Abstract
Single photon lidar (SPL) is an innovative technology for rapid forest structure and terrain characterization over large areas. Here, we evaluate data from an SPL instrument - the High Resolution Quantum Lidar System (HRQLS) that was used to map the entirety of Garrett County in Maryland, USA (1700 km2). We develop novel approaches to filter solar noise to enable the derivation of forest canopy structure and ground elevation from SPL point clouds. SPL attributes are compared with field measurements and an existing leaf-off, low-point density discrete return lidar dataset as a means of validation. We find that canopy and ground characteristics from SPL are similar to discrete return lidar despite differences in wavelength and acquisition periods but the higher point density of the SPL data provides more structural detail. Our experience suggests that automated noise removal may be challenging, particularly over high albedo surfaces and rigorous instrument calibration is required to reduce ground measurement biases to accepted mapping standards. Nonetheless, its efficiency of data collection, and its ability to produce fine-scale, three-dimensional structure over large areas quickly strongly suggests that SPL should be considered as an efficient and potentially cost-effective alternative to existing lidar systems for large area mapping.
Highlights
An alternative approach to wall-to-wall mapping is available using Single Photon Lidar (SPL)
A large number of residual noise points were observed around isolated targets that were further identified as highly reflective surfaces (Fig. 3b) using high-resolution imagery
Our results suggest that high solar noise in Single photon lidar (SPL) relative to other lidar systems may not be a limiting factor and that automated methods similar to the one developed here are appropriate for daytime acquisitions
Summary
An alternative approach to wall-to-wall mapping is available using Single Photon Lidar (SPL). Multi-photon lidar systems utilize high energy laser beams with long pulse widths[3,20] to capture energy scattered from the target (ground surface or canopy). Single photon lidar systems in contrast, transmit a shorter and lower energy pulse and detect scattered energy more efficiently (Fig. 1) This is accomplished by recording the time of flight of every photon of energy captured by the telescope per laser pulse transmitted. SPL instruments typically operate in the green wavelength (532 nm) as opposed to near-infrared (e.g. 1024 nm) used by conventional vegetation lidar instruments This is because efficient single photon near-infrared detectors are not yet widely available. SPL point clouds include more background solar returns than other lidar instruments when flown during daylight conditions This requires significant post-processing and complicates canopy structure retrieval from SPL when data are acquired in bright sunlight conditions[14]. High point densities are much easier to achieve from airborne systems than from space
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