For users of geographic information systems (GIS), several global positioning system (GPS) satellites promise advances in technology that will be useful in providing highly accurate locations on the earth's surface. Processed data allow users to map visible features such as utility poles and towers, traffic signs, buildings and trees. Indeed, anything visible to the eye can be captured by the digital imaging system. Using camera pairs mounted on top of a van, physical features of a right of way (R/W) can be located precisely with GPS coordinates.
Routing a Transmission Line
Faced with the need to build a 12-mile (19-km) line from a substation to a new customer, Florida Power Corp. (St. Petersburg, Florida, U.S.) used a van with GPSVision, which is a mobile mapping system developed by Lambda Tech International (Waukesha, Wisconsin, U.S.) to survey the route. Under the circumstances, it would have taken a traditional survey crew several days to complete the work. With the survey van, crews driving at speeds of 35 mph to 50 mph were able to complete the work in less than an hour.
In addition to two cameras mounted on the roof of the van were a solid-state inertial guidance system and a GPS antenna connected to a 7400 GPS receiver supplied by Trimble Navigation Ltd. (Sunnyvale, California, U.S.). During the drive-through, images were collected every 40 ft (13 m) with GPS and inertial data being stored as a digital record with each image. Once the drive was completed, the data were returned to the office where the GPS data were modified using the technique known as differential correction, which is required to produce error-free data.
In this respect, it must be recognized that GPS signals contain errors that reduce the accuracy of positional data. These errors are corrected by comparing the van's GPS data with data available from a stationary GPS unit whose exact position is known. When transferred to a compact disk, Lambda Tech software reads the data. The software, which runs on a Microsoft Windows PC, brings up a map of the route, digital stereo images and a user-defined list of features. A data screen shows the feature position, name and user-defined attributes. While viewing the images, the user can point with a cursor to anything within the range of the camera and accurately position the object. Data coming out of the Feature Extraction software is already formatted for the target GIS or CAD system, which in this application was AutoCAD.
How the System Works
Driving a van equipped with a combination of hardware and software, digital pictures are collected, along with GPS and inertial data, yielding positional data that are accurate enough to produce quality maps. After differentially correcting and processing the GPS data, the information is stored on CDs, DVDs or similar media. Map and database creation is accomplished by pointing and clicking the mouse at any visible object in front of the cameras to a distance of about 130 ft (40 m). An asset symbol is placed on the map and its location on the surface of the earth is logged with a horizontal accuracy of at least 3 ft (1 m). Adding attributes, measuring distances between features, outlining areas or determining clearances in the same picture can be accomplished by pointing and clicking again.
At the heart of the GPSVision system is the positioning module, which consists of geodetic quality GPS receivers, inertial guidance and a linear distance-measuring device. As an integrated system, these components allow the data-collection vehicle to maintain a submeter position even if the GPS signals are lost for up to 10 minutes, as might be the case if trees interfered with signal reception.
Using digital cameras with pixel resolution of 768 by 484 with full-frame progressive-scanning technology, the system produces high-quality stereo digital imagery. Because the cameras cover the same area from different positions, locations of features closer than 130 ft (40 m) in front of the lenses can be calculated using triangulation algorithms. Once the asset is positioned, information about the object can be added to the data stream.
Output to GIS or CAD systems is as simple as defining table cross references and pushing a button to generate data files for AutoCAD, Microstation, MGE, ArcInfo, ArcView, Small World and other popular systems. Additional software tools for random viewing, automatic stationing and offsetting features also are available as part of the GPSVision suite of programs.
Although the GPSVision technology was not considered to be a replacement for a conventional survey, it provided valuable information that was needed to launch the transmission-line project. In actuality, the system did more than a traditional survey. Not only was it faster, but the images produced information on physical features along the R/W, which provides relationships among facilities that were already in place. Cost savings were significant compared with conventional methods, with respect to labor and, especially, with respect to time.
GPSVision has four basic modules that are necessary to accomplish a survey of geographical features:
For positioning, a highly accurate inertial measurement unit is used alongside the GPS receiver and distance-measuring device.
For stereo imaging, high-resolution progressive-scan color cameras are mounted on the roof of a van to record the images of interest.
For digital recording, a digital imaging and data-storage system are employed.
For measurement and positioning software, full-function Feature Extraction PC-based software is used allowing the user to point at visible features, determine their real-world position and attach attributes interactively. Heights, widths and areas also can be measured and recorded.