Abstract
Over the past 30 years, the use of field spectroscopy has risen in importance in remote sensing studies for the characterization of the surface reflectance of materials in situ within a broad range of applications. Potential uses range from measurements of individual targets of interest (e.g., vegetation, soils, validation targets) to characterizing the contributions of different materials within larger spatially mixed areas as would be representative of the spatial resolution captured by a sensor pixel (UAV to satellite scale). As such, it is essential that a complete and rigorous assessment of both the data acquisition procedures and the suitability of the derived data product be carried out. The measured energy from solar-reflective range spectroradiometers is influenced by the viewing and illumination geometries and the illumination conditions, which vary due to changes in solar position and atmospheric conditions. By applying corrections, the estimated absolute reflectance (Rabs) of targets can be calculated. This property is independent of illumination intensity or conditions, and is the metric commonly suggested to be used to compare spectra even when data are collected by different sensors or acquired under different conditions. By standardizing the process of estimated Rabs, as is provided in the described toolkit, consistency and repeatability in processing are ensured and the otherwise labor-intensive and error-prone processing steps are streamlined. The resultant end data product (Rabs) represents our current best effort to generate consistent and comparable ground spectra that have been corrected for viewing and illumination geometries as well as other factors such as the individual characteristics of the reference panel used during acquisition.
Highlights
Field spectroscopy has long played a key role in the collection of spectral information used for a broad range of remote sensing studies [1,2,3]
ASDToolkit is a standalone application for generating estimated Rabs from the commonly acquired Rratio provided by the ASD FieldSpec series of spectroradiometers (Malvern Panalytical Company, Longmont, CO, USA)
There are several options for processing Rratio files, including multiple selection choices for the reference panel to use, an option to apply an incident angle correction factor (IACF) to account for the time offset between collection of the reference measurement and target measurements, an option to apply a spectral discontinuity correction at the cross-over wavelengths between the three spectrometers (i.e., 1000 and 1800 nm), and options to override the illumination conditions using two common lab-based illumination and viewing geometries
Summary
Field spectroscopy has long played a key role in the collection of spectral information used for a broad range of remote sensing studies [1,2,3]. As described by [2], the generation of Rratio requires a second surface, the reference panel, to be used as a bright (approaching 100% reflectance), theoretically Lambertian (i.e., diffuse) target with which the ratio is calculated. In order to generate standardized, representative, and comparable Rabs, there are multiple factors to consider, including the influence of downwelling irradiance from direct (e.g., the sun) and diffuse (e.g., the sky) sources on the target(s) of interest and the spectral characteristics of the reference material (e.g., 99% reflective Spectralon®) against which the target is measured. By applying corrections for variations in solar illumination conditions during the measurement period and the individual spectral characteristics of the reference panel used during acquisition, a more accurate approximation of the estimated Rabs can be achieved. ASDToolkit is a standalone application for generating estimated Rabs from the commonly acquired Rratio provided by the ASD FieldSpec series of spectroradiometers (Malvern Panalytical Company, Longmont, CO, USA)
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