Extraction of ground surface elevation from ZY-3 winter stereo imagery over deciduous forested areas

Forest height is an important indicator for forest biomass. Ground surface elevation is essential to derive forest height from spaceborne data including interferometric synthetic aperture radar or stereo imagery. Considering the good performance of stereo images in characterizing the vertical structure of forest with non-closed canopies, the main issue addressed in this study is whether stereo imagery acquired in winter can view the ground surface under dense deciduous forest. To make full use of information provided by different observation geometries, the three sets of matching points from different view were combined. Then the vertical distribution of matching points from stereo images was referenced to the vegetation vertical structure and the ground surface elevation from airborne laser scanner (ALS) data. The vertical distribution of matching points from stereo images over typical deciduous forest stands including sparse, disturbed and dense forests were examined. Most matching points were located on the ground surface while some points came from branches and trunks in all the three forest stands. This phenomenon was also observed from a transect of a digital surface model from ZY-3. Twelve elevation indices, including minimum, maximum, mean elevations and an additional nine percentiles of cumulative probability (E10 to E90) from the matching points of ZY-3 over 30 m × 30 m cell were compared with the ground surface elevation from ALS data. The results showed that E30 gave the best measurement of ground surface elevation with R2 > 0.99 and RMSE = 2.54 m.