When Bryant Associates of Boston, Mass., was tasked with taking elevations on more than one million points for a busy tunnel site that would serve about 50,000 commuters each day, Managing Surveyor Gary Hamilton knew his crews couldn't afford to fall short on the accuracy of their measurements and checks. That's why Hamilton selected digital level instrumentation to do the multi-year monitoring job.
In 2000, Bryant Associates was subcontracted to provide vertical deformation monitoring for 11 railroad tracks that now pass over a new Central Artery Tunnel (CA/T) in Boston. It was the first freeze-jacking project in the United States in which the ground was frozen to push the tunnel sections underneath the tracks. The data collection and monitoring efforts were done to ensure that the track alignment did not alter. This was especially critical at switches-if they became misaligned during tunnel construction, they could increase the danger of derailments.
Hamilton purchased one of the first Sokkia (Olathe, Kan.) SDL30 digital levels on the market, and it worked to his company's advantage. "We bought one, the specs met, and we actually shadowed the Central Artery crew for a week," Hamilton says. "Within about two or three weeks, we were collecting 25 percent more points than Central Artery's crews were." He gives due credit to the digital levels: "You couldn't do it without it."
Many surveyors are realizing that digital levels enable them to better serve their clients; the accuracy and speed of digital levels virtually eliminates having to return to the field after discovering errors in the office. Since digital levels were first introduced in 1992 by Leica Geosystems, their basic technology hasn't changed much, but they have been recognized by users as being far superior to their predecessors, the automatic levels. Across the board, users experience overall increased productivity of 50 percent with digital levels over conventional optical levels. This is due to increased speed and accuracy, electronic data recording, programmability and suitability on numerous applications.
Speed and ProductivityIn interviews with the manufacturers, the most noted benefit of the digital level is its accuracy. With automated, electronic technology, there are no transcription errors, according to John Florio, Sokkia's manager of product planning and support. "It's not going to record a 9 when you were supposed to write down a 5 or a 6," Florio notes. "They are less prone to observation errors because you're not relying on the human eye."
Andrew Hurley, director of product marketing for Leica Geosystems, concurs: "I remember doing a lot of traditional optical leveling over quite long distances, and you have a fear as you get to the end of closing out your run that you've made a mistake because you've read the staff incorrectly. People that make mistakes in traditional optical leveling-they're gross errors, but with digital leveling, you just point and click, and it gives you the answer. It takes out that human error, so your production is tenfold."
Hurley continues: "You're relying on the skill set of the operator to read the staff. Anybody who has ever done any leveling over any distance knows how easy it can be to make mistakes. The biggest mistake that gets made with a digital level is remembering to actually level it."
Mark Contino, marketing manager for Topcon explains: "The technology built into those instruments will give you an error message before you'll make a mistake. As distance increases between the rod and the level, the human sighting error has a tendency to increase, where the digital level technology maintains a more consistent accuracy specification. Field operations consistently demonstrate that a digital level will complete a standard level loop or open-ended leveling run with higher degrees of accuracy than a standard automatic level."
Matt Delano, survey portfolio manager at Trimble, notes that with digital levels, "You also have the ability to read much faster. With a typical automatic level you're reading nominally to a hundredth of a foot or to the nearest [couple of] millimeter[s] on a metric rod; with digital level technology the instrument is reading a code and it has the ability to read that code to a higher precision. With a precise digital level, all you're going to do is basically point at the rod, focus on it and press a button. The instrument will automatically read the code and give you a precise measurement, basically down to the level of a third of a millimeter."
Along with the accuracy of digital levels, manufacturers also point out the advantage of the instrument's speed. Sokkia's SDL30 measures within three seconds. Leica's SPRINTER series measures points in one to three seconds for single measurements. Topcon's DL101 and 103 models measure within 2 to 4 seconds, and Trimble's DiNi series records as fast as two seconds.
