关键词: 疏透度, 农田防护林, 投影误差, 影缩误差, 数字图像处理

Abstract: Both projective and contractive aberrations of shelterbelt porosity estimated by measuring photograph with the help of digitized photographic silhouettes are further studied on the basis of "Study on random error of shelterbelt porosity estimated by measuring photograph with the help of digitized photographic silhouettes". The regular patterns of "the closer the image of shelterbelt is to main point on a photograph, the less absolute value of projective aberration in the porosity is", and "contractive aberration in the porosity is irrelevant to the location of shelterbelt on a photograph" are expounded,and the fact that absolute value of projective aberration in the porosity is negatively related to both focal length of camera and distance of taking a picture is clarified. Of projective and contractive aberrations in the porosity under crown of the shelterbelt disposed respectively by 3 main patterns of rectangle, triangle and oblique line, the models are established. Based on synthesizing the results of study on the errors in the porosity, a series of measures from calculating the number of observation and taking a picture in field to measuring photograph in laboratory up to calculating the errors are advanced to shrink and restrict all errors in the porosity as far as possible,to emend both projective and contractive aberrations in the porosity under crown of shelterbelt, and to estimate the porosity according to known or given error limit.

Key words: Porosity, Farmland shelterbelt, Projective aberration, Contractive aberration, Digitized photographic silhouettes