A dataset of multiangular spectra of tree bark for three boreal tree species

The collection and preprocessing of this dataset has been reported in detail in Juola J., Hovi A., Rautiainen M. (2020). Multiangular spectra of tree bark for common boreal tree species in Europe. Silva Fennica vol. 54 no. 4 article id 10331. https://doi.org/10.14214/sf.10331. The dataset includes average multiangular stem bark reflectance spectra of three common boreal tree species found in Europe: silver birch (Betula pendula Roth), Scots pine (Pinus sylvestris L.), and Norway spruce (Picea abies (L.) H. Karst.). Bidirectional reflectance factors (BRF) at visible and near-infrared wavelengths (418-924 nm) are reported for each tree species. Multiangular measurements were performed in six different view angles along the principal plane (29, 58, 65, 90, 115, and 140 degrees), using a laboratory goniometer. The 90° view angle is perpendicular to the sample surface, < 90° view angles are backward-scattering directions, and > 90° view angle are forward-scattering directions. The goniometer utilized a Specim IQ hyperspectral imaging spectrometer. The nominal illumination angle for the measurements was 40°. The average reflectance spectrum for each tree at each view angle was calculated by averaging over all measured samples. The samples comprised of stem bark taken at different heights along the tree stem, from two individual trees per species. The samples were collected from forest stands located in Keimolanmäki, Vantaa, Finland (60°19´N, 24°50´E), in March-April of 2019, and the measurements were conducted within 12 days from the sample collection. The study stands were typical managed and even-aged forests in Finland, approximately 40 years old, and grew on a mesic site type on mineral soil. The spreadsheet "multiangular_spectra_juola_et_al_2020.csv" in this dataset is organized as follows: first column "tree_species" (pinus_sylvestris, picea_abies, betula_pendula) indicates the tree species, second column "view_angle" (29, 58, 65, 90, 115, 140) indicates the view angle, and the subsequent columns "wl417.5"- indicate the measured BRF value at that wavelength. A separate list of the wavelengths (without the prefix "wl") is provided in the file "list_of_wavelengths_juola_et_al_2020.csv". The encoding of the csv files is UTF-8.

本数据集的采集与预处理流程已在Juola J.、Hovi A.、Rautiainen M.（2020）发表的研究中得到详细阐述：《欧洲常见寒带树种树皮多角度光谱》，刊载于期刊《Silva Fennica》第54卷第4期，文章编号10331，DOI链接：https://doi.org/10.14214/sf.10331。
本数据集包含欧洲三种常见寒带树种的茎秆树皮平均多角度反射光谱：白桦（Betula pendula Roth）、欧洲赤松（Pinus sylvestris L.）以及挪威云杉（Picea abies (L.) H. Karst.）。
本数据集报告了各树种在可见光与近红外波段（418~924 nm）的双向反射率因子（Bidirectional Reflectance Factors, BRF）。
本次实验采用实验室测角仪，沿主平面设置6个不同观测天顶角（29°、58°、65°、90°、115°、140°）开展多角度测量。其中90°观测天顶角与样本表面垂直，观测角小于90°的方向为后向散射方向，观测角大于90°的方向为前向散射方向。
本次测量使用的测角仪搭载了Specim IQ高光谱成像光谱仪（hyperspectral imaging spectrometer），测量的标称照明天顶角为40°。
针对每个树种的每一个观测天顶角，研究人员对所有采集的样本取平均，得到该树种对应观测角下的平均反射光谱。
样本取自每个树种的两株独立个体茎秆不同高度处的树皮，采集地点为芬兰万塔市Keimolanmäki林区（北纬60°19′，东经24°50′），采集时间为2019年3月至4月，所有光谱测量均在样本采集后的12天内完成。
该试验林为芬兰典型的人工经营同龄林，林龄约40年，生长于矿质土壤的中生型立地条件上。
本数据集附带的电子表格文件`multiangular_spectra_juola_et_al_2020.csv`组织结构如下：第一列为`tree_species`，取值包含pinus_sylvestris、picea_abies、betula_pendula，用于标注对应样本的树种；第二列为`view_angle`，取值为29、58、65、90、115、140，用于标注观测天顶角；后续列以`wl417.5`为前缀，分别代表对应波长下测得的BRF值。
文件`list_of_wavelengths_juola_et_al_2020.csv`中单独提供了波长列表，已去除前缀`wl`。
上述CSV文件均采用UTF-8编码格式。