The Salford Advanced Laser Canopy Analyser (SALCA) is a unique dual-wavelength full-waveform terrestrial laser scanner (TLS) designed to measure forest canopies. This article has two principle objectives, first to present the detailed analysis of the radiometric properties of the SALCA instrument, and second, to propose a novel method to calibrate the recorded intensity to apparent reflectance using a neural network approach. The results demonstrate the complexity of the radiometric response to range, reflectance, and laser temperature and show that neural networks can accurately estimate apparent reflectance for both wavelengths (a root mean square error (RMSE) of 0.072 and 0.069 for the 1063 and 1545 nm wavelengths, respectively). The trained network can then be used to calibrate full hemispherical scans in a forest environment, providing new opportunities for quantitative data analysis.
ARTICLE HISTORY
The Salford Advanced Laser Canopy Analyser (SALCA) is an experimental terrestrial laser scanner designed and built specifically to measure the structural and biophysical properties of forest canopies. SALCA is a pulsed dual-wavelength instrument with co-aligned laser beams recording backscattered energy at 1063 and 1545 nm; it records full-waveform data by sampling the backscattered energy at 1 GHz giving a range resolution of 150 mm. The finest angular sampling resolution is 1 mrad and around 9 million waveforms are recorded over a hemisphere above the tripod-mounted scanner in around 110 minutes. Starting in 2010, data pre-processing and calibration approaches, data analysis, and information extraction methods, were developed and a wide range of field experiments conducted. The overall objective is to exploit the spatial, spectral and temporal characteristics of the data to produce ecologically useful information on forest and woodland canopies including leaf area index, plant area volume density and leaf biomass, and to explore the potential for tree species identification and classification. This paper outlines the key challenges in instrument development, highlights the potential applications for providing new data for forest ecology, and describes new avenues for exploring information-rich data from the next generation of TLS instruments like SALCA.
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