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What ground sample distance means
Ground sample distance, usually shortened to GSD, is the size of one pixel on the ground. A GSD of 2 centimetres per pixel means each pixel in your drone photo covers a 2 centimetre square of ground, so a 20 centimetre object spans about 10 pixels. A GSD of 10 centimetres per pixel means the same object spans only 2 pixels and much detail is lost. GSD is the single number that decides whether a drone survey can measure a crack, count plants, or map a roof, and it is set before take-off by your altitude, lens, and sensor.
The calculator above needs five inputs: sensor width, flight altitude above the ground, lens focal length, and image width and height in pixels. It returns the GSD in centimetres per pixel, the ground footprint of one photo in metres, and the area one photo covers. For example, a drone with a 13.2 mm sensor, an 8.8 mm lens, flying 50 metres above the ground and shooting 4000 by 3000 pixels, gives a GSD near 1.88 centimetres per pixel and a footprint near 75 by 56 metres. That footprint then decides how many flight lines your survey needs.
The formula and what moves the number
GSD grows with altitude and sensor width, and shrinks with focal length and image width. Double the altitude and the GSD doubles, so detail halves. Double the focal length and the GSD halves, so detail doubles, but each photo covers a quarter of the area, so the flight takes far longer. Raise the megapixel count and the GSD improves without changing the footprint, which is why high-resolution sensors suit mapping. The formula is simple: sensor width times altitude times 100, divided by focal length times image width, giving centimetres per pixel when altitude is in metres and the rest in millimetres and pixels.
Altitude needs care, because it means height above the ground below the drone, not height above the take-off point. On a hillside, a drone holding a fixed height above take-off flies higher above downhill ground and lower above uphill ground, so the GSD varies across the survey. Terrain-following flight modes fix this by adjusting altitude to the ground below. If your survey area has hills, plan the GSD for the highest ground, where the drone is closest and the GSD is finest, and check that the lowest ground still meets your accuracy needs.
Picking a GSD for the job
Choose GSD from the smallest feature you must see, not from the nicest number. A common rule is that a feature needs at least 3 to 5 pixels across it to be recognised reliably, so a 10 centimetre crack needs a GSD near 2 to 3 centimetres per pixel, while a roof survey that only needs outlines can accept 5 centimetres per pixel. Finer GSD means more photos, longer flights, more batteries, and larger datasets, so do not chase the finest number by default. Match the GSD to the deliverable: a pretty overview map, a measurement-grade survey, and a plant-counting survey have very different needs.
Remember that GSD is not accuracy by itself. A survey with a fine GSD but poor overlap, no ground control points, and a shaky drone still produces a poor map. Plan 70 to 80 percent forward overlap and 60 to 70 percent side overlap, place ground control points across the site, and fly when the light is even and the wind is low. The calculator gives you the pixel size before you fly. Good flight planning turns that pixel size into a map you can measure from with confidence.
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