Use of a series of aerial photographs to estimate growth of trees and standards
Creators: name: Wang, Yumin
Publisher: name: University of British Columbia
Year: 2011

description: Although the conventional methods of predicting growth of trees and stands by the measurements of variables on the ground yield a considerable degree of accuracy, they take a long time and are expensive. Therefore, prediction of growth of trees and stands by the use of aerial photographs as developed herein may be preferable in some situations. A survey was made of ten series of aerial photographs taken at various times during the year with the use of several kinds of photography. One series was omitted because of too much exaggeration of relief. Of the nine series, three were regarded as inadequate for growth studies due to poor photography. Repeated measurements were made for 135 sample trees on different series of photographs in terms of total height and crown width. The tree images were classified as good, medium and poor according to the visibility of both top and the base of tree, and the accuracy of height measurements was defined in terms of standard error of the mean difference from photo-measurements and the ground data. In addition to the conventional method of taking an average of four parallax readings or two identical readings, the writer set up a checking method by which a high degree of accuracy was secured. The standard error of the mean difference for 95 heights of good image trees was ± 0.23 feet. When the good image trees were classified as conifers or hardwoods, the standard errors of the mean differ- ences were ± 0.30 feet and ± 0.41 feet respectively. Generally the measurements of height for conifers appeared to be more accurate than those for hardwood. There was no consistent result indicating which species gives better measurements. For height growth estimation, Height/Age curves were used as a supplement to ground data, and a method of pooling errors was applied. The use of this method for determining the accuracy of growth prediction was assumed to be applicable, and height estimates based on direct measurements on different series of photographs yielded a considerable degree of accuracy, if good quality photographs were available. The accuracy of crown width measurements was fairly high. The smallest standard error of the mean difference was ± 0.21 feet for 53 Douglas fir and the largest standard error of the mean difference was ± 0.58 feet for 12 alders. For growth estimates of crown width, the indirect approach of using Crown width/dbh ratios was used as a substitute for ground control. It was concluded that the prediction of growth of crown width can be made from a series of aerial photographs. The application of theoretical values as criteria in determining the acceptability of growth estimates of crown width might be practicable because variation will be reduced by the joint computation of two pairs of observations. However, when Crown width/dbh ratios are used as control, there tends to be a great deal of error, partly because of the use of ratios. Accuracy of growth studies on three different groups of plots varies with the composition of stand. The group of young natural regeneration plots showed a relatively high degree of accuracy. It was concluded that where the stand is comparatively open, growth estimates on the photographs would give a useful degree of accuracy.