LIDAR (Light Detection and Ranging) is a remote sensing technology that uses a laser beam of light to measure the distance from a LIDAR sensor to distant objects. Its operation is similar to how SONAR measures distances underwater using sound pulses.
LIDAR often is used to measure the height of the ground’s surface and the height of features on the ground. In this case, a LIDAR sensor, mounted in a plane, is flown over the area to be imaged. The sensor sends out a series of laser pulses toward the Earth’s surface. Each laser pulse bounces off objects on its journey toward the ground (tree tops and branches, vegetation, roofs, stone walls, etc.) and returns skyward toward the sensor. If the pulse travels all the way to the ground without hitting anything else, it reflects off the surface of the Earth and returns back to the sensor.
The LIDAR sensor measures the time it takes each pulse to travel to the ground (or other object) and back and uses this information to create a 3-D representation of the imaged surface. The return pulses can be used to define both the “bare earth” (the ground) as well as aboveground structures (trees, houses, etc.). These distinctions are created by processing the raw data using algorithms designed to separate bare earth from an aboveground structure.
Most people who utilize LIDAR data will not deal with the raw data directly but instead will work with some of the products produced from LIDAR overflights. Examples of LIDAR-produced data include digital elevation models, topographic contours, and intensity images.
For more basic information, see the NOAA CSC LIDAR page. For more technical information, see the USGS Center for LIDAR Information Coordination and Knowledge.