Evaluation of a useful method to identify snow‐covered areas under vegetation – comparisons among a newly proposed snow index, normalized difference snow index, and visible reflectance

Abstract

Objective methods of monitoring snow‐covered areas by optical remote sensing were evaluated using synchronous observations conducted with the passage of the Landsat‐7 satellite over the plains of Niigata prefecture, one of the snowiest regions in Japan. The observations were conducted in the springs of 2002 and 2003. Snow‐covered areas were identified using three methods: (1) visible (red) reflectance, (2) Normalized Difference Snow Index (NDSI) which uses visible and shortwave‐infrared reflectances, and (3) a newly proposed snow index called S3 which uses visible, near‐infrared and shortwave‐infrared reflectances. The Snow‐Cover Ratio (SCR) was defined as the ratio of the number of pixels in snow‐covered areas to the total number of pixels in an image. The threshold value for the three indices used to identify snow‐covered areas was defined as 50% of SCR, which converged to nearly the same value regardless of the images analysed. Under clear conditions, visible (red) reflectance can identify snow‐covered areas accurately if no vegetation is present. NDSI can distinguish snow‐covered areas from mixels (mixed pixels) of snow and vegetation by referring to the Normalized Difference Vegetation Index (NDVI). S3 can distinguish snow‐covered areas from mixels of snow and vegetation without any reference data. S3 is, therefore, more useful than NDSI because it automatically distinguishes snow‐covered areas from mixels of snow and vegetation.

Acknowledgments

This work is financially supported by the Grant‐in‐Aid from the Ministry of Education, Science and Culture of Japan (No. 15300306), by the Inamori Foundation, by the financial assistance of Tokyo Geographical Society, and by the President's Research Fund of Tokyo Metropolitan University. We thank Mr. Y. Hando (local resident of Tsunan Town) for his collaboration in the forest stand observation. Appreciation goes to the Forestry Cooperative of Tsunan Town and the Ojiya Forest Office of Niigata Prefecture for selecting the observed forest of Tsunan Town. We also thank Mr. Y. Fujioka, Mr. T. Nuimura, Ms. T. Koike, and Mr. K. Teuchi for their assistance with in situ snow mapping, and Dr. D. Nakayama (Tokyo Metropolitan University) for his advice and technical assistance.