Wrangell - St Elias National Park & PreservePage 63 of 98
Laser scanning to document historic sites: what it does well and where drawings still win
High-definition laser scanning can collect data quickly and from safer vantage points, but a scan alone does little to preserve anything. John Wachtel of the National Park Service describes what the technology gives documentation teams and what it demands, drawing on Alaska projects.
Historic American Engineering Record photograph with high-definition laser scan data overlaid. NPS/Alaska Region.
From tape measures to point clouds
The NPS Heritage Documentation program has documented historic sites for more than 85 years, and its collection of drawings, photographs and written histories has passed 200,000 pieces, stored at the Library of Congress. The Historic American Buildings Survey (HABS) began in 1933 as a "make work" program for unemployed architects and draftsmen, recording structures through accurate line drawings. Teams spent weeks measuring with tape measures and building levels to within 1/8".
Figure 1. A HABS team taking dimensions in 1934. Library of Congress, Prints & Photographs Division, KY-20-19-7.
In 1966 the program added the Historic American Engineering Record (HAER), and in 2000 the Historic American Landscape Survey (HALS). Computer Aided Drafting (CAD) was a major step, turning field notes into digital files. High-definition laser scanning is a more recent one: it collects highly accurate spatial data as a 3D digital representation of a site, called a point cloud. It suits sites that are unsafe to measure by hand or that would need several people and visits. Collection runs at thousands or millions of points per second and reaches up to 300 m depending on the model, so operators can work from safer positions.
The costs of the data
Processing demands a high-end desktop: solid-state drives, a CPU with as many cores as possible, at least 32 GB of RAM and a dedicated graphics card with at least 2 GB of video RAM. Office time may be double the field time, and a project's storage averages around 50-100 GB.
| Strengths | Drawbacks |
|---|---|
| high speed, long range, sub-millimeter precision, easy field operation | high upfront cost, demanding computers, significant processing before drawing starts |
| one person can scan | small details such as window and door profiles are not captured well from typical exterior distances |
Traditional hand measuring takes three people. With several on site, some can split off to measure profiles with a profile gauge and sketch them, work scanning cannot do satisfactorily at distance. The method used in the field shapes what can be recreated in the office, so teams must balance tools against time and staff.
Not a treatment
Scanning only collects information. Data on a flash drive preserves nothing until it is processed, so a plan for the data is critical. Through informed interpretation in CAD, scan data becomes documentation that captures character-defining features and supports treatment.
Why still draw
CAD drawings remain the gold standard. They convey general and specific information in a format with established standards, they are cheap and easy to read compared with the hardware and software needed to interpret a point cloud, and plotted on vellum and stored properly they can outlast digital files, which have a shorter shelf life. Drawings come from careful review and classification of scan data: drafters isolate parts of the point cloud, then trace key features as vector lines, which can be scaled to any size without losing quality.
Figure 2. West elevation of the Kantishna Roadhouse in Denali National Park and Preserve (built 1919-1920). The point cloud is cropped in CAD to isolate elevations and details, and a draftsperson traces the features to make a line drawing, which is more legible than a point cloud. NPS/Alaska Region.
Other products can come from the same scan: scaled orthographic images, rendered flythroughs, 3D models and virtual tours. They tend to have a shorter shelf life but widen exposure and understanding.
Four Alaska projects
- Kantishna Roadhouse, Denali (spring 2017). The remote building, an example of early twentieth-century settlement in Interior Alaska, was scanned for HABS-standard drawings. Weather cut two planned days to one; fast, high-resolution scanning gave flexibility. Normally scans at several resolutions save file size; when time runs short, the technician can favor speed, and bigger files cost more office time but less than a second visit.
- Kennecott Mines National Historic Landmark (fall 2017). A pilot with Trimble, Inc. and local survey company Frontier Precision tested two scanners in a three-day survey of the mill town in Wrangell-St. Elias. Five objectives: the mill building from street level, the upper tram deck, the ore chute, interior spaces and the adjacent glacier. The mill site is nearly 15 acres of complex terrain. The Trimble SX10 excelled at long range and the TX8 at speed and resolution; using both let the team compare data in the field. All five objectives were met. The ambitious scope was deliberate, to produce a technique others could adapt.
- Richard Proenneke cabin, Lake Clark. The site on Upper Twin Lake requires a float plane. Equipment has shrunk to a scanner in a 50-pound hard case and a survey tripod, so weight limits were not exceeded. Targets were a backup, needed only to align interior and exterior scans.
Figure 5. Site plans of the Richard Proenneke Cabin and Site. The left compares the field sketch with planned scan positions and targets; the right shows registered data in plan view, colored by elevation.
- St. Nicholas Chapel, Kenai. With the non-profit ROSSIA, Inc. (Russian Orthodox Sacred Sites in Alaska), the Heritage Assistance Program scanned the Holy Assumption Russian Orthodox Church and St. Nicholas Chapel, a National Historic Landmark, in fall 2017, in a single afternoon during a drive to a conference in Homer. The data supported ROSSIA's Historic Preservation Fund grant application the next year, was used in the conference talk the following day, and helped estimate material costs, give dimensions for schematic drawings and plan the chapel's re-roofing.
Figure 6. Laser scan data of St. Nicholas Chapel in Kenai. The section cut highlights the roof construction and helped plan how to keep character-defining features such as the exposed rough sawn skip sheathing inside. Image from NPS's page.
Sources
- John Wachtel, National Park Service, "High-definition laser scanning for documenting cultural resources", Alaska Park Science 20(1): 20-27, https://www.nps.gov/articles/000/aps-20-1-3.htm; rewritten in hubnx's own words.