Part of a series of articles titled Alaska Park Science Volume 20 Issue 1 - Parks as Proving Grounds.
Article
High-definition Laser Scanning for Documenting Cultural Resources
John Wachtel, National Park Service
NPS/ALASKA REGION
Library of Congress, Prints & Photographs Division, KY-20-19-7
The National Park Service’s (NPS) Heritage Documentation program has been documenting historic sites for more than 85 years and the collection of drawings, photographs, and written histories has grown to more than 200,000 pieces stored at the Library of Congress. The Historic American Buildings Survey (HABS) was initiated in 1933 as the first of the three Heritage Documentation programs and as a “make work” program for unemployed architects and draftsmen. The objective of the HABS program was to document structures that represented American history through accurate line drawings. Using tape measures and building levels, documentation teams spent weeks measuring buildings to within 1/8” accuracy (Figure 1). The field notes would be transcribed onto ink on Mylar or vellum providing accurate plans, sections, elevations, and details of the subject building. The Heritage Documentation Program expanded in 1966 to include the Historic American Engineering Record (HAER) and in 2000, the Historic American Landscape Survey (HALS). As their names suggest, HABS focuses on documenting buildings and structures, HAER records sites and structures related to engineering and industry, and HALS documents historic landscapes.
Documentation Advancements
The process to document historic sites has evolved in recent years with the introduction of new tools and technologies. One of the most notable advances occurred with the introduction of Computer Aided Drafting (CAD), making it possible to transfer field notes into digital files and expanding the potential uses for drawings.
More recent technological advances include High-Definition Laser Scanning, which greatly increases the amount of information that can be acquired on site. High-Definition Laser Scanning (or laser scanning) is the process of collecting highly accurate and detailed spatial data resulting in a three-dimensional (3D) digital representation of the site or structure, commonly called a point cloud.
Some sites may pose safety concerns with traditional documentation methods. Others may require teams of multiple people and several site visits to obtain the basic information necessary to begin documentation. In some cases, laser scanning may address both issues. The speed at which data can be collected (thousands, even millions of points per second), paired with long-range scanning (up to 300 m depending on make and model) means that the operator can collect more information from safer vantage points.
Of course, with the increased ease of data collection comes the need to process, manage, and store all those data back in the office. Processing data generally requires a desktop computer with relatively high-end specifications. Solid-state hard drives (SSDs), a central processing unit (CPU) with as many cores as possible, a minimum of 32 gigabytes (GB) of random access memory (RAM), and a dedicated graphics card with a minimum of 2 GB of video RAM (VRAM) are all recommended. Depending on the computer used for processing and skill level of the user, the amount of time spent in the office may be double what is spent in the field collecting the data. The amount of storage space required to process a project can vary, but averages around 50-100 GB. Understanding the processing demands as well as the storage requirements may help inform the scope of a project.
Laser Scanning, a Treatment?
Although there are many benefits of conducting laser scanning for documentation purposes, it should not be thought of as a preservation treatment itself because it is only collecting information. Simply laser scanning a building does little to preserve it. It is simply data on a flash drive until it can be processed back in the office. Having a plan for the data is critical to producing meaningful and useful documentation that captures the character-defining features of a site or structure that can then be used for preservation, maintenance, and interpretive purposes. Laser scan data can become meaningful documentation through a process of informed interpretation using CAD software, and in turn that documentation can be used to facilitate treatment.
Why Drawings?
Production of a set of architectural or interpretive drawings using CAD is still the gold standard for documentation. Drawings are a simple and convenient way to communicate general and specific information in a format that has well-established standards. There is relatively little effort and cost required to read a set of full-size architectural drawings, when compared to the computer hardware and software requirements necessary to interpret laser scan data on the computer in a meaningful way. Additionally, when plotted on materials such as vellum and stored appropriately, drawings can far outlast digital files, which tend to have a much shorter shelf life.
Drawings from point clouds are the result of careful review and classification of the scan data (Figure 2). Using specialized software, drafters are able to isolate certain portions of the data, making it easier to visualize. From there, a process of tracing key features with clean vector line work begins. Vector images are different from raster images in that they can be scaled to any size without losing quality, as opposed to raster images that have a fixed resolution and lose fidelity when scaled up. Once completed, the line drawing provides a crisp, legible, and accurately scaled representation of the subject.
A draftsperson can then trace over the features of the point cloud, creating a line drawing. The line drawing is more legible than a point cloud, which is best viewed using computer software to rotate and examine from all angles.
NPS/ALASKA REGION
Added Benefits
Traditional HABS/HAER/HALS documentation projects produce a set of drawings, large format photography, and a written historical component. Each component contributes important context to the other, resulting in a comprehensive resource for managers, facility and maintenance staff, architects, historians, students, and stewards of historic properties.
In addition to the pairing of CAD and laser scanning, additional products can be derived alongside the traditional drawings. These include scaled orthographic images of the point cloud, rendered flythrough animations, 3D models, and virtual or guided online tours. When produced individually, these products tend to have a shorter shelf life, but can increase exposure, awareness, and understanding of the resource.
Sample Projects
Kantishna Roadhouse
In the spring of 2017, high-definition laser scanning was conducted on the Kantishna Roadhouse (Figure 3), located within Denali National Park and Preserve (built from 1919-1920). The building is a significant example of the exploration and settlement theme from early twentieth century Interior Alaska. The project objectives were to obtain laser scan data of the building and surrounding site in order to produce a set of architectural drawings to HABS standards.
Clockwise from left: Longitudinal section through point cloud with an overlay of the line drawing; Point cloud shown isolating the ground floor and its deflection using a colorized elevation ramp; Point cloud showing the west elevation of the roadhouse.
