Calculate ground sample distance (GSD) or a missing camera value from altitude above ground, sensor dimension, actual focal length, and image size in pixels. The model assumes a nadir view over level ground; GSD is pixel spacing, not a guarantee of mapping accuracy.
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GSD Formula
Ground sample distance, or GSD, is the ground length represented by one image pixel. The calculator uses altitude above ground, sensor dimension, focal length, and image dimension to solve for the missing value.
- GSD = ground sample distance, usually in meters per pixel, centimeters per pixel, or inches per pixel
- A = altitude above ground
- S = sensor dimension, either sensor width or sensor height
- F = actual focal length, not its 35 mm equivalent
- P = image dimension in pixels, either image width or image height
Use matching dimensions in the formula. If you enter sensor width, use image width in pixels. If you enter sensor height, use image height in pixels. Choose the quantity to solve for and enter the other four values. The calculator converts the selected units internally and returns the result in the selected output unit. A calculated pixel count is theoretical; round up to a whole number to meet the target GSD or finer.
Common GSD Ranges and Unit Conversions
Smaller GSD values mean finer ground sampling. The qualitative categories below are illustrative, rather than standardized suitability or accuracy thresholds; image quality, terrain, processing and the required task also matter.
| GSD | Approximate ground detail | Typical interpretation |
|---|---|---|
| 1 cm/px or less | Very fine | Useful when small surface features need to be visible |
| 2 to 5 cm/px | Fine | Finer sampling than 5 to 10 cm/px for the same ground area |
| 5 to 10 cm/px | Moderate | Larger ground spacing per pixel than 2 to 5 cm/px |
| More than 10 cm/px | Coarser | Coarser ground sampling; suitability depends on the task |
| Conversion | Equivalent value |
|---|---|
| 1 m/px | 100 cm/px |
| 1 cm/px | 0.01 m/px |
| 1 in/px | 0.0254 m/px |
| 1 ft | 0.3048 m |
| 1 in | 25.4 mm |
Example Calculations
Example 1: Calculate GSD
You have an altitude of 120 m, a sensor width of 13.2 mm, a focal length of 8.8 mm, and an image width of 5472 px.
The result is about 3.29 cm/px.
Example 2: Calculate altitude
You want a GSD of 2.5 cm/px, using a focal length of 8.8 mm, an image width of 5472 px, and a sensor width of 13.2 mm. First convert 2.5 cm/px to 0.025 m/px.
The altitude needed is 91.2 meters above ground.
FAQ
What does a lower GSD mean?
A lower GSD means finer ground sampling: 2 cm/px represents smaller pixel spacing than 10 cm/px. With the other inputs fixed, lowering altitude, increasing actual focal length, decreasing the sensor dimension, or increasing the matching image pixel count lowers GSD. Increasing the sensor dimension with focal length and pixel count fixed increases GSD.
Should altitude be above ground or above sea level?
Use altitude above ground, not altitude above sea level. GSD depends on the distance from the camera to the ground surface being photographed. If terrain elevation changes, the actual GSD also changes across the area.
Can you mix sensor width with image height?
Use width with width or height with height. Pair sensor width with image width in pixels, or sensor height with image height in pixels. Matching aspect ratios do not make a mixed-axis calculation valid; mixing axes generally gives the wrong GSD.
