Determination of the tridimensional shape of canopy gaps using two hemispherical photographs

The goal of this paper is to develop an approach to measuring tridimensional shape of canopy gap (TSCG), which is one of the most important factors affecting the understory light environment. TSCG may be expressed as a series of measurements: the gap size, the gap shape, and the canopy height in each azimuthal direction. The proposed method is based on two vertically separated hemispherical photographs of the canopy gap, and is thus named the two hemispherical photographs (THP) method. The small displacements between corresponding physical features on the two photographs can be used to measure the TSCG through a series of geometric equations based on the polar projection of the fisheye lens. In order to test the accuracy of this method, gap sizes were also measured by several ground-based methods, and the mean canopy height was measured by LaserAce hypsometer in a case study. The effects of two factors (the camera location and height difference between two photos) on the THP measures (the gap size, gap shape and mean canopy height) were also evaluated. Our results indicate that the THP measures of the gap size and mean canopy height have strong positive correlation and no significant difference with their corresponding alternatives. THP thus has a good accuracy. Moreover, ANOVA results show that all THP measures are not significantly affected not only by the height difference, but also by the location of the camera when the image is processed at 10° intervals in this sample. In addition, the THP measures of the gap size and gap shape are not influenced by the rugged canopy gap and the small-scale landform. In conclusion, THP is a rapid, simple, objective and accurate approach to the determination of TSCG.