Forest canopy gap fraction from terrestrial laser scanning

A terrestrial laser scanner (TLS) was used to measurecanopy directional gap fraction distribution in forest standsin the Swiss National Park, eastern Switzerland. A scanner modelwas derived to determine the expected number of laser shots inall directions, and these data were compared with the measurednumber of laser hits to determine directional gap fraction ateight sampling points. Directional gap fraction distributions weredetermined from digital hemispherical photographs recorded atthe same sampling locations in the forest, and these data were comparedwith distributions computed from the laser scanner data.The results showed that the measured directional gap fractiondistributions were similar for both hemispherical photographyand TLS data with a high degree of precision in the area of overlapof orthogonal laser scans. Analysis of hemispherical photographyto determine canopy gap fraction normally requires some manualdata processing; laser scanners offer semiautomatic measurementof directional gap fraction distribution plus additional threedimensionalinformation about tree height, gap size, and foliagedistributions