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

The laser pulses emitted from terrestrial lidar systems can 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. Very simply stated, these returns are processed, with time and positional information, to derive elevations for each of these returns. 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 the points to be classified into one of several general categories, including vegetation, buildings, water, ground, and others.

Many types of information can be extracted from a classified lidar point cloud. Two common products are bare-earth DEMs and first return digital surface models (DSMs). First return is created by identifying those points which are the first reflection from each laser pulse. These can be extracted and delivered as a separate point dataset, or converted into a raster surface. In open areas, a single point is returned.

Together, the first return and bare-earth data allow vegetation height to be modeled; this can in turn, support biomass and carbon estimates. Datasets of first return points, only, are referred to as “first-return surfaces” (raster or vector) or DSMs, which are typically raster.

Tags: Maps, Topographic, Data Products