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3D Scanning for Historic Preservation and Restoration

3D Scanning for Historic Preservation and Restoration
Luiza Plieva

The cornice profile, the brick bond, the sag of a hundred-year-old floor, the ornament that a mason cut once and nobody has measured since: these are what the restoration of a historic building has to reproduce, and they exist only in the building itself. This guide explains 3D scanning for historic preservation: how a scan records a historic building, what it captures that a tape survey does not, how it relates to the documentation standards that heritage records are held to, and what the architect, the conservator and the owner get from it.

Why historic buildings are scanned

A modern building was built to drawings that still roughly describe it. A historic building was often built to no drawing that survives, and it has moved, settled, been added to and been patched for a century: nothing in it is plumb, square or repeated exactly, and that irregularity is the thing worth preserving. A tape survey of such a building records the dimensions someone decided to take on the day, straightens what it cannot describe and leaves the ornament to a sketch. A laser scan records every surface in its line of sight as it stands. A terrestrial scanner of the Trimble X7 class, by its datasheet, measures up to 500,000 points a second with a 3D point accuracy of 2.4 mm at 10 m and 3.5 mm at 20 m (one standard deviation), and the registered point cloud it produces holds the lean of a wall, the camber of a floor and the profile of a molding in the same record, to be drawn from now or in twenty years.

For ornament that has to be reproduced, a carved capital or a cast-iron bracket, a handheld scanner of the HandySCAN SILVER class is used on the object itself at close range; its brochure rates it at an accuracy of up to 0.040 mm as a typical value for diameter measurements on a calibrated sphere, and the mesh it produces is the record from which a replacement is carved or cast. The 3D scanning for parts and objects page describes that work.

The documentation standards

Documentation that enters the national heritage record is held to standards the National Park Service maintains. Its Heritage Documentation Programs, the Historic American Buildings Survey among them, “document historic sites and structures across the United States through the creation of measured drawings, large-format photographs, and historical reports”, archived at the Library of Congress, and the programs maintain “the Secretary of the Interior’s Standards and Guidelines for Architectural and Engineering Documentation” so that documentation “meets uniform requirements for quality, content, and archival stability”. A laser scan does not replace that documentation; it is the measurement that measured drawings are traced from, and the point cloud is a record the owner keeps alongside them.

Rehabilitation work that applies for the federal historic preservation tax credit is evaluated against the Secretary of the Interior’s Standards for Rehabilitation, and the application documents the building’s condition before the work and the work as completed; the guide to 3D scanning for the historic tax credit covers what the scan adds to that record.

What the scan captures

Outside, the scan records the facades with every opening, string course and cornice as they stand, from the street and from neighboring roofs where access is granted, and the roof where it can be seen. Inside, it records the rooms with their ceilings, the stairs, the moldings and the fireplaces, and the attic and basement structure where they are exposed. Because the scanner measures surfaces, it records their condition as well as their shape: a bowed wall or a sagging floor is in the cloud as measured geometry, and a scan repeated after a year shows movement larger than the scanner’s own accuracy.

What the scanner cannot see is not in the cloud. The structure behind plaster and the foundation under a floor are documented from openings the conservator makes, from older drawings or from a separate survey, and the drawings mark them as taken from those sources. Polished stone and dark surfaces 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. Clear glass returns little of the beam, so a window registers mostly as its frame.

What the architect and the conservator get

The registered point cloud is delivered in E57, the ASTM exchange format for 3D imaging data, and in RCP/RCS for Revit, with its registration report and the panoramic images from each scan position, which on a historic building are a record in their own right. From it, the measured drawings are traced: plans, elevations, sections and the details of moldings and profiles, in DWG and PDF, with the sheet saying which lines were measured and which were not. A Revit model of the building is built from the same cloud when the design is done in Revit, at the level of development the scope names. The accuracy of the line work is stated in the U.S. Institute of Building Documentation’s Level of Accuracy terms, as a standard deviation at the 95 percent confidence level; the USIBD guide has a heritage view of its suggested levels, which generally run tighter than the standard ones, and a tighter level can be specified for the profiles that a replacement has to match, measured at short range.

The as-built drawings page describes the sheet set, and the 3D laser scanning page the scan.

Price and time

A historic building is priced on its area, its stations, which are many because its rooms are many and its ornament is dense, and on what is built on the cloud. Typical ranges from recent projects: a house or an apartment with drawings or a model, $2,000 to $4,500; interiors 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 point cloud alone starts at about $1,200. The exact price comes in a quote within one business day of your sending the address, photographs and the deliverables the project needs. The scan is scheduled as soon as the project needs it, often within days of the order, and the drawings usually follow in two to eight weeks depending on the scope. We scan in all 50 states.

FAQ

Why scan a historic building instead of measuring it by hand?

A tape survey straightens what it cannot describe. A laser scan keeps the irregularity, the lean, the sag and the ornament, as a measured record that can be drawn from at any time.

Does a laser scan meet the HABS documentation standards?

The scan is the measurement; HABS documentation consists of measured drawings, large-format photographs and historical reports prepared to the Secretary of the Interior’s Standards and Guidelines for Architectural and Engineering Documentation. The measured drawings are traced from the scan, and the owner keeps the cloud as a record of its own.

Can you scan the ornament?

A building scanner records a cornice or a capital at its own resolution; for a profile that a replacement has to match, a handheld scanner records the object itself at a brochure-rated accuracy of up to 0.040 mm, and the mesh goes to the carver or the foundry.

What does the scan not record?

What it cannot see: the structure behind plaster, beam pockets, foundations. Those are documented from openings, older drawings or a separate survey, and the drawings say so.

How accurate are the drawings?

The scanner is rated at a 3D point accuracy of 2.4 mm at 10 m (one standard deviation) by its datasheet. The line work is stated in USIBD Level of Accuracy terms in the quote, with a tighter level available for profiles measured at short range.

How much does it cost?

A house with drawings or a model usually runs $2,000 to $4,500 and the point cloud alone starts at about $1,200; larger buildings are quoted on their own. The quote arrives within one business day.

Sources

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