By Waheed Uddin, professor of civil engineering and director, Center for Advanced Infrastructure Technology (www.olemiss.edu/projects/cait), University of Mississippi, University, Miss.
For years, transportation projects have benefitted from aerial photography and ground surveys for land parcel management, route location, topographic surveys, digital terrain modeling and contour generation. Today’s infrastructure planners are fortunate to have a much wider range of tools at their disposal. Newer remote sensing technologies, such as high-resolution satellite imagery and airborne light detection and ranging (LiDAR), are being used to extract terrain elevations, create contour maps and locate infrastructure assets like road centerlines, bridges, culverts, signs and overpasses.
Such capabilities offer several advantages compared with conventional mapping methods. For example, data are collected and stored digitally, time is saved during data collection and analysis, more area can be covered, and data collection is subject to fewer topographic and weather operating constraints.
Spaceborne Satellite Imagery
The availability of cost-competitive, high-resolution, multispectral satellite imagery provides tremendous opportunities for analyzing infrastructure inventory, land use/land cover and traffic volume, as well as assessing environmental and post-disaster conditions. For example, satellite imagery was invaluable in quantifying Hurricane Katrina damage and associated economic costs in 2005. Figure 1 shows some examples of GeoEye’s 1-meter IKONOS satellite imagery of Oxford, Miss.
Satellite-based imagery applications are particularly important for remote areas and developing regions where little aerial imagery is available. For example, as part of a recent intelligent transportation system project for Karachi, Pakistan, DigitalGlobe’s 0.6-meter QuickBird-2 satellite imagery was used to help create road network and land use data for geospatial analysis. The satellite imagery presented a snapshot of road traffic density when the imagery was collected. Using a linear speed-density model and knowledge of 24-hour volume distribution, average daily traffic volume for the sample road section was calculated from traffic density. Based on statistically designed traffic sample sections, traffic volume maps were produced for Karachi’s entire road network, which serves more than 14 million people. Additional geospatial analysis of the archived imagery can provide a unique opportunity to evaluate land use changes, traffic demand trends, disaster impacts and environmental sustainability.
With the use of low-powered lasers and modern positioning instruments, airborne LiDAR technology has become a low-cost, time-efficient alternative for topographic surveys and terrain mapping projects (see “Comparing Data Collection Technologies for Infrastructure Projects” at right). LiDAR technology uses the near-infrared band of the electromagnetic spectrum and measures the time it takes for a laser pulse to travel from the transmitter to the target and back to the receiver. Because the light speed is known, the distance can be calculated. An accurate timing system is needed to guarantee the resolution, because the laser pulses are sent at 3,000 to 10,000 times or more per second. The aircraft positioning is recorded using an inertial navigation unit associated with avionics systems, a high-accuracy Global Positioning System (GPS) receiver in the aircraft, and GPS base stations installed in known locations. Thus, it’s possible to determine 3-D georeferenced coordinates for each pulse and then correct the aircraft positioning in terms of roll, pitch and heading, thereby improving the system’s accuracy. Modern LiDAR systems can operate at a laser pulse repetitive frequency of 50-100 kHz per second.