The geometry is one triangle
How much of a wall a frame covers depends only on the field of view and the distance to it. The width covered is twice the standoff times the tangent of half the horizontal field of view. At 25 ft standoff with an 84 degree horizontal field of view, that is 2 x 25 x tan(42 degrees), about 45 ft across. The height follows from the aspect ratio: on a 3:2 frame the height is two thirds of the width, so about 30 ft down. That is the tile the surface gets divided into.
Two things about that formula are worth holding onto. The relationship with distance is linear, so doubling the standoff doubles the frame in both directions and quarters the shot count, at the cost of half the detail. And the field of view figure has to be the horizontal one. The diagonal figure is what gets quoted in marketing because it is a larger number, and using it here overestimates coverage by a noticeable margin and leaves gaps in a plan that looked complete.
Overlap on a structure is a different question from overlap on a survey
A mapping flight uses high overlap because photogrammetry needs many views of every point to reconstruct geometry. A visual inspection has a different job: a human being looks at a sequence of frames and has to be able to follow the surface from one to the next without wondering whether something fell between them. Enough overlap to guarantee no gaps and give the reviewer continuity is a much lower figure, and something around a third to a half is a common working choice.
The exception is when the deliverable is a model rather than a set of photographs. If the output has to be a reconstructed three dimensional surface, the requirement jumps to survey-grade overlap and the shot count goes up several times over. Decide which deliverable you are producing before you plan the flight, because it changes the plan more than any other single choice.
Pitched roofs have more roof than they have footprint
A roof measured from the ground gives you its plan area, and the actual surface is larger by the slope factor. For a rise of p in 12, that factor is the square root of (144 + p squared) divided by 12. A 6 in 12 roof has a factor of about 1.118, so it has 12 percent more surface than its footprint. A 12 in 12 roof has about 41 percent more. This calculator applies the factor to the width dimension when you set a pitch, which is the right treatment when the flight lines run along the ridge.
A pitched roof also has two planes at an angle to each other, and a camera pointed straight down at the ridge sees both of them obliquely. For anything that has to be looked at properly rather than seen, flying each plane roughly square to its surface gives usable images and roughly doubles the flight, and that is the honest planning figure rather than the optimistic one.
Why the time is longer than the shot count suggests
Twelve seconds a frame sounds slow until you watch it happen. Each shot involves moving to a position, waiting for the aircraft to settle, checking the framing against a screen in daylight, taking the exposure, and confirming it is what you wanted. Close to a structure everything takes longer: wind wraps around a building and comes from directions it does not come from in open ground, position hold is less reliable in the shadow of a wall than in the open, and drift matters more when the surface is 25 ft away than when it is 250 ft below. A hundred shot inspection is twenty minutes of exposure time and closer to an hour of real time once packs, checks and repositioning are counted.
| Situation | Effect on the plan |
|---|---|
| Gusty conditions round a building | Longer per shot, more reframes, more discarded frames |
| Low sun on one elevation | That face needs a different time of day, not a different exposure |
| Complex roof with many penetrations | Detail shot count often exceeds the overview count |
| Deliverable is a model, not photographs | Overlap jumps, shot count multiplies |
| Anything found that needs a second look | Plan a spare pack for it |
The rest of the planning
The endurance figure this page asks for comes from the drone flight time calculator, and if the payload is heavy enough to matter, check the margin first with the drone thrust to weight calculator. If the job also includes an overhead survey of the site rather than only the structure, the drone mapping and GSD calculator sets the resolution and the drone survey coverage calculator sets the flight lines. For the pack logistics behind a long day, see the LiPo pack and charging calculator. For measuring the roof itself before you fly, the roof pitch calculator covers the slope factor used above.
The two things that actually hurt people
Lithium polymer packs are a real fire hazard, not a theoretical one. A pack that has been punctured, crushed in a crash, swollen into a pillow shape, or run flat below its cutoff can ignite without warning and burns hot enough that smothering it is not a plan. Almost every pack fire that gets written about happened while the pack was charging with nobody in the room. Charge where you can see it, on a surface that does not care, and stop the charge if a pack gets hot or gains thickness. A damaged pack should be moved away from anything that will burn and kept there; for getting rid of it, follow whatever your local hazardous waste program says, because that is a local question and not one a website should answer.
Propellers cut. They are stiff, they turn fast, and a multirotor that has armed on the ground is a running machine. A drone that loses a motor in flight does not glide anywhere; it comes down more or less where it was. Keep people out from under the aircraft and keep your hands away from the arms while the battery is connected.
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
How close should I fly to a building?
Further than feels necessary, and the reason is aerodynamic rather than legal. Wind does not behave near a structure the way it does in open air: it accelerates around corners, rolls off a parapet, and produces gusts from directions that have nothing to do with the prevailing wind. Position hold is also less reliable in the lee of a wall where satellite geometry is poor. Working further out with a longer lens gives the same detail on the sensor with much more room to recover from a drift, and it is the approach that scales to a windy day. The calculator will show you exactly what the extra distance costs in shot count.
Should I use the horizontal or diagonal field of view?
Horizontal, and it matters. The diagonal figure is larger and it is the one usually quoted, because a bigger number sounds better. Putting a diagonal figure into a horizontal coverage calculation overstates the width each frame covers, which means the planned step between shots is too large and the real overlap is less than you specified, sometimes to the point of leaving gaps. If your camera only publishes a diagonal figure and an aspect ratio, the horizontal can be derived from them, and it is worth doing once for the camera you use.
How many detail shots should I plan for?
More than the overview count on a complex roof, and that is not an exaggeration. The overview grid covers the surface systematically; the detail shots cover everything the overview raises a question about, and on a roof with multiple penetrations, flashings, valleys and equipment curbs, that list is long. The practical approach is to plan the overview properly, budget a generous number of detail shots, and accept that the real number is discovered on site. It is much easier to fly a spare pack than to come back.
Can I plan the whole flight from the office?
You can plan the shot count and the time, which is what this page produces, and you should. What cannot be planned from the office is where you launch from, what the wind is doing at the site, whether the elevation you need is lit at the hour you arrive, what is on the roof that nobody mentioned, and whether the airspace or the ground permissions allow the flight at all. Those are settled by a site walk and by checking the current requirements through the official channels before the flight. A number from a calculator is a planning figure, never an authorisation.
Does this work for something other than a building?
The geometry works for any surface you can describe as a rectangle at a distance, so towers, tanks, bridge soffits and retaining walls all fit the same arithmetic by putting the surface dimensions in and treating the standoff as the working distance. What does not transfer is the operational picture. Structures near infrastructure, near people or in places with restricted access bring constraints that have nothing to do with the camera, and some of them are constraints on whether the flight may take place at all. Work out the shots here and settle the permission question separately, in that order.