The Doming Problem, and the Ten Minutes That Fix It

If you have ever processed a drone survey and found the edges of the block sitting a few centimetres lower than the centre — or higher — you have met doming. It is one of the most common systematic errors in photogrammetry and one of the easiest to avoid.

What causes it

A flight flown entirely at nadir, at constant altitude, with a camera whose lens distortion is being solved from the imagery itself, gives the bundle adjustment an ambiguity it cannot resolve. A small error in the radial distortion model looks almost identical to a gentle curvature of the ground. The solver picks one, and it is often the wrong one.

The result is a surface that is internally consistent and globally bent.

The fixes, in order of effort

• Add oblique images. Tilt the camera to around 20 degrees for one pass. This breaks the ambiguity directly and is the single most effective change.
• Fly a crossing grid. A second pass perpendicular to the first, ideally at a different altitude.
• Use a pre-calibrated camera and lock the distortion parameters rather than self-calibrating.
• Distribute ground control at the edges, which constrains the curl even if it does not remove the cause.

The first two cost flight time measured in minutes on a typical block.

How to detect it after the fact

Hold back checkpoints across the full extent, including the corners. Plot the vertical residuals against distance from the block centre. If there is a trend, you have doming. If your checkpoints are all near the middle, you will not see it, which is one more reason to place them at the edges.

Why it persists

Because nadir-only grid flights are what the flight planning apps default to, and because the error is invisible unless you specifically test for it. The deliverable looks perfect. It measures wrong.

Processing teams see this constantly — when I hand data to UAV Data Processing the first question is whether there is an oblique pass, and if there is not, they will say what the accuracy statement can honestly claim.

The habit worth forming

Add the oblique pass by default. Not when the job seems to warrant it — always. Ten minutes of flight time is cheaper than a return visit, and vastly cheaper than a surface nobody trusts.