A solar panel installer draws his layout plan on the basis of the roof as it actually is. Not on the basis of a building plan from twenty years ago that is missing the rooflights, the ventilation ducts and the later extension. For an industrial roof, that usually means someone goes up onto the roof with a tape measure. Or someone has the roof surveyed from the air.
AAtechnics, an installer of solar installations, charging infrastructure and battery storage, asked us to survey the roofs of Van de Velde Beton in Opwijk in preparation for a PV project.
The question
The installer needed three things to draw his layout plan:
- The roof outlines, per roof plane
- The height differences between the roof sections
- Everything on the roof: rooflights, vents, rooftop structures
Accuracy required: better than 2 cm for the relative dimensions on the roof. Delivery format: an orthophoto in Lambert 72 and a DWG drawing to draw on directly.
Why a drone makes sense here
The roof plane concerned covered about 890 m², in the middle of a continuous row of halls. Surveying from the roof itself means fall protection, someone working near rooflights that cannot be walked on, and half a day's work for one person or more. From the air it is one flight, without anyone having to go onto the roof and without access to the production hall.
The approach
Preparation. The site was outside any geozone. The flight fell within the Open category; a risk assessment was still part of it. Because the business premises are freely accessible during the day and lorries drive around, we notified the site in advance and scheduled the flight after opening hours, with the site empty.
Capture. A photogrammetric flight with RTK positioning, looking straight down and at an angle, so that upstands and roof edges are also captured.
Processing. The images become an orthophoto in Lambert 72. On it we draw the roof outlines, the height levels and the objects on the roof in DWG, in layers the installer can use directly.
What we learned
Rooflights are awkward. Glass and polycarbonate reflect and let light through. In a point cloud they produce holes and noise. For the outline of a rooflight, the orthophoto is more reliable than the 3D data, and that has to be built into the method.
Relative and absolute are not the same. With RTK but without ground control points, the relative dimensions on the roof are better than 2 cm. The position in Lambert 72 is then indicative. For a layout plan, the relative accuracy is enough: the installer needs to know how large the roof plane is and where the obstacles are, not to the centimetre where the roof sits in the national grid. For a project where the absolute position matters, we add control points. That is stated in the quote in advance.
The roof does not stop where the assignment stops. The designated roof plane was in the middle of a row of halls. The adjacent roofs are captured in the same flight. If the installer later wants to use those roofs as well, the data are already there.
Result
When a roof survey for PV makes sense
- Industrial and commercial roofs from a few hundred square metres
- Roofs with rooflights, rooftop structures or height differences
- Roofs for which no up-to-date plan exists
- Situations where the roof cannot be accessed safely or freely
After installation, a thermal capture of the finished installation can serve as a baseline for the warranty period. Read when PV thermography tells you something.
Related service: Surveys