GSD (Ground Sample Distance) is the real-world distance covered by a single pixel in an aerial image, normally quoted in centimetres per pixel. A survey flown at 2 cm GSD means each pixel in the imagery represents a 2 cm square on the ground.
Formula
GSD = (sensor pixel pitch × flight height above ground) / focal length
In practice the two things an operator can change on the day are the aircraft (its sensor and lens) and the flight height. Everything else is fixed. Lower flight height gives finer GSD, more images, more flight time and more processing.
What GSD controls
- What is visible. A feature smaller than a few pixels across cannot be reliably identified. This is what decides whether a crack, a fastener or a small defect resolves at all.
- How precisely a feature can be picked. Marking a point on the imagery is limited by pixel size, so GSD sets a floor on how tightly anything can be digitised.
- Flight time and data volume. Halving GSD roughly quadruples the number of images needed to cover the same area, with matching increases in flight, processing and storage.
GSD is not accuracy
This is the most common misreading. GSD describes image resolution — how much detail is in the picture. Accuracy describes how close the model's coordinates are to reality, and that comes from GNSS positioning, RTK or PPK, ground control and the coordinate system and datum in use.
A survey can have very fine GSD and poor positional accuracy, or coarse GSD and excellent accuracy. A quote that offers a GSD figure in place of an accuracy statement has answered a different question than the one that was asked.
Choosing a GSD
Work backwards from the smallest thing the deliverable has to resolve, rather than asking for the finest number available. Volumetric surveys and site orthomosaics generally do not need the GSD that a close-range structural inspection does, and buying resolution nobody uses costs field time on every future flight.
Related
- Orthomosaic
- Photogrammetry
- RTK vs PPK
- How many ground control points do you actually need?
- All glossary terms