When you plan a drone mapping mission, the first question is usually "how high should I fly?" The answer depends on how much detail you need, and detail is measured by Ground Sample Distance, or GSD.
What GSD actually means
GSD is the real-world distance between the centres of two adjacent pixels in your image, measured on the ground. A GSD of 2 cm/px means each pixel covers a 2 cm × 2 cm patch of ground. The smaller the GSD, the finer the detail, and the sharper your orthomosaic or 3D model will be.
It is the honest measure of resolution. A 20-megapixel camera flown too high can produce a coarser map than a 12-megapixel camera flown low, because what matters is how much ground each pixel sees, not the raw pixel count.
The formula
For a straight-down (nadir) shot, GSD is:
GSD = (sensor width × flight height × 100) / (focal length × image width)
with sensor width and focal length in millimetres, flight height in metres, image width in pixels, and the result in centimetres per pixel. The factor of 100 converts metres to centimetres.
Two things fall straight out of this formula:
- GSD is proportional to altitude. Fly twice as high and your GSD doubles (detail halves).
- A longer focal length gives finer GSD at the same altitude, because the lens magnifies the scene.
How altitude and sensor choice interact
Say you fly a 1-inch sensor drone (13.2 mm sensor width, 8.8 mm focal length, 5472 px wide) at 100 m. The GSD works out to about 2.7 cm/px. Drop to 50 m and it halves to about 1.4 cm/px, but you cover a quarter of the area per image and need many more photos. That trade-off, detail against flight time and image count, is the heart of mission planning.
What GSD should you target?
- 2 to 5 cm/px: general topographic mapping, agriculture, site overviews.
- 1 to 2 cm/px: detailed survey, stockpile volumes, construction progress.
- Below 1 cm/px: close inspection work such as roofs, facades or cracks, usually flown low and slow.
A useful rule of thumb: your GSD should be at least two to three times finer than the smallest feature you need to identify. To reliably see a 5 cm crack, aim for around 1.5 to 2 cm/px.
Things the basic formula ignores
- Camera tilt. The formula assumes a nadir shot. Oblique angles stretch GSD across the frame.
- Terrain. Flight height is above ground level. Over a hill, the ground rises toward the drone and GSD becomes finer; over a valley it coarsens.
- Motion blur. Flying too fast for the shutter speed blurs pixels regardless of GSD.
Planning a flight from GSD
In practice you work backwards: decide the GSD your deliverable needs, then solve the formula for altitude. Our GSD calculator does this for you, including a "altitude for 1 cm/px" readout and the ground footprint of each image, with presets for common DJI drones.
Try it: work out your resolution and flight altitude with the free Drone GSD Calculator.