Pulses Increase with New Airborne Laser Mapping5 Mar, 2007 By: Andrew G. Roe,P.E.
Will the new LIDAR system's increased data-capture technology win over conventional system users?
A new twist to established technology could enable airborne laser mapping to be used in more applications than previously envisioned. The Multiple Pulse in Air (MPiA) technology announced by Leica Geosystems in late 2006 measures distances at higher rates than conventional time-of-flight measurements, enabling airborne systems to collect twice as much data as before.
In conventional airborne LIDAR (light detection and ranging) systems, an aircraft-mounted laser sends pulses of light to earth and measures the time it takes for the pulses to reflect back, thereby determining distances and establishing 3D coordinates for thousands of points per second. An inertial navigation system measures aircraft movement, and kinematic GPS receivers -- one on the ground and one on the aircraft -- determine the aircraft's position.
Unlike conventional LIDAR systems, which wait for one pulse to return before sending another, the MPiA technology fires a second pulse before the first one returns, establishing two coordinate locations instead of one. Using advanced electronics, "we can keep track of which return reflections are associated with which pulses," said Ronald B. Roth, airborne LIDAR product manager for Leica in Westford, Massachusetts. "You can essentially double the pulse rate."
Conventional LIDAR technology waits for one pulse to return before sending another, while MPiA technology fires a second pulse before the first one returns. (Image courtesy of Leica Geosystems.)
The higher pulse rate means airborne LIDAR, which has been used for more than a decade, could prove more effective in mapping rugged terrain, major cities with tall buildings and wide areas such as counties. With present technology, aerial mapping crews must balance flight altitudes with data density. At higher altitudes, flights are generally smoother and offer more clearance above obstructions, but the increased altitude means longer return times for laser pulses and consequently fewer measurements.
Because crews can cover twice as much area in a given time with MPiA, Roth expects to see reduced data acquisition costs and new applications for LIDAR. MPiA can produce "first-order elevation models for entire counties," he said. First-order elevation standards represent the highest accuracy standards for organizations, such as the National Geodetic Survey, and generally require low, narrow flight paths or ground surveys.
Leica has tested the technology in actual applications and has a patent pending, according to Roth. He predicts the system will be available commercially within three months. Owners of compatible single-pulse systems should be able to upgrade their equipment for a cost of approximately 10% of their original systems, he said.
In one test, Leica's MPiA system mapped a forested area during leaf-on conditions and captured an overhead utility line at a flight altitude of 1780 meters. The data set was classified into ground points (in light greenish-brown color), trees and low vegetation (in dark green) and power lines (in red). The system has also been tested on large industrial and municipal infrastructure sites.
A forested area was mapped during leaf-on conditions to capture an overhead utility line at a flight altitude of 1780 m. (Image courtesy of Leica Geosystems.)
Industrial and infrastructure sites could benefit from MPiA technology. (Image courtesy of Leica Geosystems.)
Aerial mapping firms are eyeing the MPiA technology with both optimism and caution. "It's still in its infancy," said Blaine Thibodeaux, corridor mapping manager for John Chance Land Surveys in Lafayette, Louisiana. "It will help data density, but the laser is just one piece of the system," he said. He notes that accuracy also depends on other factors, such as whether the oscillating mirrors that scan for laser pulses can operate effectively at higher rotation rates.
Thibodeaux says his firm, which has been performing ground and aerial surveying since the 1950s, will likely test MPiA later this year, but he is not sure how strong the demand will be. "It depends if clients are asking for it," he said. He also notes that conventional LIDAR already produces massive data sets, and increased data density could bog down systems. "You also have to ask if clients can handle that density. At what point does it become too much?"
As with other technological advancements, market forces will likely decide whether MPiA becomes commonplace or merely a novel solution in search of a problem. But the potential to squeeze twice as much data out of any given flight will undoubtedly cause mapping firms and end users to take notice and evaluate the possibilities.