Drone photogrammetry from photo to measurement

The drone follows a flight plan of parallel lines at a constant height (here 120 m). The photographs overlap widely, along each line and from one line to the next, so that every point on the ground appears in many images. The position of each shot is recorded; ground control points surveyed on site allow the model to be accurately tied in. For the area shown here, more than 700 photos taken at 120 m with a 1-inch, 20-megapixel sensor: about 3.3 cm per pixel on the ground.

The software detects thousands of distinctive points shared by several images: the tie points. Matching them recovers the exact position and orientation of every shot. This is aerial triangulation, which produces a first, sparse cloud.

Once the images are oriented, matching is carried out pixel by pixel: millions of points are computed, each with its position (X, Y, Z) and colour. Nearly 27 million for the area shown here. The dense cloud is the basis for all measurements.

Each point is given a category: ground, tall or low vegetation, machinery and plant, stockpiles, water, haul roads… Classification separates the ground from what stands on it, and makes it possible to compute a bare-earth model (DTM) as well as the surface model (DSM).

The points are joined into a network of triangles: the mesh. The cloud becomes a continuous surface on which slopes, profiles and volumes can be computed.

The photographs are projected onto the mesh: the model shows the true appearance of the site and can be viewed from any angle. It is the basis of 3D models of terrain, buildings and heritage.

The digital surface model gives the elevation at every point. Shaded and coloured by height, it brings out the relief: working faces, benches, bunds, stockpiles. Machinery and plant are removed so that only the ground remains.

The slope is computed at every point of the actual ground and grouped into classes (0–5°, 5–15°, 15–30°, 30–45°, over 45°). It locates banks and working faces, for safety as well as for restoration.

The photographs are rectified to remove perspective and relief distortion, then mosaicked and colour-balanced into a single image. Unlike an ordinary aerial photo, an orthophoto has the same scale everywhere: it can be overlaid on a plan or a GIS and measured directly.

Distances, areas and volumes are computed on the model: length of a haul road, area of a pit floor, volume of each stockpile. With regular monitoring, comparing two surveys shows what has changed: cut and fill, remaining reserves, quantities of stored material.

The model produces contour lines, here every metre with index contours every 5 m: the classic way of reading relief on a plan.

Contours, tops and toes of slopes, haul roads, site boundary, stockpiles and volumes are brought together on a plan, with title block, legend, scale and north arrow: topographic plan, development or restoration plan, environmental mapping.