Facade Laser Scanning for Glazing and Curtain Wall Replacement

A glazing replacement is ordered to a drawing and installed against a building, and the two rarely agree. The slab edges of an older tower were poured to a tolerance; the mullions of the existing curtain wall were shimmed into place by a crew that is long gone; the embeds that the new anchors have to meet were set decades ago. Facade laser scanning before anything is ordered turns the building as it stands into the measured base that the new system is designed and fabricated to, and this guide covers what that scan captures, how it is done, and what the design team and the fabricator get from it.

What goes wrong without a measurement

A new curtain wall or window system is fabricated to the drawings the fabricator receives. When those drawings come from the original construction set, they show where the slab edges and the columns were meant to be, and the installer finds out at the first floor how far the building drifted from that intent. The usual answer is a field survey with a tape and a total station, which gives a set of spot measurements per elevation, and every embed or slab edge between those spots is interpolated.

A laser scan replaces the spots with a surface. A terrestrial scanner of the Trimble X7 class, by its datasheet, measures up to 500,000 points a second over a range of 0.6 to 80 m with a 3D point accuracy of 2.4 mm at 10 m and 3.5 mm at 20 m (one standard deviation). The facade scanned from the street is recorded as a continuous surface in which the lean of a mullion line and the step between two floors of openings are measured rather than sampled, and the slab edges scanned from the interior perimeter are recorded the same way. The registered point cloud is then the record that the architect, the engineer and the fabricator all draw against, as described on the 3D laser scanning page.

What the scan captures

From the outside, the scan records the facade as it faces the street: the existing mullions and transoms, the sill and head lines of every opening, the returns, the parapet, the canopy and the ground-floor storefront, together with the adjacent buildings and the pavement the new work has to meet. From the inside, with the existing panels still in place, it records the slab edges, the columns and the perimeter beams where they are exposed, and the embeds and brackets where a ceiling or a spandrel panel is opened for the survey. Where the scope needs the structure behind the existing wall, the survey is planned around those openings, because the scanner records what it can see and nothing behind a closed spandrel.

A facade scan is often ordered with the interior perimeter scanned on the same visit, so that the new system’s anchor positions can be checked against the slab edges measured from inside. The scan does not see inside the existing wall or the slab, so cast-in channels and embeds that are covered stay covered until the demolition opens them, and the model marks them as unverified until then.

How the facade is scanned

Low and mid-rise facades are scanned from the street, the sidewalk and the roofs of neighboring buildings where access is granted; the upper floors of a taller building are scanned from the floors themselves, at the openings where panels are out, and from the roof. The stations are registered to each other in the field, and the registration report lists the average error per station and the overlap between scans. The whole cloud is tied to the building’s grid when the project has survey control, or delivered in a documented local coordinate system when it has not.

Glass is a surface a laser scanner handles poorly. Clear glazing returns little of the beam, so the frames and what stands behind them are measured while the panes are thin or missing in the cloud; coated and reflective glass behaves differently and is noted in the scope. Polished metal and dark spandrels are scanned in the instrument’s high sensitivity mode, which the datasheet lists for dark and reflective surfaces and which takes about twice as long for the same point spacing. A facade of a mid-rise building is usually scanned in a day or two, and the quote says how long for a taller one.

What the design team and the fabricator get

The delivery starts with the registered point cloud in E57, the ASTM exchange format for 3D imaging data, and in RCP/RCS for Revit, with its registration report. From it, the facade deliverables are drawn: the existing elevations with every opening dimensioned, the sections through the wall at each floor showing the slab edge and its distance from the facade line, and, when a model is ordered, a Revit model of the existing facade and the perimeter structure at LOD 300, where each element is represented so that its quantity, size, shape, location and orientation can be measured, or at LOD 350 where the model has to show how the slab edges, columns and existing anchors meet, the level the BIMForum specification intends for construction-level coordination. The accuracy of that line work is stated in the quote in the U.S. Institute of Building Documentation’s Level of Accuracy terms, a standard deviation at the 95 percent confidence level, and a tighter level can be specified for the anchor zones measured at short range.

What the scan does not produce is the fabrication model. LOD 400, in the BIMForum specification, is the element “with detail sufficient for fabrication, assembly, and installation”, and that is the fabricator’s model of the new system, built to the fabricator’s tolerances against the measured base. The scan supplies the base; the system’s design, its anchors and its shop drawings stay with the curtain wall engineer and the fabricator. The MEP BIM page describes the systems behind the wall when they are in scope, and the Scan-to-BIM page the model itself.

Price and time

A facade survey is priced on the number of elevations, the building’s height and whether the interior perimeter is in scope. Typical ranges from recent projects: interiors together with the facades and the site, $2,500 to $5,000; an office or a commercial floor, $3,000 to $9,000; large buildings run from $12,000 and are quoted on their own. The exact price comes in a quote within one business day of your sending the address, photographs of the facade and the deliverables you need. The scan is scheduled as soon as the project needs it, often within days of the order, and the elevations and sections usually follow in four to eight weeks for a commercial building, sooner for a house. We scan in all 50 states.

Sources

• Trimble X7 3D laser scanning system datasheet: scan speed, range, 3D point accuracy, high sensitivity mode, registration report.
• BIMForum, Level of Development Specification 2023, Part I: the LOD 300, 350 and 400 definitions.
• USIBD Document C120, Level of Accuracy Specification Guide, version 2.0 (2016): the LOA levels as standard deviations at the 95 percent confidence level.
• ASTM E2807, Standard Specification for 3D Imaging Data Exchange: the E57 file format.