Measuring individual tree height using a combination of stereophotogrammetry and lidar

Photogrammetric methods using parallaxes can be employed to measure tree heights on aerial photographs. Because it is often impossible to measure ground elevation near trees growing in dense forests, such height measurements remain prone to error. Our objective was to solve this problem by combining a stereomodel and a digital terrain model (DTM) produced by an airborne-scanning system that uses light detection and ranging (lidar). A stereopair of scanned aerial photographs was first registered to a lidar DTM. The elevation of the apex of 202 Thuja occidentalis (L.) individuals was measured by an observer on a digital photogrammetric workstation. The tree base elevations were read from the lidar DTM and subtracted from the corresponding apex elevations to calculate individual tree heights. These were then compared with the heights measured in the field. The average photo-lidar bias was 0.59 m, and the average deviation of 1.01 m decreased to 0.88 m when the bias was removed. It was demonstrated that the photographic clearness of the tree apices influences the height error, while the density of the lidar echoes under the forest canopy does not. Using this method, retrospective studies of changes in tree height become feasible by using archived aerial photographs and recent lidar DTMs.