Beyond Digital Heritage Preservation

3D Laser Scanning for Architectural Assessment and Surface Documentation

A 1.046Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE

The use of 3D laser scanning within the heritage preservation field is well known and publicized. However, the majority of projects are typically displayed as either compelling screenshots of colorized point cloud images or 3D models rendered from the data. The use of this type of deliverable for technical architectural or engineering documentation is limited. The point cloud data must be converted to a line or surface drawing that can be used within standard CAD software, and the resulting 3D models are nothing more than the drafter’s interpretation of the data.

One of the failings of 3D CAD software is it is generally based upon creating orthogonal and geometric curved shapes, it is unable to illustrate the fluctuating potato chip-like surfaces that actually exist on a building. How, coming from a technical requirement for accuracy, can one utilize point cloud data and photography to advance the understanding of a building’s condition and hence, generate a better workflow in onsite investigation and proposed restorative measures?

One real advantage of 3D laser scan data is its ability to accurately document variations in surface conditions. When properly scanned in the field, the resulting data will capture every nuance of the building’s surface. Finally we have a method that can rapidly record the variations in surface plane of a rusticated stone wall, or the bulges and tilts that exist in a seemingly flat planar wall or floor surface. Yet too often we take the richness of that data and simply convert it to a flat plane surface within a CAD or Revit model. The undulations and cracks in a building’s surface can reveal a lot about what is happening to a structure as it ages. The following are two examples of using the full potential of point cloud data to provide levels of documentation that would have been very difficult to achieve without laser scanning.

The High Bridge, part of the Croton Aqueduct, is the oldest remaining bridge in New York City. Closed to the public since 1970, it is currently undergoing restoration. Along with the bridge, restoration efforts are being focused on the massive brick vaulted gatehouses located at each end. The Manhattan side Gatehouse building is exhibiting numerous cracks and shifting of the walls. This information needed to be documented within a 3D model so that an understanding of the forces behind the movement could be calculated and proper restorative measures proposed. The curved surfaces and meandering layout of the underground water tunnels made laser scanning a perfect choice to document the existing conditions of this structure (see Fig. 1).

Using data obtained from a phase based FARO scanner, a dense 3d point cloud model was formed of the interior and exterior of the gatehouse. This data was brought into AutoCAD using kubit’s point cloud software to create 2D and 3D line drawings as a basis for documentation. The architect was able to take multiple slices of the point cloud cutting through the vault, to illustrate the movement and depth of the cracking. However, the density of the point clouds and resulting file sizes made it difficult to work with the data. Various solutions were tried including reducing amount of scan points, compression of data and conversion to a mesh surface (see Fig. 2).

The best solution was to utilize a hybrid of visual and scan data. A 3D model was prepared using 3DStudioMax, onto which was applied the scan data image, allowing the location of cracks to be accurately placed over the curving surface. A linear surface feature like cracks can be based upon an analysis of the scan data images, whereas information on the object’s surface geometry is provided from the scan data itself. The resulting models provided both 3D and 2D representations of the building’s movement; with the underlying point cloud data always available to study surface variations at any given point within the building’s surface (see Fig. 3).

3D Data, 2D Documentation
Local Law 11 is an ordinance enacted by the City of New York that requires owners of buildings over six stories in height to have the facades inspected by an architect or engineer every five years. This has resulted in a need for updated faade documents of many buildings, to serve as a basis for the inspection report and any ensuing remediation work proposed. While 3D point cloud data is not required, its use can serve as a basis for the 2D documents needed as well as giving insight into surface movement issues that exist on the facades not readily apparent to cursory visual observation.

A recent project at the Ritz Carlton Hotel in Manhattan illustrates how scanning, along with conventional photography, can be used to aid the architect in their assessment of the building. Like many of Manhattan’s older masonry buildings the Ritz Carlton Hotel is rich in detail and ornamentation, items that are often difficult to measure and draw using conventional techniques. Laser scanning will record an entire faade in one day, a substantial reduction over time required using hand measuring methods. The use of scan data as a basis for creating 2D drawings is also an efficient method when illustrating complex architectural ornament and organic shapes.

