[S-RD20] Cutting-edge sensing technology applied to geology and resource exploration

[SRD20-P04] Measuring Spectral Reflectance Variability in the Gobi Desert by Observation Angles

Baljmaa Byambakhand ²、Yukihiro Takahashi¹、Margad-Erdene Jargalsaikhan⁴、Bayarsaikhan Uudus³、Erdenebaatar Dashdondog²、Turtogtokh Tumenjargal²、Davaadulam Gombosuren⁵、*Begzsuren Tumendemberel^1,2 (1.Department of Cosmosciences, Graduate School of Science, Hokkaido University、2.Department of Physics, School of Arts and Sciences, National University of Mongolia、3.Department of Biology, School of Arts and Sciences, National University of Mongolia、4.Center for Remote sensing, Yamaguchi University, Japan 、5.Institute of Physics and Technology, Academy of Sciences, Mongolia )

キーワード：Spectral reflectance, Gobi Desert, Leaf Area Index (LAI), Observation angle

While traditional Earth observation satellites primarily capture images directly from overhead, advancements in technology now allow modern satellites to adjust their observation angles. The Gobi grasslands, characterized by limited plant diversity, sparse vegetation, or a low leaf area index, often appear as barren deserts in conventional satellite imagery. Despite this, the Gobi is renowned as one of the world's most iconic deserts, hosting a complete ecosystem with unique natural formations that support a range of biodiversity, including rare and endangered species. The landscape reveals a stark contrast: from particular perspectives, the valley radiates lush greenery, while from others, it unveils a predominantly sandy facade. This study aims to explore how the spectral reflectance of the Gobi Desert with observation angles and sunlight illumination, utilizing ground-level measurements and satellite data. Fieldwork was conducted on August 27, 2023, in the Shiveegov sum region of Govsumber province, at coordinates 46.05836N, 108.66189E, between 10:00 and 13:00, at hourly intervals. Measurements were taken using a Liquid Crystal Tunable Filter (LCTF) camera and a Handheld spectroradiometer at 20°, 40°, and 60° relative to the vertical axis, totaling 47 observations.
The findings indicate that as the observation angle (zenith) increased, reflectance values decreased in the red light wavelength but increased in the infrared spectrum. Each pixel captured by the LCTF camera contains spectral data for the imaged area, offering a comprehensive two-dimensional correlation and streamlining the data analysis process. Images captured at nadir angles predominantly reflected the spectral characteristics of sand, while those taken at oblique angles more closely resembled vegetation spectra. Notably, the Normalized Difference Vegetation Index (NDVI) values varied significantly, showing a 65.9% increase at a 60° angle compared to a nadir perspective. The results suggest that oblique imaging in the Gobi steppe can yield richer biological surface data. Incorporating satellite observations, ground-level measurements, and joint analyses of the Gobi's grassland vegetation enhances precision and provides vital, timely insights for ecosystem evaluations.