How this calculator works
Similar triangles link sensor width to the footprint on level ground. Divide that footprint by the image width in pixels. This model assumes the camera points straight down, the terrain is flat and lens distortion has been corrected. Use altitude above the surface, not above sea level, and the actual focal length rather than a 35 mm equivalent.
Formula
GSD = altitude x sensor width / (focal length x pixel width). All physical lengths must use consistent units.
Worked example
At 100 m altitude with a 20 mm sensor, a 20 mm lens and a 4000-pixel image, GSD is 0.025 m, or 2.5 cm per pixel.
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Frequently asked questions
Is GSD the same as mapping accuracy?
No. GSD describes sampling; geolocation, control points, overlap and processing affect map accuracy.
Why does altitude matter?
A higher camera covers a wider ground footprint with the same pixels, so each pixel spans more ground.
Can I use the 35 mm equivalent focal length?
Use the actual lens focal length together with the actual sensor width.