A single aerial photograph of a site is a marketing image. The same photograph taken from the same position every month for two years is a defensible record of what happened, when, and it is worth considerably more than the flights cost.
One aerial photograph of a construction site is a nice image for a hoarding. Twenty-four of them, taken from the same positions at the same intervals, are something else entirely: an incontrovertible record of what the site looked like on a specific date.
The second thing is worth far more than the first, and it costs very little more to produce.
What makes a series comparable
Repetition only works if the visits are genuinely repeatable. Four things have to be fixed:
Positions. The same GPS coordinates, the same height, the same heading, every visit. Recorded once at the first visit and flown as a mission thereafter.
Framing. The same focal length and the same composition. A frame that includes a fixed landmark at the edge is worth designing in, because it gives the eye an anchor when the images are compared.
Interval. Regular, and matched to the programme. Monthly suits most building projects. Weekly suits groundworks and fast-moving phases. Ad hoc visits produce a set of pictures rather than a series.
Time of day. Approximately consistent, so shadows fall in a similar direction. This sounds cosmetic and is not: shadow direction changes what is legible in a frame, and a series shot at wildly different hours is much harder to compare.
Do those four and the sequence can be flicked through like an animation. Miss them and you have a folder.
What the record is actually used for
Progress verification. Against a programme, an aerial record settles arguments about what had been completed on a given date without anybody relying on memory.
Delay and variation claims. This is where the money is. A dated, geometrically consistent series showing site conditions is contemporaneous evidence, and it is the kind that survives scrutiny because it was collected routinely rather than assembled after a dispute began.
Logistics planning. Crane positions, laydown areas, access routes, muck away, temporary works. Seeing the whole site at once changes decisions that are otherwise made from a site plan and a walk round.
Quantities. Flown with the right overlap, the same visit produces an orthomosaic you can measure off, or a surface model for cut and fill and stockpile volumes. See our guide to point clouds, orthomosaics and 3D models and our stockpile and volumetric surveys page.
Stakeholder and client reporting. A single aerial in a monthly report does more than four pages of narrative, and it is the item clients actually look at.
Marketing and record. At the end, the series is the story of the project, and it is the only chance anybody will ever get to make it.
The London complications
Two things make progress monitoring harder in the capital than on a greenfield site.
Airspace. Much of central London sits under restrictions that require specific authorisation, and some sites sit near helicopter routes or hospital helipads. This is entirely workable but it has to be established at the start of a programme rather than discovered at visit three.
Uninvolved people. A dense urban site is surrounded by people who are not part of the project, and the separation rules apply to all of them. In practice, launch positions inside the hoarding, agreed with the principal contractor, solve most of this, and it is another reason to set the programme up properly at the first visit.
Setting it up
The first visit is the one that matters. It establishes the positions, the framing, the flight plan and the site arrangements, and everything after that is a repeat of it. That means the marginal cost of each subsequent visit is low, and it means whoever flies visit one should expect to fly all of them.
If you are already on site and have not started, start now. A series that begins at month eight is still worth having, and it is the single most common regret we hear.
See construction progress monitoring, construction site mapping, construction drone filming, or get in touch with the site address and the programme dates.