Laser-Scanning: A comprehensive evaluation of its potential for high resolution digital surface model generation, forest stand parameter estimation, and single tree recognition.

For the forest inventory the knowledge of the available timber volume is of greatest importance which has traditionally been estimated by statistic methods from the collection of tree data in samples, i. e. at specific arbitrary positions. In recent times aerial photogrammetry has gained more importance which allows to collect certain tree data regionally. Both methods are expensive and limited in their accuracy due to methodological constraints. Since a few years airborne laser scanners have been available for commercial use and are operated by a growing number of private companies throughout the world. These instruments allow to collect both ground and (tree) canopy points three-dimensionally. From an airplane, laser pulses are sent down in strips across the flying direction. These pulses are reflected by leaves, ground, or other objects. The time between sending the pulse and receiving the echoes is measured and allows to calculate the distance from the airplane to the reflecting object by knowledge of light speed. Position and attitude of the airplane is measured continuously by satellites (GPS) and a so-called inertial measurement system. Available systems today allow to collect up to more than 80,000 object points per second. During the current project laser data were collected by three laser flights in summer respectively winter in the research forest of the Viennese University of Agricultural Sciences in the Rosalia mountains 60 km south of Vienna. A total area of about 10 sq. kilometers was collected with a mean point density of 8-10 points per sq. meter, resulting in approx. 300 Mio. ground and canopy points. The study consisted of three steps, trying to find out which information relevant to forest inventory can be reliably estimated from the laser data: 1. Statistical methods (regression analysis) were used to try to obtain the same parameters from the laser data as were known from the inventory data and special ground reference measurements. It could be shown that laser data can be used to reliably estimate tree height, proportion of conifers and deciduous trees, and crown density. 2. During the second step information was derived from the laser data that has not been used in forestry so far, particularly canopy "roughness" of forest stands and size of gaps within stands. These measures were checked against photogrammetric imagery and ground reference data that were both collected during the project. It was possible to split a forested area according to criteria such as homogeneity of tree species, crown density, tree size, or vitality. Such segmentations are of particular interest for the so-called uneven-aged forestry which is gaining growing importance especially in mountainous areas (protective forests) since there is a most versatile mixture of trees of different size, species and age. 3. As a third step single tree recognition was tried by means of the crown forms. The trees found such way are classified by the methods that were developed in steps one and two. Thus the parameters can be estimated both tree by tree and for whole stands. During the project it could be shown that laser-scanner data are very well suited to yield certain specific parameters relevant to forest inventory for a whole region which could be collected with high efforts and only in samples so far. Furthermore, several methods were developed in order to extract these parameters from the laser-data both for single trees and for forest stands. The progress in technology will reduce mission costs for the laser flights during the next couple of years down to a level that the methodology will be widely used in forestry. On the other hand it became obvious that collection of ground reference data and photogrammetric imagery will still remain important. Most promising is the combination of both image and laser data. So new techniques of forest inventory are arising at the horizon that will soon shape and ease everyday work of foresters.