Monitoring stand development within the Mt. St. Helens blast zone using a twin-camera helicopter boom

Twenty-five 6.5-ha regeneration plots were established in the Mt. St. Helens blast zone following salvage logging operations. Five different silvicultural thinning treatments have been applied to these plots. In July 1995, a twin-camera helicopter boom system was used to acquire very large-scale (1:850) stereopairs within each 6.5-ha plot. In addition, an overflight of the area was conducted to acquire standard 65% forward overlap stereo photos at a scale of 1:9,000. These overflight photos were used to locate each 1:850 stereopair within each 6.5-ha plot so that data can be gee-referenced and entered into a geographic information system. The 1:850 stereopairs are being used to make individual tree measurements and other site parameters, such as crown closure and size and distribution of understory vegetation and large woody debris. A PC-base analytical stereoplotter is being used to collect all photo measurements. Because the distance between the cameras on the helicopter boom is known, a local coordinate system of known scale can be established for each 1:850 stereomodel. Distances and heights can be measured to within 5% of field-measured values, without the need for ground control. By using an analytical stereoplotter to perform orientations on each stereomodel, accurate measurements can be made in areas with large amounts of relief displacement, even if the camera lines of sight are not parallel.