"[With digital levels] you're not transcribing manually; you can display raw and reduced readings at the same time-users can see the rod height and the elevation at the same time," Florio says, noting the added productivity offered.
Brad Tisdale, a Trimble DiNi user and program manager for the Height Modernization Project for the Wisconsin DOT, which applies digital levels for Second-Order Class 1 leveling, highlights an additional advantage to digital levels: "When it's taking those readings, it's doing stadia shots, too. With an auto level, you'll have multiple readings and then [have to] try to calculate a distance from your stadia." Paul Hartzheim, PE, a geodetic survey supervisor for the Wisconsin DOT, notes how critical this advantage is over measuring the distance by walking. "Some people have a good pace, others don't. Once that instrument is set up, it takes a backsight [and] it's so instantaneous to get the stadia shot. [The instrument] might [indicate to] come ahead a meter or go back a meter-it's instantaneous. Even if you've got a person who can pace, that's still a nice feature." On the Height Modernization project, the Wisconsin DOT is required to match backsights and foresights within 2 m per setup.
Tisdale also credits the customized programmability that most digital levels offer. "With most digital levels like ours, there are things you can set up so it rejects readings if they're out of a certain tolerance," he says. This has aided the six Wisconsin DOT crews immensely in the eight-phase project that will essentially re-level the state of Wisconsin. He adds: "As far as the readout on the instrument, it shows you how many readings it has taken, what the difference is in those readings, where it is reading on the rod [and] the difference from the instrument to the rod, so it gives you a lot of output right there instead of blindly recording data that you don't see until it's downloaded."
"The automatic display, computation and recording of the digital data is where the real speed benefit is realized when compared with manual computations and recording," Contino says. "Onboard programs provide the operator with the ability to close and balance level information before leaving the field, helping to eliminate the wasted time of returning back to the jobsite to re-measure incorrect data."
Digital levels also lessen fatigue in users. With electronic data collection, writing down data and doing calculations is eliminated. The entire process is sped up so users can move through projects more quickly and more efficiently.
Applications A'PlentyThe applications on which digital levels can be used are limitless-essentially anywhere an automatic level is used. "There are two real target markets out there [for digital levels]," Florio explains. "One [is] high productivity work, which is where you need very accurate leveling over long distances (such as on highways, bridges, causeways, railroads, runways)-long linear objects where you need very accurate grade change."
The second application is the construction market. "Traditionally, [that market] used automatic levels, then it became automatic levels and laser levels, and they're now trying to get digital level productivity and accuracy." Florio adds that the selling feature for contractors is ease-of-use. "It's easier to get someone who can hit the button and read the number directly on the screen than to get someone who can look through the [telescope and read the] rod, and do the calculations."
Hurley notes a large spectrum of applications for digital level use, including topography, dimensional control, building and road construction, civil engineering, landscaping, tunneling, mining and quarrying, and jobs for the agricultural and architectural markets.
Delano adds vertical control for photogrammetry to the list. "The advantage there is efficiency [for] longer level runs. [If] you make a blunder you have to do a long level run all over again. The reliability of using a digital level really pays off." Contino echoes this thought: "Surveyors performing floodplain surveys are often required to tie into published NGS vertical datum. This often requires lengthy level loops or level runs where a digital level can provide significant performance, ease-of-use, and accuracy advantages over standard leveling techniques."
Contino says digital levels are the "perfect complement to a GPS control system. The inherent vertical error in standard satellite-based positioning, and the added error of incomplete or inaccurate geoidal models require that GPS control points be measured with other technology such as a total station or level if they are to be considered high accuracy in the vertical component. Many surveyors who utilize GPS to set control points also incorporate a digital level to tie GPS control points to known vertical datums in a specific area."
