Hong Life CalculatorsUnderstanding the GSD calculator — concept and principle
Ground sample distance (GSD) is how much ground one image pixel covers, usually in centimetres per pixel. It decides how much detail a drone survey or aerial photo can show. This calculator finds GSD from the camera's sensor width, focal length, image width and flight height, plus the ground footprint and area of one photo and the height needed for a target GSD.
How it is calculated
Pixel size p = sensor width ÷ image width (px)
GSD = p × H ÷ f
Footprint = GSD × image width by GSD × image height
Area (ha) = width × height ÷ 10,000
Height for a target GSD: H = GSD × f ÷ p
- Sensor width and focal length in mm, height H in m; use the real focal length, not the 35 mm equivalent.
The principle
A camera looking straight down forms an image whose scale is the focal length divided by the height above ground, just as in a pinhole camera. Similar triangles relate a pixel on the sensor to the patch of ground it sees: the ground patch is the pixel size multiplied by H ÷ f. Flying twice as high doubles GSD, giving coarser detail but covering four times the area per photo. A longer lens or a sensor with smaller pixels gives finer GSD at the same height.
Worked example
Default inputs: sensor width 13.2 mm, focal length 8.8 mm, image 5,472 × 3,648 px, height 100 m, target GSD 2 cm.
- Pixel size = 13.2 ÷ 5,472 = 0.00241 mm = 2.41 µm.
- GSD = 0.002412 × 100 ÷ 8.8 = 0.0274 m = 2.74 cm/pixel.
- Footprint = 0.0274 × 5,472 = 150 m by 0.0274 × 3,648 = 100 m; area = 1.5 ha.
- Height for 2 cm GSD = 0.02 × 8.8 ÷ 0.002412 = 73 m.
| Focal length (real, mm) | Ground sample distance (cm/pixel) |
|---|---|
| 2.2 | 10.96 |
| 4.4 | 5.48 |
| 6.6 | 3.65 |
| 8.8 | 2.74 |
| 11 | 2.19 |
| 13.2 | 1.83 |
| 17.6 | 1.37 |
How to read the result
Choose a GSD a few times finer than the smallest feature you need to see. Plan surveys around the GSD you need, then check flight height against local drone rules and obstacles. Lower flights give finer detail but more photos and longer missions. Terrain matters: hills closer to the drone get finer GSD than valleys, so for uneven ground set height relative to the terrain or allow for the lowest point.
Common mistakes
- Using the 35 mm-equivalent focal length instead of the real focal length.
- Using height above take-off point instead of height above the ground being mapped.
- Confusing GSD with absolute positional accuracy, which needs ground control.
- Forgetting that oblique (tilted) photos have varying GSD.
Frequently asked questions
What GSD do I need for mapping?
It depends on the task; finer GSD is needed to see small features, at the cost of more flight time.
Does a higher resolution camera improve GSD?
Yes, if pixels are smaller for the same sensor size and lens.
Where do I find the real focal length?
In the photo's EXIF data or the camera specifications.
Does GSD equal accuracy?
No. GSD is resolution; accuracy also depends on GPS, ground control and processing.
Sources: R. P. Comer, G. Kinn, D. Light and C. Mondello, "Talking Digital", Photogrammetric Engineering & Remote Sensing 64(12): 1139–1142, 1998 (ASPRS)