Nine techniques for estimating forest overstory cover were tested for precision and differences in mean overstory cover above plots. Two observers were used to test repeatability and possible interactions of four techniques with the overstory. The most precise instrument in the spherical densiometer, but it gives higher estimates of overstory cover than all except one of the other techniques tested. There was an interaction of person and plot with the convex spherical densiometer. Techniques that project wider angles result in higher mean estiamtes of overstory cover. Techniques using angles greater than 10* are biased with respect to direct overhead (vertical) overstory cover.
Vales, David J., Bunnell, Fred L.. 1985. Comparison of Methods for Estimating Forest Overstory Cover. Ministries of Environment and Forests. Wildlife Bulletin. B-36