Terrestrial 3-dimensional measurements of tree crowns

Tharandt

2006

S. 73-85

16 Abb., 6 Lit. Ang.

Artikel aus einem BuchUnselbständiges Werk

200133473

In the frame of a PhD thesis, conducted at the former Chair of Silviculture (ETH Zürich) in Cooperation with the Chair of Photogrammetry and Remote Sensing, two alternative versions of a photogranmietric method for crown studies of standing trees were developed and investigated. Photogrammetry allows for a non-contact measurement of 3D coordinates of stem, branches and shoot points on a tree. These data can be used for the delineation of the crown orthogonal projection and for the determination of the crown increment for a given period backwards. In this project, trees wtih heights of 20-23 m from a 50 years old Scots pine high-pole stage stand were investigated. With this method, field work for data acquisition can be accomplished by one person in about 10 minutes per tree, under ideal weather conditions. We used conventional digital cameras (3.3 and 5 Megapixels), with the Option to work with either one or two cameras. In the first case, two cameras were fixed to a metal bar and mounted on a camera stand (tripod). The 3D coordinates of the projective centers were determined by measurements toward the focal tree. In the second case, nadir Images of the crown periphery and oblique Images of the tree stem and crown were acquired. Additionally, the coordinates of two reference points at the stem base were measured. Photogrammetric Image processing then was conducted using PhotoModeler Pro (http://www.photomodeler.com), a commercially available photogrametric Software package. The Images were oriented relatively and a 3D model was measured. Beforehand, camera calibration had to be conducted, which also could be performed in PhotoModeler Pro. A tree crown projection can be delineated by 30 - 80 points, whereas processing time is not influenced significantly by the number of points. Absolute orientation of the 3D model can be achieved by means of the acquired coordinates of the projective centers in the first case and by means of distance and orientation with respect to the reference points in the second case. The coordinates of the crown points were exported after measurement and further processed using MS-Excel, MS-Access, ArcGIS (ESRI) or special visualization software. The generation of a 3D crown projection model for one tree consumes about 1-4 hours for an experienced operator, additionally 1-2 hours for the delineation of the crown increment. The accuracy of the coordinates under optimal conditions can be estimated as about 1cm in horizontal direction and 1-4cm vertically. For this reason, the hitherto most accurate method for tree crown modeling was developed. It could serve also for: detection of trees slightly damaged by wet snow (small crown shift due to local exceeding of the plasticity limit of the stem), studying inter- or intra species concurrence between individual deciduous trees.