residuals. Accuracy that is not reported is not accuracy.
Two limits are stated from the start. Vegetation hides the ground: a surface under trees is not seen by photogrammetry, and those areas are either cleaned or counted as unmeasured. Discontinuity orientation read from the surface degrades as resolution drops; a joint measurement taken from a thinned model does not replace the compass measurement on site — it complements it.
Delivery
Point cloud and surface model, orthophoto, sections, survey record and accuracy report. The record states the flight date, the coordinate system used, the ground control points and which value was measured and which was assumed.
Questions on this topic
Is a drone survey enough for a rockfall design?
For the terrain model, the block inventory and the sections, yes — provided the accuracy is reported. Discontinuity measurements on the face and anchor-ground observations still need the geologist on the slope.
What accuracy can be expected?
Typically a few centimetres with a proper ground-control network; the achieved value is reported per job with the control-point residuals, because it depends on the flight, the surface and the geometry.
Sources
- Artusa survey practice (drone photogrammetry, terrestrial laser scanning), Türkiye and Oman.
- See Glossary: photogrammetry, point cloud, discontinuity orientation, design block.