The growth of GPS-driven engineering, surveying and construction has been matched by an increasing awareness of the role a network of Continually Operating Reference Stations (CORS) can play in those functions. Capable of providing high-quality GPS reference data, stations within the CORS network automatically measure — and compute error-corrections — for GPS satellite data, then make that data available to users of the network, thereby enhancing the data's accuracy and value to the user. The Tennessee Department of Transportation (TDOT) recognized a need for such a network and — looking to avoid post-purchase disappointment and setback — followed a rigorous routine to vet its list of potential suppliers. The result, a contract with Topcon Positioning Systems, was almost three years in the making but has proven highly-successful. Today TDOT's network is 22 stations strong and growing, and has helped drive a leap in both GPS-driven capability and better on-site efficiency.
Charting a CORS
The push to upgrade TDOT's survey function was mainly driven by a need to improve on-site efficiency, according to Jim Waters, the Design division's assistant director, responsible for Field & Aerial Surveys, CADD and Region 2 Design.
"GPS has been a part of this division since the late 1980s, and in the late 90s was expanded to a point where each of TDOT's four regions had their own GPS crew. We were looking, however, at the possibility of taking the next step in the evolution of that function: creating a statewide CORS network for the department's use. Doing so, we felt, could enhance and streamline the survey function."
After attending a GPS conference in Nashville, Waters says he and his team started thinking seriously about reference stations and their long-term ability to allow the department to cover more projects with the same amount of staff.
"That potential for increased efficiency coupled with a difficulty getting replacement parts for our existing, dated GPS equipment got us moving toward an upgrade."
Doing It by the Book
To ensure the department's GPS investment was a sound one, Waters chose to follow an optional verification process the state of Tennessee makes available to its various agencies. Called Testing of Qualified Products (QPL), the process is a method by which departments can effectively qualify vendors and their products in advance of purchase.
"We knew this would be a sizeable, high-profile investment and wanted to make sure of what we were purchasing up front," he says.
After an intensive supplier vetting and subsequent bidding, on October 20, 2006, nearly three years after beginning to look into an upgrade, the contract was awarded to Topcon Positioning Systems and their local dealer, Hayes Instrument Co. of Shelbyville, Tenn.
The Whole Package
The contract which TDOT awarded Topcon was an impressive one, to say the least. Today, the survey group, which had once solely relied upon aging, static-grade equipment, counts in its inventory 22 GPS Reference Stations (NET G3 receivers, G3-A1 Geodetic Antennae, and necessary Antenna Cabling); 30 Portable GPS Rovers with internal modems and radios (GR-3); 20 Portable GPS Base Stations with internal modems and radios (GR-3); 30 RE-S1 repeaters to enhance signal quality in difficult applications; nine GMS-2 GIS receivers (four for immediate use by the Design Division, four for use by the Environmental Division, and one for the GIS Section of the Information Technology Division); TopNET Software for 25 Reference Stations and 75 GPS Rovers; 17 Seats of Topcon Tools — Survey; and 11 Seats of Topcon Tools — GIS. According to David Marshall, TDOT roadway specialist supervisor in Nashville, the department wasted no time putting it to immediate and productive use.
"With the new equipment in place, one of the first projects we undertook was to establish our statewide CORS Network. Today, that consists of 22 reference stations — one at almost every TDOT District or Regional Office — with 20 more sites being considered to provide additional coverage in areas away from district offices. The equipment is rated for accuracy to 60 kilometers, but we spaced the installations at 40 kilometers to provide a level of redundancy."
Multiple Approaches
Marshall says the CORS site installations were standardized statewide and generally took no more than half a working day each to complete. Once fully established and certified with NGS as part of the National CORS Network, TDOT's CORS network will afford the department a third survey mode.
"Our CORS network will allow us to access our main server using one of the Topcon GR-3 receivers with internal cell phone modem and get RTK survey capability in that manner. However, because there are obviously dead areas for cellular coverage, we will still have two other modes on which we can rely: static, and RTK using radio communications with a GR-3 receiver as a portable base. We still believe in bringing project control points into our local project site for use in a static session, so it's good to still have that capability. However, getting solid, survey-grade accuracy in real time is certainly far more efficient than anything we'd done in the past, and we're seeing the benefits of that already."
In getting to the point they are at today, TDOT's Jim Waters says they developed a close relationship with representatives at both the equipment manufacturer level and with their local dealer, Hayes Equipment.
"Over the course of three years you really get to see how people work, and we've been very pleased with the level of service and cooperation we've gotten from both Topcon and Hayes," he says. "When you are dealing with a purchase of this size and complexity, there are going to be 'bumps' along the way and we were no exception. However, Eddie Clanton, Hayes' CEO, was our primary point person during this process, and we found them to be an organization that places a lot of value in honesty and won't stop until every issue has been resolved. We found the same to hold true for Joel Frost and Steve Briggs at Topcon; their commitment has been solid from the start and has only gotten better."
Broader Uses, Now and To Come
Although Waters is primarily concerned with the contributions the new equipment will make to their survey function, he sees a wealth of additional opportunities for it throughout the department.
"We were very interested in a GIS capability from the outset," he says. "So, in specifying the equipment, we wanted a GIS-based tool that would give us 1-meter accuracy right out of the box and could be tightened up to survey grade with corrections from the network. The GMS-2 met those requirements, and our Environmental division has already started using it for wetland delineation projects. In addition, our archaeologists and biologists have plans to use it and we see a good deal of potential for our maintenance staff to use it for inventory. I feel we've only just begun to realize this equipment's true value."