How to Determine Which Laser Scanning Registration Method is Right for You
On-Site Registration
On-site registration is an important facet of the registration process. When you are working on a project, do you want to wait until you get back to the office to make sure that registration is going to work? What happens if you go out and do over 150 scans, and come back to the office with no idea if anything is going to work and you look at the data and realize, “Oh, I missed scans here, I missed some doorway scans here, and I totally missed this room.” That’s not a good feeling when you have to go back and return to the site and fill in the holes that you missed the first time in the field.
On-site registration allows for real-time processing and registration of scans in the field. That means you receive feedback right away as you start adding scans to your project, and know from the beginning if the scans are going to register to each other. Field time isn’t increased with proper computing power like a laptop or a tablet, but project efficiency is increased. It gives you a sense of confidence of a job well done.
Time for another example. Let’s say you’re scanning a 4,000-foot tunnel with a total of 168 scans, spheres for registration, and survey control. Without on-site registration, you have to ask yourself questions such as are you sure all scans have sufficient overlap? Are you sure all spheres are within range of the scanner? Are you sure you haven’t spaced some scans a little too far? Are you confident registration will work at high accuracy?
All of these questions that you must ask yourself in the field, you won’t be able to answer if you’re not running on-site registration. You have to wait to go back to the office, hope for the best and cross your fingers. That real-time feedback of registration is reassuring and will certainly be beneficial.
Conclusion
Registration is one of the most important aspects of a successful laser scanning project. While new technologies provide immediate benefits and quick processing times, they also hold their own set of unique challenges. Specifically, choosing a registration method that’s best suited for you and your project. Each process features benefits and limitations, and considering and understanding both is a critical step in providing an accurate data set for your client.
SCANNING FONTHILL CASTLE
Originally published by Gene V. Roe, PhD, P.E., PLS, founder and author, LiDAR News.
Modified with new information here for CEU purposes.
Credit: TRA Studio
Figure 1 (left): Current view of Fonthill Castle in New York City. Figure 2 (right): The completed exterior scan.
Conserving a landmark building is always a challenge, but when that building’s design is as complicated, both on the interior and exterior, as the Fonthill Castle, located in The Bronx’s Riverdale neighborhood in New York City, an extra set of issues are presented when it comes to documentation.
The castle was built in 1852 for a famous Shakespearean actor, Edwin Forrest, and his wife, and is a Gothic Revival-style building consisting of a cluster of six octagonal towers at varying heights, with no corridors or wasted space, built of hammered grey stone. Five of the towers emit from a three-story central tower. Forrest later sold the lot to the Sisters of Charity of New York, who relocated the Academy of Mount Saint Vincent to the Bronx from Manhattan. The castle has been a part of the college ever since.
TRA Studio Architecture, a firm based in New York City, was hired for the project. They were responsible for redesigning the castle to accommodate a new school for the college; however, the castle was in great need of restoration and infrastructure upgrades. Their end goals were to reconstruct some of the building and adapt the space for exhibitions and events, with the ultimate goal of reasserting the castle as central to the expanding campus.
Eric Yang, an architectural designer at TRA, was given the assignment of documenting the existing conditions of the castle. He had been investigating the idea of using 3D laser scanning on previous projects, and decided that the castle was the right project to test out this approach.
The decision was made to hire an experienced 3D laser scanning consultant from the New York City area. Yang contacted John Smits, the founder of New York City-based Actus3D and architect with over 30 years of experience, who had an established track record of using 3D laser scanning on a number of challenging projects. Since this would be TRA Studio’s first time using 3D data capture, Yang wanted to get a first-hand understanding of the process, so he convinced Smits to let him be his assistant.
The project consisted of accurately documenting the interior and exterior dimensions of the castle as well as the conditions of the structure itself. The masonry needed to be repaired in a number of places, and there were cracks that indicated the possibility of structural issues. The interior spaces were very complex, and there were multiple levels that presented a number of measurement challenges.
The decision was made to scan the exterior and take photographs, so the scans could be colorized to better highlight any damaged areas. Smits adjusted the density of the scans as collected to match the requirements for detail. Spheres and checkerboard targets were placed in key locations (target-based) to ensure that the scans could be tightly registered into an integrated 3D model. Additional control surveys were not required.
Smits used a popular manufacturer’s laser scanner to capture a total of 19 scans outside and 91 scans inside. The light weight and portability of the scanner allowed the project to be completed in just two days with the two-person crew, which would not have been possible using traditional survey methods. Plus, the scanning eliminated the need for return visits. Yang agreed, saying, “The ornate details of the castle would have been impossible to document with traditional survey procedures.”
To correct for some slight rotation in the scans in the more remote areas, cloud-to-cloud registration was used. Smits found that, overall, the dimensions obtained were accurate to within an inch, and in most cases, it was likely closer to half-an-inch.
“Overall, I was very impressed with the 3D data capture,” Yang says.
To give him the ability to view the point clouds, Smits provided a complete copy of the processed and aligned data. The scanned data was used by TRA Studio to create a series of fly-throughs, which helped to inform the client of the progress of the project.
Based on the results from their first use of 3D laser scanning, TRA Studio has seen the benefits to this powerful new approach to building documentation. Yang notes, “We were definitely impressed with the 3D documentation capabilities of the scanner … overall, we will use 3D scanning in the future on projects with this level of complexity.”
Alexis Brumm is an experienced magazine editor with familiarity in various construction industries, including heavy construction, drilling, surveying, geospatial, and the crane and lift industries. www.linkedin.com/in/alexis-brumm-b3a82732
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