Revision Note (2023): High-definition terrestrial laser scanning technology has evolved rapidly over the last decade, and with it, Heritage Documentation Programs' (HDP) use in the field. This page has been updated to reflect HDP's current assessment of the technology, and the ways that our programs incorporate laser scanning into our cultural resource survey work. Use the guidance linked at the bottom of this page to create field records for HABS/HAER/HALS surveys that involve laser scanning.
Introduction
HDP now uses laser scanning on nearly every project. However, we continue to supplement digital data collection with traditional hand measuring techniques, and to process both into two- and three-dimensional measured drawings using computer-aided design (CAD). Although our methods continue to evolve, the interpretive role of HDP drawings remains unchanged: to make the site or structure understandable to the general public, and to reveal the industrial processes, patterns of use, and the cultural values embedded within.
Not only are the measured drawings more easily understood than point clouds, they ensure the long-term permanence of the information. While the Library of Congress continues to explore sustainable methods and formats for "born digital" records to mitigate the significant back-end costs involved in the storage and frequent upgrading of files, for now it still requires documentation in hard copy. Likewise, scan data does not meet the Secretary of the Interior's Standards and Guidelines for Architectural and Engineering Documentation requiring that documentation be reproducible and durable long-term, as well as clearly and concisely produced.
Using the Laser Scanner
As of 2021, HDP uses two scanners for high-definition laser scanning. Each has strengths that tend to complement the other’s weaknesses. One is a long-range scanner with tilt-compensated data and highly customizable scan parameters. The other is a mid-range scanner that sacrifices range and compensation for portability and speed. In our current workflow, we often use these machines in conjunction with a total station to establish higher-accuracy control networks, and a survey-grade RTK GPS to provide geo-location information and additional control. This combination of equipment means that, unlike a decade ago, an HDP field team can now efficiently laser scan a structure and its site in their entirety: exterior to interior, cellar to attic. Laser scanning fits easily into the workflow of most projects, which range from architectural monuments and industrial complexes to engineering structures and cultural landscapes.
The benefits of the laser scanner have remained largely the same over time: high accuracy, a range that can record data at difficult to reach heights and locations, and time-savings in the field. It is indispensable for measuring structures that would otherwise require challenging and costly means of access, or that are unsafe. A laser scanner’s accuracy is unachievable using hand-measurements alone. The exactness of the point cloud can reveal the deflections in walls and sagging in floors and roofs or, depending on the resolution setting, variations such as surface texture and cracks.
The newest generation of scanners continue to improve in speed. Scans that once took an hour now only take several minutes at the same coverage and resolution. Small form-factor scanners are now so fast and portable that, when used in conjunction with a high-accuracy control network, they can be used to scan a building interior room-by-room as quickly as a team measuring by hand.
Cons of Using the Laser Scanner to Create Documentation
The biggest disadvantages of the laser scanner remain the technology’s fundamental reliance on a direct line of sight to collect data. A laser scanner cannot record elements that are obscured by adjacent features or vegetation, or are otherwise out of its line of sight, such as the tops or undersides of structures. Also, because the scanner records by reflectivity, it has difficulty detecting elements that are dark in color or that are mirror-like, such as glass, water, chrome, or highly-polished finishes. Likewise, it cannot actually "read" building material or discern the subtleties of age and construction technology; these distinctions must be noted by the recorder.
Long-range terrestrial laser scanners are still generally less effective than hand measuring when it comes to recording architectural details. Unless special close-range, high-density scanners are used, the point cloud image results in an undefined edge that makes it difficult to accurately measure complex molding profiles.
Ensuring the accurate registration of multiple scan positions, and thus the overall accuracy of a project, can also be difficult. This is more tied to process and experience than shortcomings inherent in the technology. Achieving high accuracy registrations is dependent on good planning of both scanner and control target locations by the operator. A working understanding of the basics of land survey (also known as geomatics) is helpful to anyone operating a terrestrial laser scanner regardless of the intended use of the data, as many of the same principles apply.
Need for Long-Term Permanence
HDP and the Library of Congress require that documentation be produced on durable materials to ensure the long-term permanence of the information. This translates to the need for hard copy—such as vellum or archival bond drawing sheets, field notes hand-written on graph paper, large-format film negatives, and historical reports on archival bond paper—rather than relying solely on electronic mediums. While promising to save time and money, recording technologies such as laser scanning currently lack the permanence and, to a large extent, the quality of traditional mediums used to create HDP surveys. Concerns about catastrophic loss of files and the back-end costs involved in the frequent transfer or upgrading of electronic files to keep them current and guard against corruption likewise remain a deterrent. The Library of Congress, other government agencies, and private sector visual information specialists continue to explore sustainable methods and formats for "born digital" records (as well as for CAD files or laser scans). However, the wide range of proprietary software and equipment makes it an impractical and costly proposition. Thus, while laser scans are vital to HDP field work, they are not considered formal documentation.
Laser scan data often serves as the basis for final deliverables such as measured drawings and 3D models. HDP is developing written guidelines for producing laser scanning field records. Creating field records is an important part of carrying out a survey that meets the Secretary of the Interior’s Standards. To read or download a PDF copy of these guidelines, please use the following links:
- Guidance for Producing Laser Scanning Field Records - forthcoming
- High-Definition Scanning (HDS) Field Record Template (pdf)
Conclusions
Even with the most recent technological advances, HDP still does not use laser scanning for recording exclusively. Staff members combine its use with traditional hand measuring techniques to create comprehensive and well-informed documentation. The scan data is combined with the hand-measured details in CAD to delineate a hard line for the otherwise indistinct edges created by the point cloud data. Along with issues of the permanence of the data long-term, another reason the programs combine recording methods is that laser scanning does not engage the recorder in the same manner that hand-measuring does. Thus its use can undermine the hands-on experience advocated by HDP and others interested in the study and documentation of historic sites. Thorough field work and research are an important part of the process, aimed at providing not just a permanent record, but an approach to the study and understanding of historic buildings. While hand measuring, field team members note the building materials and construction technology, and record observations about changes in the fabric of the structure. It is best practice to incorporate such observations into the scanning process as well.
Last updated: October 30, 2023