TOPOGRAPHYC SHADOW INFLUENCE ON OPTICAL IMAGE ACQUIRED BY SATELLITE IN THE SOUTHERN HEMISPHERE

ABSTRACT During image acquisition, is usually chosen scenes with a lesser cloud cover to avoid loss of spectral information. However, when training samples are collected for image classification, the user finds shadowed areas. Such situation is similar to the presence of clouds since spectral information of these classes is the same in all optical bands of the sensor. This fact becomes more pronounced in mountainous relief areas due to shadow projection on the terrain, which can vary among all seasons during the solar year. With the goal to obtain images with a lower presence of shadow, it was simulated, under the same relief conditions, shading variation in function of latitude (0° to 40° S). Solar radiation models were processed for the days and times passages of the Landsat TM and ETM+ satellite on the Southern Hemisphere. It was verified that over 30° S and 40° S latitudes, a loss of shading area varying between 27% to 91 % and that images should be preferentially taken between October and February. For latitudes comprising 0° and 10° S, the loss was considered negligible, when we set a 10% threshold of loss in the total valid area in an image. According to the amount of radiation in a terrain, South and West areas received less direct solar radiation over the year for all analyzed latitudes in the modeling.

摘要 在图像采集过程中，为避免光谱信息损失，通常会选择云量较低的场景开展拍摄。但在收集图像分类训练样本时，用户常会遭遇阴影区域问题。这类场景与云覆盖情况类似——传感器的所有光学波段中，阴影区域的光谱信息均保持一致。在山地地形区，这一现象更为突出：地形会投射阴影，且全年随太阳年季节变化产生差异。为获取阴影占比更低的遥感影像，本研究在相同地形条件下，模拟了南纬0°至40°区间内随纬度变化的阴影分布差异。针对南半球陆地卫星TM（Landsat Thematic Mapper）与ETM+（Landsat Enhanced Thematic Mapper Plus）卫星的过境日期与时刻，构建并处理了太阳辐射模型。研究证实，在南纬30°至40°区域，阴影区域面积损失率介于27%至91%之间，且影像拍摄的优先时段为10月至次年2月。对于南纬0°至10°的区域，若将影像有效总面积的10%作为损失阈值，则阴影损失可忽略不计。根据地形接收的辐射量测算结果，在本次建模分析的所有纬度区间内，南向与西向区域全年接收的直接太阳辐射均相对更少。