The Long & Short of It (and the Fast and Gradual)
The advantages of using 3D laser scanning technology are being seen more often in the surveying and engineering world than even just a couple of years ago. More clients are specifically requesting laser scanning services for a variety of different sectors including architecture, plant facilities, educational institutions and municipalities. Manufacturers continue to do a good job offering diverse product lines that satisfy the varied needs of clients. Each scanner on the market offers features for specific applications and, as with any new technology, it is best to analyze the benefits and drawbacks of each before a purchase is made and the unit is put into practice. Here we will explore some of the advantages and disadvantages of phase-based scanners versus time-of-flight (TOF) scanners.
Scoping out the DifferencesWith the aspects of physics aside, the two main differences between phase-based and TOF scanners are speed and range. Phase-based scanners are often 100 times faster in their data acquisition rate, capturing hundreds of thousands of points per second versus a few thousand a second for TOF scanners. Range-wise, phase-based scanners measure around 50 meters while TOF scanners can measure between 200 meters and 1,500 meters. These differences play a major role in deciding which scanner is best suited for a project.
Last year, Midwestern Consulting in Ann Arbor, Mich., began using a phase-based scanner through a rental agreement with Avatech Solutions of Madison Heights, Mich. The multidisciplined firm had been using TOF scanners for six years and was interested in expanding its services with another type of laser scanner. “Whenever we are scanning indoors with ranges less than fifty meters, we plan on using a phase-based scanner,” says Mark Vander Veen, SIT, project surveyor at Midwestern Consulting. “To be able to capture a very dense, full field-of-view scan in a matter of a few minutes makes phase-based scanners the right choice when you are in a complex site.”
To demonstrate the qualities of both scanner types, Midwestern scanned a tunnel using both a TOF scanner and a phase-based scanner. In four hours, the TOF scanner covered 300 feet of tunnel in 10 scans, but the point cloud data was not very detailed. The phase-based scanner, on the other hand, covered 1,500 feet in 25 scans and produced point cloud data that was detailed enough to see if a light switch was on or off. In this application, the phase-based scanner proved to be the better option.
“Our phase-based scanners are the workhorses in the industrial environments,” says Dr. Stuart Gordon, senior surveyor at AAMHatch, a spatial information provider based in Australia. “The time-of-flight scanners are the ‘artillery’ that we fire into (or up to) structures from farther away.” These workhorses (the phase-based units) are known for producing huge amounts of data, which can create point clouds in excess of 1 billion points. Often these large databases have to be split into more manageable sizes for clients to work with them. Data storage and archiving can also be a challenge.
“We have already filled our 2.5-terabyte server,” says Chris Siebern, PE, PS, senior engineer at Dynasty Group, a consulting engineering and surveying firm in Chicago. “We have to rely on our tape backup system for long-term storage.”
While some clients still request 3D models, the amount of detail captured with a phase-based scanner allows for designers and engineers to work with a point cloud rather than converting it into a 3D model. Getting the point cloud to the end user can sometimes be a chore. “Often we have to sneakernet it to our clients,” says Jason Adamowicz, project manager at Ghafari Associates LLC of Dearborn, Mich., referring to the practice of walking external hard drives to the offices of his clients.
This high level of detail produced by a phase-based scanner can be beneficial, making point clouds that look like digital photos. “People are always intrigued when we show them data from the phase-based scanner,” Midwestern’s Vander Veen says. “They always think it is just a static image. Then when we navigate around in the data, they are amazed.” He adds, referring to Leica Geosystems HDS’ (www.leica-geosystems.us) TruView software, “And now we have the ability to easily share this data with our clients.”
“For our manufacturing clients,” Ghafari’s Adamowicz explains, “we set up the TruView data so that the data is accurate relative to the plant coordinate system. Engineers can pick on any point in the viewer and get the plant coordinates they need.” These files are smaller than complete point cloud databases so no sneakernetting is required. Rather, the data can be shared on an office network or the Internet.
Another way of working with a smaller file size from a laser scanner is to reduce the point cloud while still in the field. Scanning with a Trimble (www.trimble.com) GX 3D TOF scanner that uses the manufacturer’s SureScan technology is one way. “We have designed solutions that capture the right amount of points to be manageable by most computers,” says Trimble’s Omar-Pierre Soubra. “More importantly, users can capture more intelligent points, which makes the deliverable faster with virtually little processing.”
The Trimble GX with SureScan automatically adapts its parameters to capture an evenly partitioned grid of points for the entire range of the scanner. “This results in less setups of the scanner in the field and in manageable data sets on your computer,” Soubra explains.
Typically, phase-based scanners cost more than TOF scanners, but many firms find the cost is worth the value. Dynasty Group frequently uses both technologies on their projects. “Our phase-based scanners capture lots of detail in a short amount of time, typically eight to ten minutes per setup,” Siebern says of Dynasty’s Leica HDS6000 phase-based and Faro (www.faro.com) LS880 phase-shift laser scanners. “These are details that you sometimes did not know you needed until there is a scope change.”
Dynasty’s TOF scanner, a Leica ScanStation, is used for civil projects such as highways and bridges--typically anything beyond 70 feet in range, according to Siebern.
Buidling exteriors are another good application for a TOF scanner. “With wide open spaces, we can spread apart our targets and scan locations allowing for [fewer] setups,” Ghafari’s Adamowicz says.
Phase-based scanners are sometimes impeded when operating outside in bright sunlight. Direct sunlight on surfaces can affect the return signal to the scanner’s detector. But users have had success scanning up to 250 feet away. “We’ve used a Faro LS880 to scan a battleship from tip to tail in one setup,” says Ed Oliveras, a technical product manager at Faro Technologies. “The workflow with a phase-based scanner is different than a time-of-flight scanner. You can be in and out of a jobsite in a third of the time,” Oliveras says.
Projects that require scanning tall structures (75 feet or more) benefit from TOF scanners as well. Midwestern continues to use its TOF scanner about every other day to scan cell phone towers.