A Private VRS Network Takes the Strain Out of Phoenix Surveying

"I used to figure one person in the office to keep up with one survey crew. But now it takes me, a receptionist and two draftsmen to keep up with one crew. If I added another crew, I’d probably need three more draftsmen."

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That’s John Davis, owner of Davis & Davis Surveying in the Phoenix metro area, and he’s talking about the tremendous gains in field crew efficiency he’s seen since subscribing to AZGPS, Arizona’s first private RTK network and one of the first in the nation to provide Global Navigation Satellite System (GNSS) capabilities. Davis was slow to adopt GPS of any kind, but AZGPS won him over. "I’ve been surveying since 1956, and I’ve used every instrument there is," he said. "I’ve looked at GPS for years, and I’d call up dealers and get demos, but I was never convinced that GPS would do me any good, until Travis and Boz came out. I made them give me a very extensive demo, which they did. I had them run a section I’d run previously with a total station, and they were within four hundredths on every corner."

That convinced Davis of the accuracy, but it was the speed of the work that really sold him. "To run a section in town takes four to six hours with a total station," Davis said, "and my guys can run the same section in an hour or less with AZGPS. Same with FEMA certs ­ they take an hour now, instead of all day. One day I gave my crew three sections to run and they left at 8:30 and were back by 12:00, with all the work done!"

Praise like that from Davis and other Phoenix area surveyors is music to the ears of Travis Thompson and Vern `Boz’ Turner, who started AZGPS in 2004. Thompson was a survey manager at HEC Engineering, which has four offices in the Phoenix metropolitan area, and spent a lot of time developing the firm’s GPS processes and equipment. He saw an opportunity for a private network, and is now a partner in AZGPS, LLC, an independent firm. From 10 users at launch, AZGPS now has 86 regular subscribers, and a dozen or so users who subscribe by the day for peak periods. Travis worked with a marketing firm for a while, but "the timing seemed wrong" and most new subscribers come via word of mouth.

And with fans like John Davis out there, word of mouth is pretty darn effective.

Building a Network
AZGPS is a Trimble VRSTM network that began when Thompson and Turner linked up four Trimble NetRS® Reference Stations in December of 2004, just four days before Christmas. By March 18, 2005, when AZGPS was officially released at ACSM in Las Vegas, another four stations had been added for full coverage of the Phoenix metropolitan area. AZGPS network coverage is now more than 1,300 square miles (3,360 square kilometers).

The basic idea behind the Trimble VRS network is that AZGPS’s reference stations continuously send data to a central server, and the server continuously calculates area and atmospheric corrections for the region covered by the network. Surveyors tap into this system with a rover and a cell phone link, which transmits the rover’s position in the form of a National Marine Electronics Association (NMEA) position string. The central server then treats the rover as a VRS, calculating unique error corrections for an area within a few meters of the rover. This eliminates inaccuracies caused by distance from the base stations. "The specs say that this yields accuracies of two centimeter horizontally and three centimeters vertically," says Thompson, "but if users constrain their site correctly they can cut that in half, and I think I’m being conservative when I say that."

The system is seamless from the user’s viewpoint, says Thompson. "All they have to do is turn on the data collector, the phone, and the receiver and in a matter of three keystrokes they can have a fixed ambiguity solution." The cell phone actually provides an Internet connection for data transfer between the rover and the central server.

The reference stations also communicate back to the central server using direct Internet connections. Because this link is so vital, AZGPS switches Internet Service Providers (ISPs) as needed, and each receiver has two Internet options for backup. "ISP lags can be as bad as an actual outage," says Thompson. "If the data from the VRS server has as little as a two-second latency, the software throws out the solution." But latency hasn’t been an issue. Thompson hears about problems that other networks can have, but "we just don’t seem to have them. Our uptime is definitely over 99 percent." And because they’re working with an Internet link, subscribers can transfer data and check e-mail in the field.

AZGPS also provides for those doing static surveys by maintaining a web server where compressed Rinex, 24-hour Rinex, and raw DAT files are always available for download. With his strong math background and experience in static GPS, Thompson likes to stay on top of the data, explaining, "I post-process the data from our network on almost a daily basis. It’s a good check on our network, and with my background, I like to check the software." Thompson uses Trimble Business Center software for this task, because it "allows me to process the network, gives me a direct connection to GPServer, and lets me pull raw data from inside the software." In addition, the software’s GNSS data processing power and intuitive interface provide additional benefits for AZGPS management. Because he stays on top of the details, AZGPS subscribers seldom have to do post-processing themselves ­ though it’s nice to know they have instant access to the raw data if they need it.