Delano also notes that digital levels are ideal for monitoring applications "particularly because of their ability to read at very high precision very quickly and very reliably." Long level lines on highways benefit from digital level use because the more accurate readings reduce the possible random errors on backsight and foresight readings, thus accumulating less error over long distances. "If I'm taking measurements to a hundredth of a foot with a regular level, and I do a hundred turns and I have an error of even a half of a hundredth of a turn, it adds up really quick," Delano notes. "If I'm using a digital level and I'm reading down a thousandth and a half, it's not going to accumulate nearly as fast. My error is going to accummulate at a factor of three to five times less. I could achieve the same outcome with an optical level with a micrometer, but if you look at the cost difference of a high-end optical level system at twenty-five hundred bucks versus a real high-end digital level at five to six thousand dollars, think of the cost of the labor when it takes two to three seconds to take a measurement with a digital level and it's taken five times that long to do it optically."
Cost ComparisonsAlthough digital levels have many strong advantages over the automatic, optical level, the factor of acquisition cost has prevented some surveyors from purchasing them. "Until recently the price [of a digital level] has been about two and half times the cost of an engineering grade level," Florio says. "Traditionally, the engineering grade levels don't fail really"¦ until someone actually physically damages or destroys it. And the method of work with those products hasn't [caused] a significant drive to purchase digital levels; not a lot of people saw the productivity gain equating with that cost." Until recently, that is. Consumers are beginning to take notice of digital levels more and more these days, as more projects dealing with long distances or very large amounts of data, or both benefit from electronic data collection.
"Our work as surveyors is our data," Delano says. "The value of what we do really lies in the quality of our measurements. If the quality of our measurements is poor, then the quality of our work is going to be poor. That's the bottom line. If you think about what it costs to put a two-man field crew [including equipment and overhead] on a site, [you're looking at] at least $500-$1,000 conservatively. It doesn't take too many errors to pay for a digital level."
"The price of digital levels has slowly reduced while the capabilities, performance and accuracy have improved," Contino says. "Now a customer can implement the power and performance of a low end digital level for [less than] $1,500. Most digital level operators will claim a productivity increase of anywhere from 30 to 60 percent over a standard level. If your typical charge-out rate for a two-man crew is $1,000 a day, and you see a 50 percent productivity increase over the operation of a manual automatic level, you can pay for a Topcon DL-103 digital level in three days of leveling work-not to mention what it costs to send the crew back out for a day or two to re-run a level loop to find the transposition error in the field notes."
Tomorrow's LevelsDigital technology has provided much opportunity for technological advancements, and digital levels have followed on this progression. The ability to use CCD technology to make precise measurements of a level rod has had a dramatic impact on surveyors who make use of digital levels. They all have a similar implementation: to "grab" the image of the rod where the cross hair(s) intersect it and resolve the rod reading by analyzing the image. Some limitations still exist, which are potential areas for further development of digital level technology, including the ability to make accurate measurements when a smaller portion of the rod is visible and the technology to allow digital levels to be used in poor lighting conditions. Enhancements to today's products will offer more for tomorrow's user. What are the predictions?
"Right now digital levels are limited to functioning optically. They have to have light like the human eye," Florio says. "As CCD sensors get better, you can reduce the amount of rod to read, or you can reduce the amount of light needed to take a measurement. Imagine digital levels that could read in darkness, underground like in mining."
Another capability Florio highlights would be if the resolution of the CCD is improved, users could get more accurate distance measurement. "And if you add some of these encoders that are being used in total stations these days, you have the chance to make a "half total station'," Florio says. "The most important [fact] is that CCD technology is developing. It's allowing those devices to be more accurate, to have more range and to work in worse lighting conditions."
Hurley forecasts digital levels to be slightly smaller and somewhat less costly in the future. He believes that contractors will adopt digital levels more in the future and that the capability, perhaps within the next five years, will exist for information to be relayed back to the office via an Internet server.
"Surveyors use a level the way a carpenter uses a hammer," Delano says. "At some point the digital level, like the data collector, will become a regular tool that every surveyor will have."