The formula, and the factor of a thousand
Ground sample distance is the ground distance one pixel covers. It comes straight out of similar triangles: the sensor and the lens on one side, the ground and the altitude on the other. In its most useful form,
GSD (cm/px) = sensor width (mm) x altitude (m) x 100 / (focal length (mm) x image width (px))
The 100 is the entire trick. Sensor width and focal length are in millimetres, so they cancel; altitude is in metres, and multiplying by 100 converts that metre to centimetres. Leave the 100 out and your GSD is a hundred times too small. Convert the altitude to millimetres instead and forget the centimetre conversion and it is a thousand times out. Almost every wrong GSD anyone has ever published is one of those two mistakes.
Worked: a 1 inch sensor 13.2 mm wide, an 8.8 mm lens, a 5,472 pixel wide image, flown at 100 m. 13.2 x 100 x 100 = 132,000. 8.8 x 5,472 = 48,153.6. The quotient is 2.74 cm per pixel. Sanity check it the other way: the ratio of sensor width to focal length is 1.5, so the footprint is 1.5 x 100 = 150 m wide, and 150 m spread over 5,472 pixels is 2.74 cm each. The two routes agree, which is what a sanity check is for.
Footprint, spacing and interval are one chain
Everything after GSD follows mechanically. The footprint is GSD times the pixel dimensions, which is the same as the similar-triangles result and a good place to catch an error. Line spacing is the footprint width times one minus the side overlap: at 70 percent side overlap on a 150 m wide footprint, the lines sit 45 m apart. Shot spacing is the footprint height times one minus the forward overlap: at 80 percent on a 100 m footprint, a frame every 20 m.
Divide that shot spacing by ground speed and you have the interval the camera has to sustain. This is where plans break in practice. At 8 m/s and a shot every 20 m the camera has 2.5 seconds, which is comfortable. Push the speed to 15 m/s and it has 1.3 seconds; raise the forward overlap to 90 percent as well and it has 0.67 seconds, which many cameras will not hold shooting raw. The fix is to slow down, not to hope.
How much overlap is actually needed
More than beginners expect and less than the maximum settings suggest. Photogrammetry reconstructs geometry by finding the same feature in many images from different positions, so the number that matters is not the overlap percentage but how many frames see any given point. High forward overlap with generous side overlap is the common starting configuration for general mapping, and it is the configuration most planning software defaults to for good reason.
| Subject | Overlap tends to want | Why |
|---|---|---|
| Open ground, hard surfaces, good texture | Moderate is enough | Features match easily between frames |
| Dense vegetation, uniform crops | High, both directions | Everything looks like everything else |
| Water, sand, fresh snow, plain roofs | High, and often still poor | There is nothing for the matcher to lock onto |
| Buildings and vertical structure | High, plus oblique passes | Facades are invisible from straight down |
Overlap is cheap to increase in planning and expensive to fix afterwards. A survey flown with too little overlap cannot be rescued in processing; it has to be reflown.
Altitude above what
The altitude in this calculation is height above the surface you are photographing, and terrain does not care what your launch point was. Fly a constant altitude over a hillside and the GSD changes continuously along the slope: the uphill end is closer to the camera and has a finer GSD, the downhill end coarser. If a job specifies a GSD as a maximum, the coarsest point on the site is what has to meet it, which means planning to the lowest ground unless you are following terrain. Trees, buildings and stockpiles do the same thing on a smaller scale.
From here, the drone survey coverage calculator takes the line spacing above and turns it into flight time and pack count, and the drone flight time calculator supplies the minutes per pack that count depends on. For close work on a structure rather than a survey grid, the drone inspection shot planner works the same optics at a standoff distance instead of an altitude. If you are preparing prints from the output, the photo print calculator covers the other end of the pixel chain.
What this page does not tell you
Nothing on this page is a statement of law and no number it produces is evidence of compliance. Drone flight in the United States is regulated federally, the rules for recreational flying and for flying as part of a business are not the same set of rules, and they are revised. There are requirements around who may operate, what has to be registered, what has to broadcast identification, and where you may fly at all; controlled airspace requires authorization before you enter it, obtained through the official channels. None of those are described here on purpose, because a stale summary on a calculator page is worse than no summary. Check the current requirements at the FAA and use the official airspace tools before every flight, not once when you bought the aircraft.
State, county and city rules sit on top of the federal ones and cover things federal rules do not: where you may take off and land, what happens over a park or a beach, and privacy. They vary between neighbouring towns. Find out who owns the ground you launch from and what they allow.
A planned altitude, a planned standoff or a planned flight line from this calculator is an engineering figure for a camera and a battery. Whether you may actually fly it is a separate question with a separate answer.
Questions people ask
Is a smaller GSD number always better?
Better in resolution, worse in almost everything else. Halving the GSD means halving the altitude, which quarters the area each frame covers, so you need roughly four times as many images and about twice as many flight lines, and the flight takes correspondingly longer. Processing time and storage grow with the image count too, and often faster than linearly. The right GSD is the coarsest one that still resolves the smallest thing the job has to identify, with a margin. Deciding that first and flying to it is much cheaper than flying everything at maximum resolution.
Should I use the true focal length or the 35 mm equivalent?
The true focal length, always, and paired with the true sensor dimensions. The equivalent figure is a convenience for comparing fields of view across formats and it is meaningless in this formula unless you also substitute the 35 mm frame dimensions for the sensor. Mixing an equivalent focal length with a real sensor size is a common error and it produces a GSD that is wrong by the crop factor. If a spec sheet only gives you an equivalent, either find the real focal length or use 36 x 24 mm as the sensor and the equivalent focal length together, which is arithmetically consistent even though it does not describe the physical camera.
Which GSD does the calculator report as the headline figure?
The coarser of the two axes. On most cameras the pixels are square and the two figures come out identical, which is a useful check that the sensor dimensions and pixel dimensions you entered are consistent with each other. If they differ noticeably, one of the four numbers is wrong, usually the sensor height, or the camera is not shooting at its full native sensor area. Reporting the coarser axis means the headline figure is the one you can promise.
Does the shutter speed matter for a mapping flight?
It matters more than people expect, because the aircraft is moving while the shutter is open. At 10 m/s and 1/500 s the aircraft travels 2 cm during the exposure. If the GSD is 2 cm per pixel, that is a whole pixel of smear on every frame, and it degrades feature matching as well as the visual quality. The calculator shows this as blur in pixels when you enter both a speed and a shutter speed. Keeping it well under a pixel is the goal, and the awkward case is low light, where the exposure the camera wants is far longer than the exposure the flight can tolerate.
Can I just fly higher to cover the site faster?
Higher does cover more per frame, and the coverage gain is quadratic, so it is a strong lever. What it costs you is resolution, in direct proportion, and that is the trade to make consciously against what the job needs. What you must not do is treat the calculator as authorising an altitude. It reports the optics of a height; it has no idea what airspace you are in, what your operating authorisation covers, or what the current rules require. That question is settled with the FAA and the official airspace tools before the flight, not with a number from a website.