2.3.2. Processing multiangular spectra of trees

AH Aarne Hovi PF Petri R. Forsström GG Giulia Ghielmetti MS Michael E. Schaepman MR Miina Rautiainen

This protocol is extracted from research article:

A dataset composed of multiangular spectral libraries and auxiliary data at tree, leaf, needle, and bark level for three common European tree species

**
Data Brief**,
Jan 30, 2021;
DOI:
10.1016/j.dib.2021.106820

A dataset composed of multiangular spectral libraries and auxiliary data at tree, leaf, needle, and bark level for three common European tree species

Procedure

Multiangular spectra (Section 2.2.1) were processed into estimates of directional scattering coefficients (DSC* _{tree}*(Ω), [sr-1]). Note that due to the biconical geometry of the measurements (Fig. 1), our processing results in an approximation of true DSC

The computation of DSC* _{tree}*(Ω) has been reported in Hovi et al. [2] and in Forsström et al. [1]. For completeness, we provide the basic computation steps also here. For derivation of the measurement equations and estimation of uncertainties in DSC

where DN* _{tree}*(Ω) and DN

Eq. (1) assumes that DN* _{tree}*(Ω) and DN

where DN* _{tree}*(Ω) and DN

Despite the corrections, there remained jumps between the detectors of the spectrometer. In addition, there was high-frequency noise present close to 350 nm and close to 2500 nm. To remove the noise, the spectra were smoothed with a Savitzky-Golay filter [8]. Finally, the sensor jumps were removed by multiplying the spectra obtained by the SWIR1 and SWIR2 detectors by correction factors, which were obtained by comparing the difference of DSC between SWIR1 (1001 nm) and VNIR (1000 nm), and then by comparing the remaining difference between SWIR2 (1801 nm) and SWIR1 (1800 nm). We provide both original (DSC_{tree,}* _{raw}*), as well as jump-corrected and filtered (DSC

An estimate of the tree's hemispherical reflectance (*R _{tree}*), i.e., the fraction of intercepted radiation scattered into hemisphere, can be obtained from the DSC

where *i* are the view azimuth angles, *j* are the view zenith angles, *w _{j}* are the Gauss-Legendre weights for each view zenith angle. Note that here we have separated positive and negative zenith angles into separate azimuths. Thus, there are 12 instead of 6 azimuth angles. Multiplication with 2

This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

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