AUTOMATIC ESTIMATION OF INDIVIDUAL TREE POSITIONS FROM AERIAL PHOTOS

An automatic method is suggested far estimating the positions of indiv idual trees from panchromatic aerial photos of even-aged homogeneous s tands of Norway spruce (Picea abies (L.) Karst.). The scanned photo is smoothed by a two-dimensional isotropic Gaussian kernel such that the number of local maxima above the most frequent grey level is approxim ately equal to the number of trees. The positions of trees at ground l evel are estimated by use of a displacement model incorporating the an gle to the sun, the camera position, and estimated tree heights. Param eters of the model are estimated from data for a thinning experiment w ith six thinning treatments and field measurements of the individual t ree positions at ground level. The displacement model is used for matc hing trees and maxima. It is found that for medium and heavy thinning about 95%, and for light thinning about 85%, of the trees can be detec ted and that the root mean square residual error in the displacement m odel is about 65 cm. For estimation of tree positions at ground level additional errors due to uncertainties in the heights may become impor tant, in particular for trees far from the nadir point.