Drone surveying is the use of drones to photograph the ground from a low height, and of software that turns the photographs into accurate maps, 3D models and measurements. It is used at mines, building sites and road projects, and in villages to draw up land records.
How does it work? The surveyor first draws the area on a tablet. The software plans a flight in parallel lines, so that each photo overlaps the next by about 70 to 80%. The drone then flies the lines by itself and takes a photo every second or two. Its satellite receiver and its inertial measurement unit (IMU) record where the camera was and how it was tilted for each photo. For accuracy to a few centimetres, surveyors also mark a few targets on the ground and measure their positions with a precise satellite receiver.
Back in the office, software matches the same features, such as a rock or the corner of a building, across many overlapping photos. From the different angles, it works out the 3D position of millions of points. This method is called photogrammetry. Newer tools also use geospatial AI to recognise objects in the images, such as stockpiles, trucks and buildings. The results are a single map-accurate photograph of the whole area, called an orthomosaic, and a 3D model of the ground. Some drones carry lidar instead of a camera. Lidar is a laser scanner that measures distance, and it can reach the ground through gaps in trees.
Take, for example, a cement company's limestone mine near Chittorgarh in Rajasthan. Every month, the company must know how much rock has been dug out and how much sits in each stockpile. A team of surveyors on foot used to take about a week. Now a drone covers the 3 square kilometre mine in about 90 minutes, in six flights, taking about 4,000 photos. By the next day, the software has built a 3D model accurate to about 5 cm. It compares the model with last month's. The difference shows that 1.8 lakh cubic metres of rock were dug out, and that the main stockpile holds 25,000 cubic metres. The figures go to the mine's planning team and into its monthly production report.
Villages are the largest use in India. Under the government's SVAMITVA scheme, drones have photographed the inhabited areas of lakhs of villages. Each house and plot is drawn from the images, and its owner is given a property card, a legal record of the land. The card can then be used, for example, to take a bank loan.
India's Drone Rules, 2021, set by the civil aviation ministry, govern these flights. The country's airspace is divided into green, yellow and red zones. Flights in green zones need no permission, while yellow and red zones need approval. The surveyor plans and watches each flight from a ground control station.
Drone surveys are sent on to a geographic information system (GIS), to civil infrastructure design software for roads and railways, and to construction progress monitoring. Earth observation satellites cover far larger areas, but in less detail.