Wisconsin Geospatial News

LIDAR unveiled

Multiple choice: LIDAR technology is how many years old?

5, 10, 15, 20, or 30.

If you selected “30” you are right!! According to Mike Renslow, the basic ideas and equipment behind LIDAR (Light Distance And Ranging) were declassified from U.S. military use in 1972. Renslow, who works for Spencer P. Gross, Inc. in Portland, Oregon, was in Madison on June 21 to speak to a regional meeting of ASPRS.

So, you might ask, why has it taken so long for LIDAR to become a useful tool for civilian mapping? The answer lies in the speed of computers which can now handle the collection and analysis of datasets that in four hours collection time can total 25 gigabytes.

LIDAR basically involves painting a pulsing laser beam back and forth across the ground from an aircraft. The time needed for the beam to reflect back to the sensor on the plane is measured, which results in a distance. From this raw data, a collection of points representing a surface model can be collected. (The plane’s position is also computed for each pulse, using GPS)

When mapping a forested area, a LIDAR unit’s light beam can be reflected back from multiple surfaces starting with the tree canopy and down to the ground. These “multiple returns” can be intrepreted into information helpful to understanding forest structure.

When measuring water depths, two LIDAR beams are used. One (in the near infrared band) is reflected from the water surface or absorbed; the other (532 nm) in the visible blue part of the spectrum, penetrates and reflects back from the bottom — in some cases from as deep as 30 meters.

More information: Technical background on LIDAR error budgets