TR 78 and GSD: The Numbers Behind a Compliant Drone Facade Inspection
Blog
11 February 2026
TR 78 and GSD: The Numbers Behind a Compliant Drone Facade Inspection
Two numbers decide whether your drone facade inspection is a compliance document or an expensive photo album.
The first is a standard: TR 78. The second is a measurement: 0.15 centimetres per pixel.
Most building owners have never heard of either, which is unfortunate, because they are the difference between data BCA accepts and data it doesn't.
What TR 78 is
TR 78 is a Singapore Technical Reference covering building facade inspection using unmanned aircraft systems. It comes in two parts:
TR 78-1 — building facade inspection using Unmanned Aircraft Systems (UAS)
TR 78-2 — specification for quality management and the application of artificial intelligence
Alongside it sits SAC Technical Note FI 01, setting the specific requirements for accreditation of inspection bodies carrying out building facade inspection using UAS.
A Technical Reference is not law in itself. Its force comes from what it is attached to.
Why it matters: the accreditation chain
Here is how the pieces connect, and it is worth understanding because the marketing language in this industry is loose.
The Singapore Accreditation Council opened an accreditation area for building facade inspection using drones under its Inspection Body scheme with effect from 12 July 2023. The accreditation provides assurance on inspections carried out by drone service providers and inspection companies for building facade inspections, and it is recognised by BCA for the periodic facade inspection regime.
On the regulatory side, the Commissioner of Building Control has approved the use of unmanned aircraft systems operated by accredited UAS service providers for the visual inspection of building facades. The industry is encouraged to engage only accredited providers, and accreditation status is verifiable through the SAC website.
So the chain runs: TR 78 sets the technical requirements → SAC accredits inspection bodies against them → BCA recognises that accreditation for PFI.
Which produces a practical test for building owners. Not "does this provider have drones and a CAAS licence?" — many do. The question is: is this provider an accredited inspection body for building facade inspection using UAS, and can I confirm it on the SAC website myself?
A CAAS permit means the operator may legally fly. Accreditation means the inspection has a documented quality system behind it. They are different things, and providers occasionally blur them.
The other duties TR 78 doesn't remove
Accreditation of the UAS provider does not displace the Competent Person. Under the PFI regime:
The responsible party appoints a Competent Person — a Professional Engineer or Registered Architect — who may be assisted by a Facade Inspector, both having passed a BCA-approved Certificate of Facade Inspection course.
Before carrying out visual inspection using accredited UAS, the CP notifies the Commissioner of Building Control by indicating the UAS details in the prescribed e-Form.
The CP must be present, or have their supervised Facade Inspector present, during UAS operations.
The CP must personally review all inspection outputs — photographs, images, readings and reports — and consult specially trained persons where necessary.
The CP conducts a full visual inspection and a representative hands-on inspection for each elevation.
The drone accelerates the visual survey. It does not replace the professional, and no accreditation changes that.
GSD: the number underneath everything
Ground Sampling Distance is the real-world size of one pixel in your image. If the GSD is 1.5 millimetres, then each pixel in the photograph represents a 1.5mm square of your building.
It is determined by three things: sensor characteristics, focal length, and distance from the subject. Fly further away or use a shorter lens, and each pixel covers more wall. Fly closer or use a longer lens, and each pixel covers less.
That is the whole concept, and it governs everything about inspection image quality.
BCA's requirement
For UAS facade inspections, BCA asks for the original images of any defects captured during the inspection to be furnished on request. Those photo files must be free of any annotation, in .jpg or .png format, and at a resolution of 0.15 cm/pixel or lower.
Note what "or lower" means here: lower GSD is better. 0.15 cm/pixel is 1.5 millimetres per pixel — the ceiling, not the target.
Note also the requirement for unannotated originals. Your provider must retain and be able to supply the raw captures, not just the marked-up report images. Ask about their data retention policy; it is a question almost nobody asks and occasionally the answer is uncomfortable.
What 1.5mm per pixel gets you
A hairline crack in concrete is typically 0.1 to 0.3mm wide — narrower than a single pixel at the required GSD.
That sounds like a failure until you understand how detection actually works. A crack does not need to fill a pixel to be visible; it needs to change the value of pixels it passes through. A dark line across a light surface creates measurable contrast even when narrower than the pixel it occupies, and it becomes reliably identifiable when it runs across several pixels in sequence.
In practice, at 0.15 cm/pixel with adequate lighting and a sharp capture, fine cracking is detectable, and everything larger — spalling, tile displacement, sealant failure, corrosion staining — is comfortably resolved.
What breaks it is not the specification but the execution: motion blur, poor light, excessive standoff, or heavy compression. A technically compliant GSD from a blurred frame is worthless.
Why GSD is worth understanding beyond inspection
The same concept governs drone mapping and photogrammetry. Survey deliverables — orthomosaics, digital surface models, point clouds — are all quality-limited by the GSD of the source imagery, and feed directly into BIM and digital twin work. When a mapping provider quotes accuracy, GSD is the underlying constraint.
This is why inspection capture and photogrammetric capture combine so naturally. Imagery captured to a facade inspection standard is high-quality survey imagery, and one flight can serve both purposes.
Five questions this article should equip you to ask
Are you an SAC-accredited inspection body for building facade inspection using UAS? (Then check the SAC website independently.)
What GSD will you deliver on my building, and at what standoff distance?
Do you retain unannotated original defect images, in what format, and for how long?
How does your workflow align with TR 78-1, and what does TR 78-2 mean for any AI-assisted defect detection you use?
Who is the appointed Competent Person, and have they filed the UAS notification? (See the 12-week compliance plan.)
Any provider genuinely working to these standards will answer all five without hesitation. Hesitation is the signal.
The bottom line
TR 78 and SAC accreditation tell you the inspection has a documented quality system behind it. GSD tells you the images are good enough to find what you are paying to find. BCA's 0.15 cm/pixel is the specific threshold your data has to clear.
Three things to check, all verifiable, none requiring you to be an engineer. In a market where every provider claims high-resolution inspection, they are the only claims that mean anything.
