In the initial stage, the main function of airborne lidar was to make topographic maps using the echoes bounced back from the ground, while the role of airborne lidar in forest resource investigation was ignored. In recent years, the use of airborne lidar technology in forest resource surveys has been gradually used by some European forestry resource management experts. Airborne lidar data is usually used in the three-dimensional structure of the forest, but the most important thing is the use of monitoring and forest resource investigation. For traditional optical remote sensing technology, if you want to obtain the three-dimensional structure parameters of forest trees, you need to use its auxiliary information, otherwise you can only obtain two-dimensional information. For airborne lidar data, you can directly obtain the three-dimensional structure parameters, making the forest vertical The estimation of distribution data can be realized directly through three-dimensional visualization. This method is very effective for obtaining individual tree parameters, such as tree crown height and tree height.
1 Use airborne lidar data to measure tree height
By calculating the distance difference between the laser echo from the ground and the top of the tree canopy, the airborne lidar system measures the tree height. As an important data reflecting site quality and volume, the acquisition of tree height information is very useful. Some studies have found that the average height of the first echo of the airborne lidar in the pine forest area and grassland is greatly related to the average height of the target object. However, the laser echo probability is very low at the top of the needle industry canopy, which causes a problem that the height of the tree detected by the airborne lidar is lower than the true value. Some research institutions conduct systematic research on 36 forest stands. The final conclusion is that there is a 91% huge correlation between the height of the tree in the field survey and the height of the forest obtained. However, the height of the tree in the field survey is generally 4.1~5.5 m higher than the height of the forest.Later, the average tree height weighted by the height of the individual tree's breast height and the cross-sectional area is used, so the calculated tree height is still 2.1~3.6 higher than the actual tree height. m. Finally, multiple grids of equal area are distributed in the forest stand, and the height of the tallest tree in each grid is taken as the average tree height. Finally, the error between the actual tree height and the measured tree height is reduced to 0.4~1.9 m. It is currently believed that this method is
The tree height is very accurate.
2 Measure forest density
After identifying the top of the canopy to obtain the trees per unit area is the forest stand density. Therefore, the focus of the study is the division of the canopy. First, we need to use the airborne lidar data to form a canopy height model, and then select the changing window to search for the local maximum value, and use the largest laser point in the elevation value as the top of the canopy. Circular and rectangular shapes are the main shapes of the search window. The height of the book affects the size of the window. As the height of the tree increases, the crown will increase and the search window will be larger. Researchers use the changed window to find the local maximum to identify the tree. The top is used to estimate the trees and trees of each sample plot. The final result shows that the correlation coefficient between the estimated value of the broad-leaved tree plot and the pine plot and the actual value is 0.26. Here, the dominant tree species
And dominant tree species have the highest survey accuracy.
3 Estimation of forest biomass and stock volume
If you want to use airborne lidar technology to detect stock volume and stand biomass, then the general method used is to use statistical analysis to establish a regression model for estimation. Diameter at breast height, trees, and tree height are all model parameters. Therefore, using airborne lidar data to accurately survey the diameter at breast height, trees, and tree height is a link that must be carried out to obtain biomass and accumulation parameters. In addition, the DBH, height and volume of trees have a certain correlation with the growth law of trees. The basic basis of the height information retrieved by lidar for forest biomass modeling is the special constraint function of this tree. However, tree species are closely related to the geographical environment and the related growth equations of trees. Usually, due to the lack of local tree growth equations, statistical regression schemes are adopted. Research generally believes that airborne lidar data can be combined with ground surveys to survey the stock volume and tree height of forest areas with high-tech resolution. Using the local maximum method to obtain the crown width and the height of a single tree is the first task in the study, and then calculate the breast height and cross-sectional area and diameter at breast height through the regression model, and calculate the volume on the basis of the existing biomass and volume calculation formulas. Calculate separately with biomass, and the final estimation accuracy is higher than that measured only with airborne lidar data.
4Application prospects of airborne lidar technology in forest survey
41 Limitations of airborne lidar technology
Even though airborne lidar has certain advantages in obtaining plant information and forest landforms, its practical application still has certain limitations, such as the systematicness of the theoretical system, the lack of data processing software and data resources, and it is comparable to other remote sensing technologies. Compared with, the cost is higher, and for airborne lidar technology, because discrete sampling is used, if the sampling error at the top of the tree will cause the tree height estimation to be wrong. In order to avoid this problem, it is often necessary to increase the sampling density and reduce the flying height, which leads to an increase in funds. Airborne lidar technology is a technology that intersects multiple disciplines, including measurement, remote sensing, signal processing and other disciplines. If you want to give full play to its advantages and functions, you must make all disciplines collaborate with each other.
4.2 Application prospects of airborne lidar technology
The development of airborne lidar technology has broad prospects and can be used in new measurement systems in many fields. It provides a new solution for obtaining spatial information with high temporal and spatial resolution. In terms of forest resource survey, its advantage is that it can directly measure the height of vegetation and obtain the vertical structure information of forest areas, which makes up for the deficiencies of other remote sensing methods. The fusion of other remote sensing data and airborne lidar technology can provide more comprehensive and accurate forest parameters, and is conducive to the development of new quantitative remote sensing technologies.