VNIR Characteristics of Rock Wall Hues at Danxia Mountain, Northern Guangdong, China

Abstract

China's Danxia landforms are characterized by their indicative red color. Portable ground object spectrometers are widely used owing to their small size, ease of operation in the field, rapid measurements, and simple operation. In this study, we analyzed the characteristic parameters of the reflection spectra of the Danxia rock wall surfaces (i.e., red surfaces, orange surfaces, and pure white spots) and different types of plant cover, as well as their quantitative relationships with color tones, using the visible-near-infrared (VNIR) reflectance spectra of the rock walls. The results indicate that the absorption depths and areas of the red and orange surface samples were highly correlated at 525 and 500 nm, as were the redness and orangeness of the samples. The redness and orangeness of the indoor and outdoor samples were relatively consistent with the absorption depth and area growth rates; however, the indoor samples had larger absorption depth and area values. The absorption depth and area of the pure white spots at 530 nm was highly correlated with redness, but the red surface and pure white spot spectra exhibited considerably different redness values, absorption depths, and absorption areas at 525–530 nm. Thus, these are advantageous parameters for distinguishing between the two surface types. The absorption positions of the red surfaces, orange surfaces, and pure white spots in the visible spectrum differed considerably, which is likely related to changes in the iron mineral types and contents present in the strata. Compared to the red surface absorption position at 525 nm, that of the orange surface shifted in the shortwave direction to 500 nm, which may be related to lower hematite and higher goethite contents. The absorption of the pure white spots at 430 nm may be caused by their pyrite content. The absorption depths of black, blocky, and green plants at 670 nm were considerably correlated with Normalized Difference Vegetation Index(NDVI). The NDVI values and absorption depths of the three plant types had widely differing range distributions, which can serve as a basis for distinguishing among the three plant types. The absorption depths of non-white spot and white spot plant coverings at 670 nm also increased with increasing NDVI values. The absorption positions of pure white spots in the visible spectrum were 430, 530, and 690 nm. After plants attached to the white spots, the 430 nm absorption position disappeared while that at 530 nm shifted in the shortwave direction. The absorption position at 690 nm was converted into that of chlorophyll. This study provides a reference for quantitatively investigating the color tones of the Danxia rock walls using VNIR reflectance spectroscopy.

Keywords

• danxia mountain
• vnir
• red rock walls
• hue
• ndvi
• northern guangdong