What is GSD (Ground Sample Distance)?
GSD is the number that quietly decides what your survey can and cannot answer. It is set in the air, before any processing, and no software can recover detail that was never captured. This guide explains what GSD is, what controls it, and how to pick a sensible target instead of always flying lower.
One pixel, measured on the ground
Ground Sample Distance is the distance on the ground represented by one pixel of the image. A GSD of 2 cm means each pixel covers a 2 × 2 cm square of the world.
It is a sampling interval, not a guarantee of accuracy. A 2 cm GSD survey with poor control can be less accurate than a 5 cm survey with good control — GSD tells you the finest detail that could be captured, not how correctly it is positioned.
What sets it
GSD follows from flight height, the camera's focal length and its sensor pixel size. Fly twice as high with the same camera and your GSD doubles.
That is the whole trade-off: lower flights give finer GSD but cover less ground per battery, need more images, and take longer to process. Higher flights cover more but sample coarser.
How GSD limits the DEM
The elevation model you derive can never carry more real detail than the imagery it came from. A DEM gridded finer than the source GSD looks smoother, but the extra cells are interpolation, not measurement.
In practice the useful DEM resolution is at or slightly coarser than the GSD. Going finer inflates file size and processing time without adding information.
Anything smaller than a few pixels across is not reliably resolved. If you need to see a 10 cm feature, a 10 cm GSD is not enough — you want it several pixels wide.
Choosing a target
Work backwards from the smallest thing you must measure, not from the finest number your drone can achieve.
Bulk earthworks and stockpiles: volumes average over large areas, so moderate GSD is usually plenty. Effort is better spent on good control and complete coverage of the toe of every pile.
Detailed site models, breaklines, kerbs and drainage: finer GSD, because you are resolving edges rather than averaging surfaces.
Corridor and progress mapping: a consistent GSD between flights matters more than an especially fine one. Comparing surveys flown at different resolutions adds a difference that has nothing to do with the ground.
The part people skip
GSD varies across a single flight. It is computed for a nominal height above the take-off point, so on sloping terrain the high ground is closer to the camera and sampled finer than the low ground. On a site with significant relief, the quoted GSD is an average, not a promise.
Terrain-following flight modes exist precisely to keep GSD uniform. If your site has real relief and you care about consistency, that is a more valuable setting than simply flying lower.
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