Drones for photogrammetry: multirotor, fixed-wing and VTOL

A mapping drone is a camera with a flight plan. The plan decides overlap, height and lines; the aircraft decides how much ground it can cover before the battery runs down, how steady the camera is held, and how well it knows where each photograph was taken. This page compares the three kinds of aircraft, explains the positioning that has changed the economics of ground control, and says what to look for in the payload. How the photographs then become a map is in the aerial photogrammetry guide.

Three kinds of aircraft

• MultirotorFour or more rotors; hovers, takes off from a truck bed, holds a line in wind, points its camera anywhere. The working drone of most sites. Limited by battery to 20–45 minutes a flight.up to ~100 ha a flight · $1,500–15,000
• Fixed-wingA small aeroplane: efficient, so it flies an hour or more and covers square kilometres. Needs a launch (hand or catapult) and somewhere flat to land, and cannot hover or look sideways.several km² a flight · $10,000–40,000
• VTOL fixed-wingTakes off and lands vertically on rotors, then cruises on wings. The fixed-wing's coverage without the landing strip; heavier and dearer than either parent.several km² a flight · $15,000–60,000

Side by side

The flight at the top of this page is the fixed-wing case: a quarry and its surroundings in one 26-minute flight from 250 m, parallel lines end to end. A multirotor would have flown the same site at 100 m in two or three batteries for a finer pixel, and could have added oblique lines along the faces.

RTK and PPK: knowing where the photograph was

Every drone records where it took each photograph. A plain GNSS receiver puts that within a metre or two, which is enough to start the processing and not enough to survey with. RTK and PPK bring it to a few centimetres by correcting against a base station of known position: RTK live, over a radio or network link during the flight; PPK afterwards, from the recorded logs. The accuracy is the same, and PPK needs no link in the field.

The payload

The camera is the survey; the aircraft is its transport. On a built-in camera look for the sensor size (a 1-inch, 20 MP sensor is the usual floor for survey work; 4/3 and full-frame payloads at 20–45 MP are the step up), a mechanical or global shutter, a fixed focal length and the ability to lock exposure. On an interchangeable payload, the same, plus the mount’s stiffness: a camera that moves against the aircraft between photographs is a camera the solver cannot model. The cameras page goes through all of it.

Survey camera

20–45 MP, 1-inch to full-frame, mechanical shutter, fixed lens. The photogrammetry payload.

Oblique rig

Several cameras angled at once, for façades and city models in one flight.

LiDAR

A laser scanner with its own GNSS and IMU, often with a camera beside it to colour the cloud. For ground under vegetation.

Multispectral and thermal

Narrow-band and heat-sensing cameras for crops, water and roofs; mapped with the same photogrammetry on top of the ordinary camera.

What the flight plan asks of the drone

The plan is a set of parallel lines at a height, with a photograph every so many metres along each. The aircraft has to fly the line straight at a steady speed, hold the camera level, and trigger at the right places; a good drone does all three without the pilot touching the sticks. Three things to set and then leave alone: the height for the pixel you need, the overlap (75% front, 65% side on open ground, more over vegetation and buildings), and a margin of one line beyond the area of interest. The 101 has the planner; the flying guide has the checklist.

Rules and the practical things

A survey flight is a regulated flight. Nearly everywhere that means registering the aircraft, holding a pilot certificate, staying below a height ceiling (120 m or 400 ft above ground is common), keeping away from airports and people, and keeping the aircraft in sight. Flights over a working site may need the site’s own permission and a lookout. The rules differ by country and change often enough that the current ones should be checked where you fly, not here.

The practical things: batteries for the whole plan plus a spare, memory for the photographs, an RTK fix before launch and a base station log saved for PPK, a landing spot clear of dust, and the targets laid and surveyed before the first photograph. Then the drone flies the plan and the pilot watches the sky.