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Report from Rhode Island DOT
7400/Vision-RIDOT/V2
RIDOT--DOT With a Vision
by Michael W. Michelsen, Jr.
The last time the Rhode Island Department of Transportation (RIDOT) [Providence, R.I.] had updated pavement information for their more than 40,000 miles of state highway in 1994, they had used video technology. This time (ed note: summer 1998), the department's Traffic and Safety Management Section used Global Positioning System (GPS) technology--and saved $ 20,000 over initial cost estimates.
"There's no question we saved considerable time and money," said Joe Bucci, supervising civil engineer at RIDOT. "The GPS data was dead-on accurate. And we got our data for all of the numbered routes and principal arterials in less than six months." For the project, RIDOT used GPSVision, a tool developed by Lambda Tech International [Waukesha, Wis.] and the 7400MSi GPS receiver from Trimble [Sunnyvale, Calif.].
The videotaped field data had allowed RIDOT to collect information on pavement condition, a good use of that technology, Bucci explained. But GPSVision was able to provide pavement condition data and right-of-way information. "The GPS component gave us the digital data that allows us to not only have the video images, but also to take measurements from those images," said Bucci. "And because all the data was in a format that fit into our GIS, we can distribute all of it throughout the RIDOT internal network."
Mapping With a Vision
GPSVision combines Trimble's 7400 GPS positioning technology, an Inertial Navigation System (INS), and digital imaging, allowing users to map streets, highways and even railways--at a fraction of the time and cost of traditional data collection techniques.
GPSVision was developed by Lambda Tech International in 1994 to meet the need for a better way to map facilities. Lambda Tech International was founded by digital mapping professionals to help government agencies and a wide variety of private clients build interactive GIS databases to a level of accuracy never achieved before.
Data imagery allows users to map visible features such as utility poles, traffic signs, buildings, trees, even potholes and more. "Anything you can see with the cameras we can locate with GPS coordinates, and we can do it faster and more accurately (submeter/horizontal) than in the past," said a Lambda representative. "We can map railways as easily as city streets. And after the data is collected the user can repeatedly go back to map additional features on an incremental basis. So it's flexible as well as accurate."
The GPSVision mobile mapping equipment is a transportable unit mounted inside and atop a vehicle. The system can be installed in a vehicle without special tools in about four hours.
The GPSVision mobile mapping system consists of four basic modules:
Positioning: A highly accurate, inertial measurement unit (IMU) alongside the GPS receiver.
Stereo Imaging: High resolution progressive scan digital color cameras.
Digital Recording: A digital image and data storage system.
P.C. Software: Full-function feature extraction software that allows the user to point at visible features, determine their position and attach attributes interactively. Heights, widths and areas can also be measured and recorded.
A Happy Accident
"When we hired Lambda Tech to provide the data gathered by the GPSVision vehicle, we knew we were going to improve our data management capabilities," Bucci said. "What we hadn't counted on was the capability it would give us to populate our GIS with the improved data."
RIDOT uses Arc/Info and ArcView software (ESRI, Redlands, Calif.) for its GIS program.
Picture This
The pictures created from the data are in stereo; users also know the exact position and direction of every picture, they explained. "We combine that with the camera lens parameters and a baseline to perform traditional photogrammetry at locations on those images," he said.
Picture this: A computer screen shows you the stereo images taken by the GPSVision vehicle. Let's assume that in both images you see an object such as a street sign. By pointing your cursor to the object in each screen and clicking the mouse, the object's X, Y, and Z coordinates (accuracy is less than one meter) are immediately fed into the target GIS format. "Anything you can see in the images can be captured very accurately by the GPS receiver,".
Before and After
"Prior to receiving our data from GPSVision, we had a system that gave us what we needed--as far as it went," Bucci said. "Unfortunately, the videos took a lot of time to use, and making measurements from those tapes was very laborious." They also had to tell the computer what area they wanted to see and then find the tape it designated, he explained. And then they still had to fast-forward or reverse through the tape to get to the section they wanted.
The 7400 Element
Trimble's 7400 receiver is the positioning component for GPSVision, Orvets explained, though it hadn't always been that way. "We started this project using Trimble's GPS Pathfinder Pro XL, differentially corrected to get the submeter accuracy," he said. "But we changed it to the 7400 because as the other elements of GPSVision changed we wanted to increase accuracy. The 7400 gives us a data point every second and allows us to update our IMU on a regular basis."
According to Lambda Tech's Dr. He, the GPS part of the equation is critical. Users need to know the cameras' precise locations to transfer the infrastructure's positions to the surface of the earth. "Without great accuracy in this positional element, there would be no way to accurately measure anything in the system," he explained. "The measurement of any image taken by the cameras would be relative to the accuracy of the GPS coordinates."
Hammersley emphasizes the importance of the system's measurement capability. "Not only can we position where things are, but the accuracy of the GPS positioning allows users to do accurate measurements," Hammersley said. "For example, measure the length of a street and you're getting accuracy along the lines of 15 centimeters. We can do things like measure distance from the street to a signal very easily."
A Matter of Results
"We are very pleased with the GPSVision product," Bucci said. "Now, not only do we have the digital record, but we can share this data with anyone who has access to the RIDOT network. Prior to using GPSVision, anyone who wanted to see pavement management data had to either come to our offices or make a field trip."
"GPS is an excellent tool for pavement management," Bucci explained. "I believe there's nothing like it for providing positioning data. In the past we used GPS only with our GIS. GPSVision has given us a whole new vision for using GPS technology."
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