Q&A: Can 3D Scans Save Notre Dame?

By Dan Verderosa, April 24, 2019
IC professor Michael “Bodhi” Rogers discusses how 3D laser-scanning technology can be used to repair historic buildings.

After an accidental fire caused heavy damage to Paris’ Notre Dame Cathedral on April 15, 2019, experts say intricate 3D laser scans of the building could prove instrumental to its restoration. Ithaca College physics professor Michael “Bodhi” Rogers has been using 3D laser scanners to create digital models of historic structures for years, including Abraham Lincoln’s summer cottage in Washington, D.C., and Ireland’s Trim Castle. IC News spoke with Rogers about using 3D scanning technology to help repair damage to historic buildings.

IC NEWS: How can 3D scanning technology be used in the repair and rebuilding of damaged buildings like Notre Dame?

ROGERS: When my team and I conduct a scan of an historic structure, we take positional readings every 5 millimeters inside and out of the building. The resulting data is referred to as a point cloud. The point cloud can be used to make detailed measurements of the damaged portions of the structure from all sides. When the damage is minor, such as to the detailed molding around a door, the data can be sent to a computer-controlled milling machine to reproduce the damaged piece of molding.

The level of detail from a high-quality laser scan surpasses traditional architectural and historic preservation drawings due to the three-dimensionality of the data. When an architect or preservation specialist creates an architectural rendering, they make a choice as to what measurements to record. For example, the size of a window is typically recorded as two measurements: height and width. Yet that window might not be square or the same size on the top as the bottom. If that window was destroyed, these fine details would be lost. Because our laser scans preserve every detail to within 5 millimeters, we can go back and take as many new measurements, from any angle or perspective, in the unfortunate case of massive damage like that Notre Dame sustained.

IC NEWS: How do 3D laser scanners work?

ROGERS: An architectural 3D laser scanner emits pulses of laser light. The pulse of laser light leaves the scanner, travels to the object being scanned, reflects off that object, and finally returns to the scanner. The time it takes to travel from the scanner and back is used to calculate the distance of the object from the scanner. Modern architectural 3D laser scanners can precisely measure the vertical and horizontal angles where the scanner is pointing. This data is used to calculate a positional reading of the object in the x, y and z-axes.

Modern scanners can pulse 50,000 to 1,000,000 times per second while spinning vertically and horizontally. This allows the scanner to record the locations of objects inside of a room every 5 millimeters, scan the entire room and take photographs that can be used to give the point cloud color.

IC NEWS: What advantages does 3D scanning give over traditional architectural renderings?

ROGERS: The best way to think about a point cloud is that it is a three-dimensional photograph. Instead of looking at a flat photo or two-dimensional architectural rendering, you can look at a point cloud from any angle. You can even “fly-through” the building by passing through walls and floors to look around rooms, the roof and the outside of the building.

A man standing next to a 3D scanner inside a house

A student uses a 3D scanner inside the home of American Revolutionary War general Philip Schuyler. (Photo provided)

IC NEWS: What challenges do you face when scanning a structure?

ROGERS: Just like when sunlight strikes a building and casts a shadow, the laser scanning process also results in shadows. The laser pulse cannot pass through objects, so the scanner needs to be positioned to minimize these shadows. The scanner then needs to be moved to a new location to fill in the areas that were not able to be scanned from the first location.

Another challenge is working in tight spaces. The scanner sits on a tripod that is 4-5 feet tall, but we also have a tripod that is 2-3 feet tall in order to better handle tight spaces, such as narrow stairs, cellars and attics.

A final challenge is managing the number of scan locations. An historic building such as Notre Dame can require 100 or more scan locations. Our documentation of Trim Castle in Ireland required about 150 individual scan locations. When scanning we use a scan grid system, which means that all of the scans come together quickly, but each scan needs to be examined and unwanted data need to be removed once we are back in the laboratory. Examples of data that need to be removed are birds, the sun and objects recorded through windows, as well as tourists, animals and cars that may be in the area during scanning.

IC NEWS: What other uses are there for 3D scanners?

ROGERS: In addition to historic preservation, laser scanners are used at construction sites, during bridge repair, at crime scenes and at automobile accidents. The laser scanners we used are recalibrated every year, and the manufacturer provides a legal document describing that recalibration. This document allows for scan results to be presented as evidence in a court of law.