How this instrument works
Ground sample distance (GSD) is the real-world distance represented by a single pixel in an aerial or satellite image, usually expressed in centimetres per pixel. A GSD of 3.75cm/pixel means each pixel in your photo covers a 3.75cm by 3.75cm patch of ground -- the smaller the GSD, the more detail your imagery resolves, which is why drone-mapping and photogrammetry projects plan their flights around a target GSD.
GSD depends on four things: the camera's sensor width, the flight altitude above the ground, the lens's focal length, and the image width in pixels. Flying higher increases GSD, since each pixel covers more ground; a longer focal length or narrower sensor decreases GSD, since the same sensor area is spread across a narrower footprint; and a higher-resolution sensor decreases GSD too, since the same footprint is divided among more pixels.
This calculation is central to planning drone surveys, agricultural mapping, and orthomosaic projects: mapping software like Pix4D and DroneDeploy uses the same GSD formula to recommend flight altitudes that hit a target resolution, and most photogrammetry projects specify their accuracy requirements in terms of GSD before ever flying.
- Enter Sensor width (mm) -- your camera's physical sensor width, from its published specifications.
- Enter Flight altitude (m) -- how high above the ground, or the takeoff point, the drone or aircraft is flying.
- Enter Focal length (mm) -- the lens's focal length used for the capture.
- Enter Image width (pixels) -- the horizontal resolution of the captured images, in pixels.
- Read Ground sample distance (cm/pixel) beneath the inputs -- lower numbers mean finer ground detail per pixel.
Worked example -- DJI Phantom 4 Pro at 100m altitude
Enter a 13.2mm sensor width, 100m flight altitude, 8.8mm focal length and 4000px image width -- the published camera specifications of a DJI Phantom 4 Pro's 1-inch sensor, flown at 100 metres with a 4000-pixel-wide image. Ground sample distance comes out to (13.2 x 100) / (8.8 x 4000) x 100 = 3.75 centimetres per pixel -- each pixel in the resulting photo represents a 3.75cm patch of ground.
Questions
How does flight altitude affect ground sample distance?
Flying higher increases GSD -- each pixel covers more ground, so image resolution in terms of ground detail per pixel gets coarser. This calculator's formula scales GSD directly with altitude, so doubling flight altitude doubles GSD, while halving altitude halves it, all else held constant -- which is why survey flight plans specify a maximum altitude to hit a target GSD.
What GSD do I need for my mapping project?
It depends on the application: agricultural mapping often works fine at 3-5cm/pixel, general orthomosaic mapping typically targets 2-3cm/pixel, and detailed infrastructure or construction inspection may require sub-centimetre GSD. Check your project's or client's accuracy requirements first, then use this calculator to work out what altitude and camera combination will hit that target.
Why does a longer focal length lower the GSD?
A longer focal length narrows the camera's field of view at a given altitude, so the same sensor captures a smaller patch of ground spread across the same number of pixels -- each pixel therefore represents less ground distance, lowering GSD. This is the same trade-off as zooming a lens in: a narrower field of view packs more pixels onto each square metre of ground, at the cost of a narrower total footprint per photo.
Does this formula assume the camera is pointed straight down?
Yes -- this is the standard nadir, straight-down, GSD formula used throughout drone-mapping and photogrammetry references. An oblique or angled camera view produces a GSD that varies across the frame, larger toward the horizon than directly below the aircraft, and needs a more involved formula that accounts for viewing angle; the nadir formula here gives GSD for a single straight-down capture.
How is GSD different from image resolution in megapixels?
Megapixels describe how many total pixels a sensor captures, while GSD describes what real-world distance each of those pixels represents once you factor in altitude, focal length and sensor size -- the same 20-megapixel camera can produce a fine GSD flown low and slow, or a coarse GSD flown high and fast. GSD is the figure that actually determines how much ground detail your mapping output resolves.