Located near the end of the Denali Park Road, the remote setting provided some unique challenges. The project originally called for two days of scanning on site, but this time was reduced to one day due to weather conditions. The ability to scan at high speeds and high resolutions affords flexibility in these situations.
In normal scenarios, multiple scans at multiple resolutions are acquired in order to reduce the number of unnecessary points collected, reserving the higher-resolution settings for the focal subject. By being selective of the data acquired in the field, overall file size is reduced and time is saved back in the office. However, if time becomes limited in the field due to unforeseen circumstances, such as inclement weather, the technician has the option in the field to modify the collection method in favor of time. Processing the data back in the office may take slightly more time and effort as a result of the increased file sizes, but this is likely more cost effective than planning a second site visit.
Kennecott Scanning Effort
In the fall of 2017, a pilot project was initiated by a multidisciplinary team, in partnership with Trimble, Inc. and local survey company Frontier Precision. The project sought to test the capabilities of two laser scanners at the Kennecott Mines National Historic Landmark (NHL) site, located within Wrangell-St. Elias National Park and Preserve. The NHL includes the Kennecott mill town, a sprawling collection of structures built for the purposes of processing copper ore and sustaining a workforce necessary to accomplish this task in a remote location. Five objectives were set and prioritized for a three-day on-site survey: General survey and scan of (1) the mill building from street level, (2) the upper tram deck, (3) ore chute, (4) interior spaces, (5) and adjacent glacier. The mill site spans nearly 15 acres of complex terrain and provides generally narrow line of sight corridors, which required careful planning by the team to make the most of the time available on site.
The equipment being evaluated was the Trimble SX10 Scanning Total Station and the Trimble TX8 3D Laser Scanner. The remote setting of the location, combined with the scale and topography of the site, presented an interesting challenge and opportunity to put the devices to the test. While the SX10 excels at long range, the TX8 excels at speed and high-resolution scanning. By incorporating both units on the same project, the team was able to register and compare the resulting data while still in the field and present their findings to park staff.
The project resulted in a successful achievement of all five objectives, as well as a satisfactory evaluation of the benefits and drawbacks in certain applications for both devices (Figure 4). The pilot project also served as an extreme example of what a documentation project could attempt. The objectives were broad and ambitious by design in order to produce a widely applicable technique that could be adopted by a range of professionals and modified for various sites and scenarios.
The right side shows the scan data overlaid. This project was tied to real-world coordinates using Survey Grade GPS and a local Bureau of Land Management control network.
Richard Proenneke Cabin and Site
While high-speed scanning is certainly useful in reducing time spent on site, it does not help if you are unable to get to the site itself. In order to document the Richard Proenneke cabin and site located on the shore of Upper Twin Lake within Lake Clark National Park and Preserve, transportation by float plane is required.
The size and amount of equipment necessary for conducting laser scanning has decreased over the years, and is now at a point where the bare minimum equipment required is the scanner itself (in a 50- pound hard case) and a survey tripod. For certain projects, targets are also set up around the site to assist in the alignment of different scan positions. However, if significant overlap exists between scans, then alignment can be achieved without targets. For the Proenneke project, targets were used as a backup for alignment, but were only needed to align interior and exterior scan data (Figure 5). Due to the relatively small amount of equipment needed (compared to earlier versions of this technology), regional transportation such as float planes can be used without exceeding weight and space limitations.
This facilitates both fieldwork and data registration back in the office. The right image shows the final registered data in plan view using an elevation-based color scheme.
St. Nicholas Chapel
The NPS Alaska Regional Office’s Heritage Assistance Program (HAP) provides technical assistance to stewards and owners of historic properties listed, or eligible for listing, on the National Register of Historic Places. Often these stewards of historic buildings are pursuing funding grants to assist preservation work and these groups may not possess baseline documentation or condition assessments for these resources, which is typically required for grants of this nature.
Working with the non-profit ROSSIA, Inc. (Russian Orthodox Sacred Sites in Alaska), an organization dedicated to the preservation of Alaska’s Russian Orthodox Churches and iconography, the HAP staff provided technical assistance in the form of laser scanning (Figure 6). During the fall of 2017, high-definition laser scanning was conducted on the Holy Assumption Russian Orthodox Church and nearby St. Nicholas Chapel, a National Historic Landmark site in Kenai, Alaska.
Scanning was conducted in a single afternoon while driving to a professional conference in Homer, Alaska to present on laser scanning. The ability to conduct laser scanning documentation within existing travel highlights potential cost savings, as well as the mobility of the equipment. Not only was the dataset useful for ROSSIA in applying for a Historic Preservation Fund grant the following year, but they were also incorporated into the presentation for the conference the following day. The scan data of the St. Nicholas Chapel helped to estimate material costs, provide accurate dimensions for developing schematic drawings, and facilitate planning efforts for the re-roofing of the historic chapel.
A Powerful Tool
The use of high-definition laser scanning to document historic resources has proven a powerful tool in the field and in the office. Benefits include high speed, long range, sub-millimeter precision, and ease of operation in the field. Drawbacks include high upfront cost, demanding computer resources, and significant time spent processing the data before production of drawings can begin.
There is also a difference in how the resource is experienced by the team when in the field. Traditional hand measuring techniques generally require three people (one to hold the tape at zero, one to call out the dimension, and one to record), whereas scanning can be conducted by a single person. Having multiple individuals on site allows the team to split up after measurements are taken and collect profile details of windows, doors, and other small-scale features. These features are best measured by hand while in the field using a profile gauge (Figure 7). With laser scanning, these smaller-scale features may not be captured satisfactorily from the distances that are typical for exterior scanning. The method used to collect information in the field has an effect on the collector’s ability to recreate it in the office. For this reason, there is a balance that must be struck between the tools used to collect the data and the time and staff available to conduct the work.
Last updated: May 19, 2021