Abstract
Vegetation coverage is one of the well-known parameters indicating the growth of rice plants. It is usually calculated as a ratio of the extracted visible green area of plants, the green visibility ratio, in a camera image using the vertical photography. However, early in the season, growth is mostly vertical and therefore a near-vertical observation angle doesn't observe vegetation coverage sensitively. But, a near-horizontal angle of observation can observe vertical and horizontal growth and allow early season monitoring of vegetation coverage. In this paper, the green visibility ratios were calculated by the visible green areas in different observation angles (the range of the observation angle 0.0 degrees to 22.0 degrees) using camera image data taken from vertical photography during the growing period. The relationship between the green visibility ratio and the observation angle was analyzed. As a result of this analysis, the green visibility ratio was increased with the increase in the observation angle and the according to the progress of growth. In addition, the range of the observation angle with the green visibility ratio almost equivalent as the vegetation coverage was decreased rapidly with the progress of growth in 40 to 50 days after transplanting.
