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What is the difference between bare-earth and first return lidar?

The laser pulses emitted from airborne lidar systems may be reflected from man-made structures, vegetation, or the earthen surface. A single pulse may reflect from upper, middle, and lower tree canopy as well as the ground beneath, resulting in multiple measurable returns from that pulse. The elevation of the reflected surface(s) is derived based on the amount of time it takes for an emitted pulse to return to the sensor, sensor position, and other parameters. The full set of discrete returns and their corresponding x, y, z coordinates are stored in what is referred to as a ‘point cloud’ dataset. The USGS requires lidar points to be classified into one of several general categories such as vegetation, buildings, water, and ground.

Many types of information can be extracted from a classified lidar point cloud. Two common products are DEMs produced by interpolation from lidar points classified as ground, and DSMs produced by interpolation from first-returns detected from each laser pulse.  Together, the first return and bare-earth data allow vegetation height to be modeled; this can in turn, support biomass and carbon estimates.

Tags: Maps, Topographic, Data Products