Thompson added GNSS capacity to the network by updating nine reference stations to Trimble NetR5 receivers with new Trimble Zephyr Geodetic 2 antennas to take advantage of GLONASS and future planned constellations and frequencies. The new receivers/antennas enable users to track GPS L1, L2, L2C and L1 and L2 GLONASS signals, and are ready to track GPS L5 signals when available. For users, the conversion increased satellite availability with very little fuss. Greg Thompson, the survey manager at Wilson & Company in Phoenix, works with a Trimble R8 GNSS receiver and says, "The GNSS capability improved our work by giving us more satellites and better coverage, enabling us to get difficult shots ­ I sure wish we’d had GNSS when I had to locate 4,100 manholes in town!" And Davis, also an R8 user, says that coverage is so good at times that he has too many satellites in the constellation, and will turn off a few to speed calculations, thus reducing shot times. Thompson is also reaching out to other VRS networks, and recently agreed to work with a cooperative of 11 Trimble receivers in California. Both sets of users have full access to each other’s networks and raw data. Thompson expects this network to eventually add up to 26 receivers to AZGPS.

Positive Subscriber Experience
Customers like working within the AZGPS region so much that some, like Davis, no longer take work outside the coverage area. "It’s too much trouble," he says, "and working within the network area is too easy and too fast." Since he never needs a base station, Davis’s crews work with just a Trimble R8 GNSS rover, and a total station that he estimates is used for about 5 percent of field work, mainly near buildings.

For his part, Greg Thompson still works outside the network coverage, with Trimble 5700 and 5800 base stations, but agrees that AZGPS has improved workflow within its coverage. "It’s very
nice not to have to set up a base station, especially in bad areas of town. There were times when we’d leave crew with a base station, simply to prevent theft."

Davis and Greg Thompson both agree that AZGPS provides superior customer service. Says Thompson, "They’ve even loaned me equipment at times to keep me running. I had a short in an older phone cable, very hard to pin down, and they just gave me a phone with Bluetooth capability until I got it worked out." Similarly, Davis describes the many times "Travis and Boz" have helped him with problems, and the initial training they provided to get him started. "If we have problems we give them a call, and they always get back to us. Can’t beat that team!"

In addition to saving the purchase cost of a base station and the headaches of having one stolen, users also estimate that they save at least an hour a day on setup time. "I thought at first that I should have gotten a base station as well as a rover, but now I don’t even want to see a base setup. They take 20 or 30 minutes; in the time it takes to set up a base, we can finish a job."

Davis does mention some pitfalls that affected his early experience with GPS, including grid versus ground errors, international feet versus U.S. feet, and some scale factor issues in his first few jobs. But after working through these, he’s never looked back. "Even my office time has been reduced. Since I can’t blow an angle, I don’t have to spend time balancing my traverses. Same with level loops. And I have no need for postprocessing ­ I’ve never used it."

In addition to high accuracy and efficiency, users also mention safety as a consideration. Time spent in right-of-way is reduced, and since operators aren’t looking through or at a total station, they are more alert to changing conditions.

Ultimately, of course, subscribing to the network has to be cost effective. And there’s no doubt that it is, at least according to Davis. "Before AZGPS we were doing $150,000 a year, and since then I’ve been doing $300,000, with the same crew. I pay them for eight hours a day, but I have a hard time keeping them busy that long. We do boundary, topo, site plans, land splits, construction staking, you name it, we use GPS for everything."

New Infrastructures Create New Opportunities
Today, RTK networks like AZGPS are being implemented in many regions around the globe. The term "VRS" is widely used to describe such networks, but the term and the technology originated with Trimble.

In the future, network usage may accelerate as coverage increases, in a manner similar to the exponential growth of cell phone coverage; in fact, there are currently more than 200 Trimble VRS networks around the world and more are being created. As fast, accurate positioning over broad areas becomes more common, more users may realize that subscription networks are the latest fundamental utility and demand even more rapid expansion.

So, like cell phone coverage a few years back, instant access to precise position is still something of a rarity, but is likely to be seen as a business necessity in the near future. If private VRS networks work as well elsewhere as AZGPS is working right now in Phoenix, the future might be arriving sooner than expected.

Craig R. Dylan is a freelancer with land surveying background, who specializes in writing for the AEC industry.

A 3.454Mb PDF of this article as it appeared in the magazine—complete with images—is available by clicking HERE