However, while laser scanning will produce the dense point cloud data that is required to create such documents, the visual clarity of the data is often an issue in maintaining the accuracy of the drawn deliverable. The use of color images taken from laser scanners can help clarity, but its quality falls short of the crispness that is delivered by today’s high quality digital cameras. Digital photography can be an effective tool to enable better visual understanding of the data being transcribed. The combined use of both point cloud data and color photography is a way to use complementary techniques that produce a visually enhanced, accurate 2D faade document (see Fig. 4).

Use of a "photoplan" in conjunction with a standard 2D architectural faade drawing provides a level of visual communication that is easily understood by both laymen and professionals. The use of high quality photos in this manner adds value to a heritage project, as it enable one to have a record of color, materials, texture and the current state of conservation. Using any number of programs available on the market, multiple digital photographs of the faade can be rectified and aligned to create a scale image of the faade. Point cloud data can serve as a backup to those photos, creating a smart data set behind the sharp visual image. Software exists that will in essence wrap a photograph over the point cloud data, creating a 3D photographic image. The underlying point cloud data can be removed, enabling a lighter weight file for use in visual presentations and web browsing (see Fig. 5).

The caveat in these methodologies is that they are dependent upon the skill and expertise of the person drafting the image from the data. Once a line is drawn, it represents one’s interpretation of the data. In order to achieve a reasonable level of accuracy, it is important to have a well processed set of point cloud data and photos as well as personnel trained in processing and understanding the data. For this project a mix of street level scans with high resolution scans from adjacent building roofs and terraces allowed preparation of drawings with a high level of discernible detail. By selectively bringing parts of the scan data into CAD at high resolution, the details can be properly drawn. Digital photographs can serve as both a reference, or if properly rectified, as a basis for the drawing itself.

Beyond its use as a drawing base, the 3D capability of the scan data can be used to provide forensic study of the faade’s surface. By manipulation of the data against true horizontal and vertical planes within AutoCAD using kubit’s software, portions of the faade can be mapped to create a topographic representation of the surface variations that exist on the building. This mapping can detect depressions or bulges in the faade such as those that may exist at a window head caused by rust jacking, or a surface anomaly in a wall from a stress crack. It can indicate if a wall is leaning or tilting off vertical. Identification of these anomalies at an early stage of the inspection helps to focus the engineers and architects efforts, as well as advise the owner as to the possible extent of problem areas and remediation costs (see Fig. 6).

The use of laser scanning in Architecture, especially in the area of heritage buildings, has tremendous potential for allowing both a greater understanding of the building’s fabric and its true dimensions and geometry. This will provide increased accuracy in the information delineated within bid documents, reducing the "unknown factor" that results in inflated bids and hidden change orders.

For heritage projects, 3D scan documentation provides a true authentic record of the building at a specific point in time. The archived images and data collected provide a detailed level of information that can be used to study the building as well as becoming the basis for actual reconstruction of the building or portions of the building at a later date if needed. The multi-dimensional data allows one to create a replica that matches the subtle touches of the building more than ever possible using conventional 2D records. Many heritage buildings surviving today were products of the latest technological advances of their time. Today, with the help of our latest modern technology, we can effectively restore and preserve those buildings.

John Smits, AIA has over 30 years’ experience in the architecture and construction industry, as Principal of an A/E office to directing development projects in NYC. He has developed methodologies for applying 3D laser scan data specifically to the architecture, engineering and construction professions, focusing on its use in both documentation and forensic study of buildings. A frequent lecturer to professional groups and organizations on laser scanning technology, he currently runs Actus3D, a laser scanning service and consulting company in New York.

Lourdes G. Cerezuela is a young Spanish architect specializing in Heritage Preservation using 3D laser scanning, studying and working around the world. After completing her education in Spain, she joined an archeological mission in Syria, and then continued various projects in the UK, Italy, and Armenia. Currently interning with a NYC based Architect and Laser Scan Company.

A 1.046Mb PDF of this article as it appeared in the magazine complete with images is available by clicking HERE

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