What the GSD is
The ground sampling distance (GSD) is the size of one pixel on the ground. A GSD of 2 cm/px means each pixel of the photo covers 2 × 2 cm, so nothing smaller than a few centimetres can be measured in the map or the 3D model made from it. It depends on three things: how high you fly, the lens and the number of pixels.
How the calculator works
Spec sheets rarely give the real sensor size and focal length, but every drone lists the 35 mm-equivalent focal length and the photo size in pixels. The equivalent focal length is defined against the 43.27 mm diagonal of a full-frame sensor, so:
- photo diagonal on the ground = height × 43.27 ÷ equivalent focal length;
- the aspect ratio splits that diagonal into the width and the height of the footprint;
- GSD = footprint width ÷ photo width in pixels.
For the mapping grid, the long side of the photo lies across the flight line, as in most mapping apps:
- distance between photos = footprint length along the line × (1 − front overlap);
- distance between flight lines = footprint width × (1 − side overlap);
- time between photos = distance between photos ÷ speed.
Worked example
A Mini-class camera with a 24 mm equivalent lens, 12 MP photos in 4:3 (4,032 pixels wide), at 100 m:
- ground diagonal: 100 × 43.27 ÷ 24 ≈ 180 m, so the footprint is about 144 × 108 m;
- GSD: 144 m ÷ 4,032 px ≈ 3.6 cm/px (about 1.8 cm/px in 48 MP mode, 8,064 pixels wide);
- with 75 % front and 70 % side overlap: a photo every 27 m and flight lines 43 m apart;
- at 5 m/s: one photo every 5.4 s.
The GSD is proportional to the height: at 50 m the same camera gives about 1.8 cm/px in 12 MP mode, but each photo covers a quarter of the area, so the flight takes roughly four times as many photos.
How much overlap
Pix4D, a photogrammetry software maker, recommends at least 75 % front and 60 % side overlap for a general mapping grid, at least 80 % on both for flat fields, and 85 % on both over forests and dense vegetation, where matching points between photos is hard. More overlap means more photos and a longer flight, so plan the battery and the wind: a mapping flight crosses the area many times, upwind on every other line.
Wind and light
- Wind: flying into the wind on every other line changes the speed over the ground, and with it the spacing of photos taken at a fixed interval. Use distance-based triggering if your app has it, and check the wind against your drone's limit at the height you will fly.
- Motion blur: at a given shutter speed, the drone moves farther during the exposure when it flies faster; keep the movement during one exposure well under one GSD.
- Light: even, overcast light gives the most uniform maps; low sun leaves long shadows that change between flight lines (see the golden hour calculator to avoid it).
Sources
- DJI Mini 4 Pro specifications (24 mm format equivalent, 8064×6048 maximum image size): dji.com
- Pix4D, recommended image overlap for nadir mapping: support.pix4d.com
Frequently asked questions
What is a good GSD for drone mapping?
It depends on what you need to see. A few centimetres per pixel is common for site surveys and volumes; finer detail, such as cracks, needs a GSD below 1 cm/px, which means flying lower or using a longer lens.
How do I get a smaller GSD?
Fly lower, use a longer equivalent focal length or shoot more pixels (48 MP instead of 12 MP). Halving the height halves the GSD.
How much overlap do I need for photogrammetry?
For a general mapping grid at least 75 % front and 60 % side overlap, as Pix4D recommends; 80 % or more over flat fields and 85 % over forests.