How traditional surveying and laser scanning can work together
High-definition, 3-D laser scanning is all the rage in the survey and mapping world, and with good reason. This technology has allowed surveyors to enter new markets and help with everything from recreating historic structures in museum exhibits to determining the causes of crane accidents or a levee break. Nevertheless, just because we have this new technology doesn’t mean we always have to use it—or that it’s even the best method for reaching a project’s goals. Many projects are still best served by traditional survey methods, while others can benefit from laser scanning or even a hybrid approach.
Laser scanning is especially useful for roadway projects. Scans can be combined with GPS and GIS systems to be imported into state plane coordinate systems.
Laser focused
Some cases require nothing more than conventional surveying. However, others benefit from the many advantages laser scanning can offer.
Safety—Let’s say a transportation agency is reconstructing an intersection and has hired surveyors to create a topographical map of the site. Conventional surveying is used for this sort of mapping all the time. However, this particular intersection experiences heavy traffic volume, making the safety of the survey crew critical. With laser scanning equipment, data on the pavement elevations, striping, abutments, and vertical clearances with bridges can be collected from the safety of the shoulder or median, thereby avoiding the need to send technicians into the road using reflector equipment. This option also saves the crew from closing lanes to traffic, seeking permits, or working under traffic-mandated time restrictions.
Comprehensiveness—It’s an old joke in the engineering world that surveyors never get enough information or the right information. With laser scanning, however, a surveyor picks up literally everything the scanner sees and it’s kept, quickly and easily, in the electronic data records. As a project advances, the design needs and requirements may change, meaning those seemingly unnecessary data may now be relevant and useful. On the other hand, scanning a large area may reap more information than was probably intended by the original limits of the survey, which clients may not be willing (or able) to pay for.
Detail—Laser scanners can detect a great deal of information that is difficult for conventional survey methods to gather. Overhead wires and their sag, the language on traffic signs and their locations, roadway striping—all of these features are detailed clearly in a laser scan, all at once. With traditional survey methods, the crew would need to survey the site as well as take photos and locate these additional features afterwards.
Speed—Laser scanning is undoubtedly faster than traditional surveying thanks to the breadth and detail it can collect in one fell swoop. However, formal survey controls must still be established before scanning takes place. In other words, this new technology doesn’t eliminate the need for good traversing, leveling, base controls, and other basic foundational elements of surveying. It simply means crews can use their inherent knowledge of good survey technique in concert with enhanced equipment to get even faster, more detailed results.
Tried and true
Laser scanners are now being used for unconventional situations, such as levee breaks, to provide data on deformation in great detail.
Because those good survey techniques are the key ingredients to any survey effort, conventional survey methods certainly still have their place in the land development industry. If you need basic cross sections of a roadway’s slope or super elevation, for instance, the massive volumes of data collected by a laser scanner simply might be unnecessary for the project. Instead, it might be more cost effective to send out a traditional survey crew. Locating topography on small lots with very little detail is also probably best suited for traditional methods, and most likely, more cost effective. Moreover, sites with heavy vegetation may fair better using traditional methods because important points will be obscured. Surveyors and project engineers should work together in these situations to determine the needs of the project and the best way to fulfill them in keeping with the project’s budget and schedule.
Similarly, large topographic mapping projects are still best suited to photogrammetry since such high-altitude images are mapped more efficiently in the air than on the ground or with a scanner. Airborne LiDAR scanning is increasingly being coupled with or even replacing photogrammetry in some areas, however, depending on the size and scope of the project and the level of detail required.
One example of such a combination is a recent large-scale highway realignment project in Florida, where both aerial photography and terrestrial scanners were used to document more than 4,000 feet of roadway. A GPS system was also established at each scanner point so that the entire project was recorded in the state plane coordinate system.
The scans were collected at planning-grade accuracy and superimposed on both the existing aerial photography and the proposed realignment, showing the existing road, the proposed road, and the texture of both, all in one view. What’s especially useful is that since nearly all state agencies now work in the state plane coordinate system, their GIS programs also incorporate that system, so many of these surveys can be imported into state and municipal GIS databases.
Another benefit of laser scanning is the ability to manipulate the view. This crane scan, for instance, can be rotated 360 degrees to get a view from virtually every perspective.
