Aerial survey of land and sites around Bideford

The ground towards the estuary mouth is dune, links and grazed burrows, and it is not stable in the way a field is. Sand moves, marram spreads, blowouts open and close, and the line between what is vegetated and what is bare shifts season by season.

That makes it interesting ground to survey repeatedly and poor ground to survey once. A single capture is a snapshot of a moving system. Two or three flown on the same plan across a year or a decade show the movement, which is usually what somebody actually wants to know.

Photogrammetry works by matching detail between overlapping photographs, and clean dry sand in flat light has very little detail to match. Large uniform faces of it can come back thin or noisy in the model even when the flight was flown properly.

Low sun helps, because it puts texture across the surface. So does capturing dunes with vegetation and paths running through them rather than an unbroken sheet of sand. Where a stretch has come back poorly, the report says so instead of presenting a smooth surface that was never measured.

The old town climbs very steeply from the quay, and the ground on both banks of the Torridge has a lot of height packed into a short horizontal distance. A flight plan drawn as though the parcel were level will sit too close to the ground at the top of the slope and too far from it at the bottom.

Sloping ground is planned as separate runs or with terrain-aware heights. It also produces long shadows morning and evening, which is worth remembering when a winter visit gets pushed to the end of a short day.

There is industrial and marine ground on this estuary as well as farmland: yards, slipways, hard standing and storage areas where material is stacked and moved. That is volume work of the ordinary kind, with one extra wrinkle, which is that a tidal yard can have water across part of it depending on when you arrive.

As everywhere else, the base surface is agreed first, the plan is written down so a repeat visit matches, and anything the water covered on the day is stated rather than estimated.

Salt on the wind is a nuisance for kit rather than a hazard for the survey. Lenses need cleaning more often, and anything left open on a tailgate in an onshore breeze gets a film on it that shows up in the frames as haze.

The practical answer is to work out of the vehicle, check the glass between flights and re-fly anything that looks soft rather than handing over a hazy capture. It is a small thing, but it is the sort of small thing that separates a usable model from a disappointing one on this coast.