The Latest in Laser Scanners
October 30, 2006
Laser scanning, also known as 3D imaging, terrestrial LIDAR, high-definition scanning and 3D laser scanning, is becoming less of a specialized service and more a part of everyday surveying. Part of the reason behind the growing acceptance of this technology is that today's laser scanners, to borrow from some of the manufacturers' taglines, are "friendly" and "think like surveyors."
Laser scanners are most often used for as-built or topographic surveys. Typically, laser scanners are not used for boundary surveys except in urban areas or for ALTA/ACSM land title surveys. The inclusion of laser scanners in the 2005 ALTA/ACSM minimum standard requirements for locating certain features on a parcel of land shows just how accepted laser scanners have become within the surveying community. In the past, laser scanner manufacturers tried to offer surveying-like features to make their scanners appear to be surveyor-friendly, but it is only in the last twelve months that the line between surveying and scanning has grown very thin.
Benefits of Modern ScannersEach new generation of laser scanners comes with new features geared toward surveyors. For those surveyors trying to use a laser scanner on everyday surveying projects, the biggest improvement in scanning technology is the addition of survey-grade dual-axis compensators, which correct for errors in the vertical and horizontal angles due to deviation of the vertical axis. Both the Leica Geosystems HDS (San Ramon, Calif.) ScanStation and the Trimble (Sunnyvale, Calif.) GX 3D scanner offer hardware with survey-grade dual-axis compensators. Among the advantages that this new technology brings to the laser scanning workflow are the following four benefits: reduced target numbers, reduced labor, increased productivity and reduced scanner drift.
Reduced TargetsWhen performing laser scanning services at my firm, Midwestern Consulting of Ann Arbor, Mich., our typical procedure requires four targets spread apart in the scanner's field of view to ensure an accurate post-processed alignment (or registration) of the point clouds. Sometimes it seems like more of an art than a science to place targets in the optimal positions to attain the most accurate data. Using four targets per scan can require significant pre-project planning, as well as a considerable amount of time in the field to set them up. Relocating targets to ensure a good line of sight to the scanner adds even more time to the process. In the past, we might have had to manage hundreds of targets on a large project that required not only scanning at each setup, but also geo-referencing with a total station.
However, using a scanner with a dual-axis compensator reduces the number of targets needed for each setup during the scanning process. A scanner with a survey-grade dual-axis compensator can be set up and leveled over a known point, allowing us to use traditional surveying workflows such as traversing, resectioning and backsighting. This eliminates the need for most scan targets. When new control points need to be set, rather than traversing with a total station, we can save field time by traversing with the laser scanner. These new control points are usually set while we are waiting for the scan to be complete. When we encounter situations where targets cannot be spread out in the scanner's field of view, such as in tunnels, narrow corridors, bridges, overpasses, river banks or around building corners, a dual-axis compensator adds flexibility in the way we set control points. Rather than having to maintain good geoemetry between four targets per scan, we can now have a linear network of control that easily fits in an area where there is a limited field of view.
Reduced LaborOn projects not originally set up as scan jobs, integrating or adding laser scanning is much smoother. The scanner can be set up where the total station was originally set and the same backsight point can be used. For a recent Midwestern project, we surveyed several cellular phone towers with traditional methods and later had to return to get more information on the antennas. Only one person was needed to set the scanner up on the project coordinate system and, using the full dome (180o horizontal and 270o vertical) field of view, only about half an hour was needed to obtain accurate elevations of all the antennas. On new projects, we can send a one-person crew to scan and establish control, which frees up our survey crews for other work. Even though we now have the option of not using a total station in conjunction with the laser scanner, it is still good practice to use conventional survey practices to provide an independent check on the point cloud data.
Trimble's PocketScape software supports both scanning and survey workflows and shows the dual-axis compensator in action. Leica's Cyclone software has a built-in traversing routine that creates drawings showing angles and distances for each setup and allows for real-time registration of the point cloud as the user is scanning. Once a traverse is completed, closure and precision reports can be created and analyzed. The traverse data can then be adjusted in the field and checked before going back to the office. This is a tremendous advantage because surveyors can be confident that their data is accurate before leaving a jobsite, which also reduces the time spent processing the data back in the office.
Additionally, crews do not have to carry all of the equipment needed for multiple scan targets to the jobsite. When they take the scanner to the field, they do not have to bring as much equipment in the truck; likewise, when working on out-of-state projects, they do not have to ship as much equipment.
Increased ProductivityThe new improvements to laser scanning systems have positively impacted productivity on the jobsite. On a recent highway project, Midwestern saw significant productivity increases when we began using a laser scanner with a dual-axis compensator. Originally, we used a scanner without dual-axis compensation and scanned from the shoulder with four targets on both sides of the six-lane highway, which was divided by a concrete median wall. One person had to drive to the other side to set up and move targets as needed. Once we switched to a scanner with dual-axis compensation, we only needed to set up on a known point and backsight a target. We could then stay on one side of the highway, increasing not only our productivity, but also our safety.
Using a laser scanner with a dual-axis compensator also allows crews to obtain real-time coordinates in the field without having to post-process the scan data. During several past projects, we considered using our laser scanner for construction monitoring and quality assurance surveys, but used other technologies because it was too labor-intensive to get real-time data from the point cloud. For instance, on projects where we monitor the installation of complex structures such as steel trusses that are being lifted by cranes, we have always monitored a few discrete points using a reflectorless total station. For future applications of this type, we will be capable of scanning the whole truss in real-time to monitor the entire surface, which will allow us to see how a truss deflects when under any load.
The ability to have real-time coordinates also means that the time spent in the office registering multiple point clouds together into one uniform point cloud no longer takes hours or days, but rather minutes. Riegl USA (Orlando) offers an internal sync timer option for its scanners that stamps the point cloud with data from an external GPS receiver and an IMU sensor, which translates into automatic point cloud registration.
Reduced Scanner DriftAnother significant improvement in today's scanner offerings is the ability to compensate in real-time for being out of level, but still within the dynamic operating ranges (+/-5' for the ScanStation; +/- 6' for the GX). Scanner drift is a particular concern on projects with long setup times, such as when a crew must scan in every direction using a scanner that has a full dome field of view. Unlike a total station that weighs around 15 pounds, a laser scanner can weigh as much as 40 pounds, which puts a lot of stress on the tripod. Depending on what type of surface the scanner is set up on, settling may occur. Without a dual-axis compensator, scanner drift goes unnoticed until the end of a scan. Now, with real-time compensation, a scanner will alert the user if it has tilted out of range.
Growing Acceptance of Laser ScanningAs acceptance of laser scannning grows, users need to remain aware of the technology's current limitations. Even though manufacturers are moving towards producing laser scanners that behave like total stations, a scanner is not a replacement for a total station. Surveyors still cannot efficiently stake out points with a scanner, and certain topographic surveys are not scanner-friendly. Sites with a lot of vegetation or sites where very dense point data are not needed will be left to survey with a total station or GPS.
Overall, however, laser scanners are widely accepted in the surveying community and provide exceptional benefits on projects where a point-to-point survey will not capture all the necessary detail or when the added safety of remotely measuring with a laser scanner is needed. As manufacturers continue to roll out products with more total-station-like features, surveying crews will continue to reap benefits from using this advanced technology. The reduced need for targets, reduced need for labor, increased productivity and reduced drift made possible by the latest models of scanners are only the beginning of the advantages that will be offered as this technology continues to evolve. It is exciting to see the advances in technology that have occurred in the past ten years and one can only imagine where we will be in the next ten years. It is my hope that by then most of our clients will be requesting 3D laser scanning services.