Large-scale 35-mm aerial photographs for assessment of vegetation-management research plots in eastern Canada

Abstract

Two 35-mm cameras were mounted on a boom and suspended from a tethered helium-filled blimp to obtain nominally vertical aerial photographs (1:828 and 1:414 contact scale) of vegetation-management research plots. Photo and ground estimates of woody plant crown area (m2/ha) and rootstock density (number/ha) were compared for several experimental vegetation-control treatments. Horizontal point-sampling estimates of total crown area made directly on 1:93 scale prints (enlarged from 1:414) correlated strongly with equivalent estimates made on the ground (n = 62, r2 = 0.97). An estimated 20 ground-truth plots were required to adequately quantify photo bias and correct subsequent prediction of actual total crown area on the plots studied. Much of the observed photo bias could be attributed to the undersampling of small rootstocks. Exclusion of individual rootstocks less than 0.10 m2 in crown area (or, equivalently, <56 cm in height) resulted in a 1:1 relation between the two sampling methods for estimates of both total crown area (r2 = 0.98) and rootstock density (r2 = 0.97). If data for rootstocks in smaller size classes are not needed, uncorrected photo estimates may be appropriate for evaluation of treatment response. Ground-sampling costs averaged $200 (Canadian) per plot, compared with photo costs of $104 per plot (without ground truth) or $150 per plot (with 20 ground-truth plots). Smaller scale photos (1:828 contact) cost 11% less than the larger scale tested, but resulted in significant undersampling of individual rootstocks less than 0.2 m2 in crown area (or, equivalently, <80 cm in height).