Abstract：Due to the limitation of field condition，single scan line method has been applied to estimate randomly distributed joint trace length on occasion，namely，sampling bias correction of random joint trace length. In order to obtain more precise joint trace length，it is proposed that (1) the method of probabilistic delineation of orientation data is used to delineate randomly distributed joint trace to obtain the predominant joint clusters，(2) each observed joint trace length and mean value of trace length are obtained for every cluster，(3) a probabilistic numerical method is used to model the probabilistic density function (PDF) of trace length for each clusters，and (4) according to the PDF of trace length for each cluster，semi-trace length scan line method is applied to correct the sampling bias of observed trace length for each cluster and to obtain the corrected mean value of trace length. The presented research results prove reasonable by actual engineering project.
