Keywords: 
• forest mensuration /
• total station /
• two－side intersection /
• free station method /
• 3D coordinates /
• tree

Abstract: In forest inventory，poor visibility caused by trees and shrubs often lowers the positioning accuracy and efficiency，sometimes even makes the survey difficult to carry out. Free－station method of total station can work well in poor visibility conditions. Due to relatively low accuracy required in forest inventory and in order to calculate conveniently in field，two－side intersection method is more suitable. When observing two sides，the method of missing line measurement can timely detect accuracy of measured length. It is unnecessary to aim straight at the mark and measure of instrument height if three－dimensional coordinates of the instrument center are directly calculated. Practices indicate that the side length error of inventory unit can be less than 5 cm，the relative error is less than 1/200，and closing error of sample plot is less than 1/2 of averagee distance between adjacent trees.［Ch，2 fig. 3 tab. 12 ref.］