New Zealand Journal of Forestry (2000) 45(2): 25–29
©New Zealand Institute of Forestry
Accurate stem measurements key to new image-based system.
J. G. Firth , R. K. Brownlie and W. W. Carson
Christchurch, New Zealand: New Zealand Institute of Forestry.
PhotoMARVL is a new system of dendrometry that enables accurate measurements to be made of the stem of individual trees. These data allow improved log grade estimates to be calculated using the MARVL3 pre-harvesting inventory procedure. Heights of features which are visible on the stem can be measured to an accuracy of better than + /- 10 cm, while stem and branch diameters can be measured to + / -1 cm. PhotoMARVL can also provide a reliable 3D measure of stem sweep. The method involves taking high resolution, oblique, stereoscopic photographs of the sample tree with a motorised 35 mm film camera. Six key parameters are recorded at the time of photography and these are used to define a measurement plane passing through a reference position on the tree stem.
Features of interest on the stem are identified stereoscopically and their X, Y and Z coordinates recorded. The coordinates can be processed to determine the dimension and location of features that ultimately govern the value of the log products that could be derived from the tree. Although the time required to PhotoMARVL a tree may be several minutes longer than that required to carry out a MARVL assessment, the data it provides will be more accurate and extensive.
PhotoMARVL is valuable as a broad-based mensurational tool and its output is inherently suited to tasks such as the construction of taper equations, the determination of internode lengths and the quantification of tree form. The technique could also be used in a double sampling role to improve the accuracy of MARVL assessments. Significant advantages of the technique are that PhotoMARVL data can be obtained non-destructively and without the need to climb trees. In addition, the images can be archived to provide a detailed record of tree and stand quality. This paper describes the field and analysis procedures involved in carrying out a PhotoMARVL assessment as well as the principles of the photogrammetry employed.
Keywords: Dendrometry; Forest Inventory; MARVL; PhotoMARVL; Pre-harvest Inventory; Terrestrial Photogrammetry.