GSD Calculator: Ground Sampling Distance for Drone Mapping
Ground sampling distance (GSD) is the size of one image pixel on the ground, and it is the first number to settle when you plan a drone mapping flight. Use the calculator below with your own camera: enter a flight altitude to get the GSD, or a target GSD to get the altitude, and read off the footprint of one photo and the spacing between photos and flight lines for your overlaps.
Ground sampling distance (GSD) calculator
Enter your camera and either a flight altitude or a target GSD. The calculator gives the GSD, the ground footprint of one photo and, from your overlaps, the spacing between photos and flight lines.
Formula: GSD = H · s / (f · n)
GSD = H × s / (f × n): H = height above ground, s = sensor width, f = focal length, n = image width in pixels. Footprint = GSD × pixels. Spacing = footprint × (1 − overlap).
Camera pointing straight down over level ground, image width across the flight line and image height along it (swap the two values to rotate). The GSD of a finished orthophoto or elevation model is a processing choice and is often coarser than the photo GSD. Photos per hectare ignore the edges and turns of the flight plan.
To see what a given GSD delivers, process a flight into an orthomosaic: Orthomosaic Software
What is ground sampling distance?
GSD is the distance on the ground between the centres of two neighbouring pixels in a nadir photo. A GSD of 2.74 cm per pixel means each pixel covers about 2.7 cm of ground. A smaller number is finer detail: you can pick out smaller objects, and the photos need to be taken lower and more of them.
GSD describes the photos, not the finished map. The orthophoto and elevation model made from them can be written at the photo GSD or at a coarser cell size, and the positional accuracy of the result depends on much more than GSD alone.
The formula, with a worked example
GSD = H × s / (f × n), where H is the height above the ground, s the sensor width, f the focal length and n the image width in pixels. Equivalently, GSD is the flight height times the pixel size on the sensor, divided by the focal length.
Take a 1-inch-class camera with a 13.2 mm wide sensor, an 8.8 mm focal length and 5,472 pixels across, flown 100 m above the ground. GSD = 100 × 13.2 / (8.8 × 5,472) = 150 / 5,472 m, which is 2.74 cm per pixel. One photo then covers 150 m across the flight line and 100 m along it. GSD scales with height: fly twice as high and each pixel covers twice the ground.
| Height above ground | GSD | Footprint of one photo | Photos per hectare |
|---|---|---|---|
| 30 m | 0.82 cm/px | 45 × 30 m | 123 |
| 60 m | 1.64 cm/px | 90 × 60 m | 31 |
| 100 m | 2.74 cm/px | 150 × 100 m | 11 |
| 120 m | 3.29 cm/px | 180 × 120 m | 7.7 |
| 150 m | 4.11 cm/px | 225 × 150 m | 4.9 |
From GSD to a flight plan
The footprint of one photo is the GSD times the image size in pixels. The distance between exposures along a line is the footprint along the flight times one minus the forward overlap, and the distance between flight lines is the footprint across the flight times one minus the side overlap. With 80 % forward and 70 % side overlap the example camera at 100 m takes a photo every 20 m on lines 45 m apart, which is about 11 photos per hectare.
Because the ground area each photo adds shrinks with the square of the height, halving the height of a flight needs about four times as many photos for the same area. Photos per hectare in the calculator ignore the edges and turns of the flight plan, so a real flight takes somewhat more.
GSD is not accuracy
A smaller GSD makes the photos sharper on the ground, but it does not by itself place the model correctly: absolute accuracy comes from positioning and ground control, and only independent check points measure it.
The processing preset sets the GSD of the products. Balanced writes the orthophoto and elevation models at twice the photo GSD and Maximum writes them at full resolution, so the number in the calculator is the best the photos can offer, not what every product contains.
Choosing a GSD
Start from the smallest thing the deliverable must show or measure, then work back to a height. If painted ground control targets should be found automatically in the photos, make the disk at least about seven times the GSD across: 0.35 to 0.5 m at a 4.7 cm GSD.
Then check the cost. A lower flight raises the number of photos and the flight time sharply, and on uneven ground a single flight height gives a different GSD on the hill and in the valley, so keep the camera at a constant height above the terrain and not above the take-off point.
Common mistakes
Using the 35 mm equivalent focal length. The formula needs the real focal length of the lens. Paired with the real sensor width, the equivalent value gives a GSD several times too small.
Using the sensor's diagonal or its "inch" name as its width. A 1-inch-type sensor is 13.2 mm wide, not 25.4 mm. Take the width from the camera datasheet; the sensor classes in the calculator are only typical values.
Measuring height from the take-off point. Over sloping terrain the height above the take-off point and the height above the ground can differ enough to change the GSD noticeably. The calculator expects height above the ground you are mapping.
Frequently asked questions
What is GSD in photogrammetry?
Ground sampling distance is the size of one pixel on the ground in a nadir photo, for example 2.7 cm per pixel. It is set by the flight height, the focal length, the sensor width and the image width in pixels.
How do you calculate GSD?
GSD = height above ground × sensor width / (focal length × image width in pixels). With a 13.2 mm sensor, an 8.8 mm lens and 5,472 pixels across at 100 m, GSD is 150 / 5,472 m, about 2.74 cm per pixel.
What GSD do I need for mapping?
It depends on the smallest feature you must show or measure and on the accuracy you must deliver. Work from the deliverable to a GSD, and then check the cost in photos and flight time; a lower flight needs about four times the photos for half the height.
How does altitude change GSD?
In proportion. Twice the height above ground gives twice the GSD, which is a coarser image, and a quarter of the photos per hectare.
Why is my orthophoto GSD different from the calculator?
The calculator gives the GSD of the photos. The orthophoto and elevation models are written at a resolution chosen in processing; Stream Capture's Balanced preset writes them at twice the photo GSD and Maximum at full resolution.
Which sensor width should I enter?
The physical width of the sensor from the camera datasheet. A 1-inch-type sensor is 13.2 mm wide. The sensor classes in the calculator fill in a typical width, which you can overwrite.
Does a smaller GSD mean higher accuracy?
Not by itself. A smaller GSD gives finer detail, but accuracy depends on positioning, ground control, overlap and processing. Check the result against independent check points.
See what your GSD delivers: process a flight into an orthomosaic
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