Snow Surveys by aerial photographs

Abstract

On the occasion of the heavy snow fall in January, 1963 in Hokuriku district the Geographical Survey Institute started its snow surveys by aerial photographs on a scale of 1 : 20, 000. Snow depth was measured at snow fissures on the slopes or exposed snow sections near the water on the photographs, using Stereotope, the third order instrument. At the same time the data on snow depth were collected at the observatories in the area. On the basis of these materials and stereoscopic photointerpretation the snow depth map 1 : 200, 000 was compiled, on which the isodepth lines of snow cover were illustrated at one meter interval. Distribution of snow avalanches as well as the prevailing wind direction on the ground were also analyzed by stereoscopic photointerpretation.After this, maps of the same kind, but more improved, were compiled for many areas. In this paper, referring the snow surveys along the planning routes of Tokai-Hokuriku, Chugoku-Odan and Tohoku-Odan Expressways, the method of show depth measurement, classification of mountain slopes for predicting the occurrence of snow avalanches and the method for estimating the force of snow avalanches are explained. To measure the snow depth on aerial photographs, the so-called relative height method is mainly used, that is to measure the relative height of snow surface from the ground using facilities such as airphoto signals for snow season. At the same time as indirect method the comparison of exposure of roads, railways as well as houseroofs in the snow and non snow seasons were also introduced. The deepest snow depth in the past was estimated from the photographing date by correlation analysis.The main results obtained are the classification of mountain slopes based on the occurrence ratio of snow avalanches and results of field surveys (Table 5), the map of snow depth and snow avalanches distribution (Fig. 11), and of the danger-degree distribution of snow avalanches (Fig. 12).