Best of both
Combined use of aerial photography and scanning on the Florida roadway illustrates how blending the new with the old is creating even more possibilities for survey and mapping applications. Conventional scanning equipment is undoubtedly cumbersome, fragile, and big (although it has its designed purpose). But electronic total stations are now available with built-in scanners that combine traditional and newer technologies. These instruments measure angles and distances and set base controls like a traditional total station, but can also pick up scans without the need to break down one set of equipment and set up the other. The ease, speed, and convenience of this system are undoubtedly valuable.
But this equipment is often only useful in limited cases. Because it more or less combines only some of the features of the two types of equipment, it doesn’t offer the full scope of each—its range, 360-degree scanning capabilities, and speed are limited compared with a full scanner. But for a small project, such as surveying the face of a building, the hybrid station is ideal and often a cost-efficient way to approach the project.
For example, prior to reflector-free equipment, surveyors would obtain measurements by sending teams on the roofs while observing them from the ground or measuring angles from window frames to building edges to determine distances. Using scanners, a crew can measure the entire façade and determine the relationship between any appurtenances. By running the boundary lines into the site, surveyors can easily determine the position of a building with respect to the property line anywhere along the side of a building rather than only at grade or on the roof. This is especially useful when buildings are built right on the property line, where, in fact, the building may be a half inch clear of the line at one point or a half inch over at another.
Another excellent example of blending traditional surveying with laser scanning is the recent survey of Penn Station in New York City. For this project, a conventional boundary/topographic survey was conducted at street level, and formal survey control was established on the New York City subway levels and Metropolitan Railroad levels below. From this control, surveyors located supplemental survey with the scanner on the street level. They then scanned the track levels, locating all of the columns, tracks, frogs, and switches. The end result was an as-built, 3-D model of all levels.
Back to the basics
No matter which type of survey equipment a crew uses, positional accuracy is paramount to its success. The stronger and more reliable your baseline data, the more accurate and useful the resulting scanning data will be. In scanning technology, components known as "dual access compensators" have been introduced to help strengthen positional accuracy by helping to keep the equipment level. When you dip a telescope vertically, for instance, those small errors in leveling translate into dramatic errors in accuracy. The compensator helps mitigate that within the instrument, but it doesn’t set that sort of control around the site. Additionally, while the compensator allows the scanner to occupy points and measure angles and distances while requiring fewer targets for registration, a crew still needs to measure those angles and distances accurately and redundantly and perform all of the fundamental steps of any survey effort.
In essence, using laser scanning technology is a lot like using a calculator. The technology certainly makes your task easier, but you still need to enter the correct information to get the correct results. But with the right training, a sound understanding of the principles of surveying, and good communication with the rest of the project team, laser scanners, traditional equipment, or the new hybrid systems can all be used effectively to serve your projects well.
Ken Stigner is a vice president and leader of the mapping and survey division for the East Coast region of Stantec. He can be contacted at kenneth.stigner@stantec.com.
SIDEBAR
Tricks of the trade
Interested in purchasing laser scanning equipment? Consider these tips before making the decision.
Be prepared—The upfront costs for laser scanners are high. And it’s not just the scanner itself—the scanner is expensive, peripheral equipment is expensive, and training is expensive. What’s more, the learning curve, especially in an office setting, is fairly steep. For a firm to truly assess whether adding laser scanning to its repertoire is viable, first find key, ambitious staff who are willing to learn and put in the time and effort to make the purchase worthwhile.
Use it—If you buy the equipment, don’t wait for a perfect project to use it. Just do it. That may mean collecting information initially by traditional methods and then scanning the site as backup. With this kind of practice, you will gradually make it part of your workflow and a useful option for all projects.
Get in-house training—Rather than send your crews to classes, hire experts to conduct in-house training on using laser scanners. Useful in their own right, outside courses tend to be geared more toward generalists (architects and engineers) who want to learn more about the technology and its basic functions. With in-house training, the instructor can go on-site to your projects and apply the training specifically to your work.
Hire a subconsultant—Enlisting the help of a survey firm on your next project will give your team exposure to laser scanning and allow your staff to ask questions and learn the process of collecting and manipulating scanning data. Be sure to hire surveyors to do the scanning rather than other professionals who also do laser scans.
