3D Scanning: Laser Scanner Versatility Factors, Part I
Professional Surveyor Magazine - October 2009
Because of a high-speed laser scanner’s relatively high cost ($70k - $150k compared to $10k - $35k for a conventional instrument), organizations considering purchasing one often put a premium on a scanner’s overall versatility to effectively address various applications and site and project logistics. This article discusses several scanner versatility factors and how to assess them against your organization’s needs.
Note that many factors contribute to a scanner’s versatility:
• Personnel versatility
• Accuracy at range for the scene
• Accuracy at range for targets
• Ability to resolve fine details at range
• Absolute range
• Deployment cost
• Field of View
• Dual-axis, tilt compensation
• Environmental capabilities
• Camera imaging
• Fit with time windows
• Compatibility with standard survey accessories
• Power supply options
• Control interface and data storage options
• Field QA capabilities
• Miscellaneous factors (several)
Part one of this series will cover the first nine factors on the above list; part two will cover the rest. Part three will cover software versatility factors.
Before diving into technical factors, I’d like to discuss a business factor: personnel versatility. The more freedom that an organization has in deploying field staff, the better. If you’re limited to one specialist and he/she is unavailable, your scanner may sit in the closet. So, look at the instrument’s user interface and workflows to see how they match up with your staff’s skills.
Today, scanners are increasingly available with surveyor-friendly features and workflows. These include features like onboard, graphic, total-station-like controls, tribrach mounting and a laser plummet, HI marks, GPS and prism attachments, dual-axis tilt compensation, etc. These features, in turn, enable surveyor-friendly workflows such as setting up over known points, resectioning, and traversing.
All scanners also need to use scan targets, so look also at workflows involving targets. Are the associated procedures and supporting instrument features, such as cameras, easy to learn or will they require specialized staff?
Accuracy at Range for the Scene:
Accuracy is the first filter for a scanner’s suitability for a project—is the scanner accurate enough? Many high-definition surveying applications involve structures and hardscape where accuracy requirements can often be ¼” (6mm). Can the scanner meet this requirement? If so, out to what range?
Assessing a scanner’s accuracy should include understanding the accuracy of a single scan point, since these are regularly used in making deliverables (see August 2009 Professional Surveyor Magazine, “Scan Point Accuracy
”). A corollary to accuracy at range is precision at range, which characterizes the noise in scan data. Low noise data is valuable for surface modeling in monitoring applications that compare surface changes over time.
Accuracy at Range for Targets:
An equally critical versatility question is, “At what distance can I safely place targets to meet a project’s accuracy requirements?” Targets are used to stitch scans to each other and for geo-referencing. When scanning projects go sour, it’s often in the registration step, so the ability to freely place targets in optimal locations and accurately capture them is a big plus for versatility.
The farther away that you can place a target and accurately capture it, the more versatile the scanner and the more accurate the registration will be. Some scanners are limited to target placements of <60’, while others can accurately capture targets out to 300’ or farther. To assess this capability, place a scan target at a distance, scan it, locate it with a total station, and compare the coordinate results.
Ability to Resolve Fine Details at Range:
A small diameter laser beam combined with a scanner’s ability to conduct fine, localized scanning at long range (e.g. <1mm point spacing at 500’ range) makes a scanner more versatile for extracting fine features, such as architectural details, bolt head geometry, etc.
The desired scene/structure to be surveyed must be within the scanner’s range where reflectorless laser returns can be recorded. To date, scanners with the longest range capabilities have not been suitable for many high-accuracy applications that comprise much of today’s scanning market.
The lower the deployment cost, the better from a versatility standpoint, as you will be more likely to win projects, gain clients, and/or earn more profit. Deployment costs are based on multiple factors:
• A scanner’s productivity, translated into the number of staff required, cost of staff, and the number of labor hours needed for a project
• The scanning system’s total purchase cost and related ownership costs (e.g. maintenance) as allocated to a project cost estimate
• Ancillary project-related costs, such as transport costs, travel and expenses, etc.
Note that I have lumped a scanner’s productivity into a deployment cost factor. For laser scanners, productivity is a complex subject. Productivity factors include a scanner’s field of view, its useful range, the number of instrument setups and target placements required, setup and tear-down time, scan time, time for camera imaging, time for target finding and scanning, time for geo-referencing, and QA checks, etc. (see Professional Surveyor Magazine, January
2007 for a three-part series on this topic).
One way or another, organizations account for the ownership cost of a system (including accessories and add-ons) within the system’s deployment cost. A common way to do this is to assign a daily or hourly value to it—let’s say $1,500/day for scanning system A. So, scanning system B with a 50 percent higher cost than A would be $2,250/day, scanner C with a 50 percent lower cost of ownership than scanner A would be $750/day, and so on.
Vendors often promote specific productivity features that each claim makes their system the best, but users need to look at the bigger picture of overall deployment cost and versatility to determine which system is best for them.
Field of View:
A scanner’s maximum field of view per scan affects field productivity and versatility. All scanners have 360º horizontal field of view, but scanners vary in their maximum vertical field of view capability. Some scanners can scan around them and scan overhead (full dome) in a single scan, while others cannot. For ones that cannot, in order to scan overhead, the scan head needs to be tilted, and a separate scan from this position needs to be accurately registered to its companion side facing scans. This process often involves placing and scanning extra targets. In many cases, it’s not easy or feasible to place these extra targets in locations that provide good geometry for accurate registration.
Users who have used scanners with full-dome capability and scanners without it regularly report that full dome scanners provide valuable versatility. Project examples include many common applications for scanning: the undersides of bridges and overpasses, building interiors, process plants, factory interiors, tunnels, ceilings and roof trusses, caves, ship and aircraft interiors, tall building exteriors, rock faces, and cell towers with limited set-back space to place a scanner, etc.
Poor portability can be a show stopper for certain projects. For example, can the scanner and its accessories fit through a manhole on a ship? Can you put it and its accessories on your back to climb to a remote archaeological site?
Dual-axis, Tilt Compensation:
This capability lets scanner users set up over known points, resection, and traverse, while also tightening up registrations. These workflows require much fewer scan targets, which adds field flexibility, because at some sites it can impractical to place scan targets or to place them where you want them (like good PDOP for GPS) for accurate registration/geo-referencing.
These are a few of the many factors that determine the overall versatility of any scanning system. Since many surveying companies or departments want to be able to tackle a wide variety of opportunities, scanners with the best overall versatility have been the most popular types of scanners in the market. Part two of this series will explore more versatility factors of scanners, and part three will be on versatility of laser scanning software.
Read Part 2
Read Part 3
About the Author
Geoff JacobsGeoff is senior vice president, strategic marketing for Leica Geosystems, Inc.
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