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Application of Airborne Lidar in Forest Resource Investigation

1 The composition of the airborne lidar system

The main components of the airborne lidar system are the global positioning system, laser scanning system, monitoring and control system, and inertial measurement device. Together they constitute the airborne lidar system. Among them, the core part of the airborne lidar is the airborne lidar sensor. The sensor is mainly used to transmit and measure laser pulses and receive the echoes of the pulses that bounce back due to obstacles. It is undoubtedly the Global Positioning System that provides positioning services for the airborne lidar system. Inertial measurement devices are mainly used to monitor the flight status of the platform where the airborne lidar system is located. While analyzing the airborne lidar data, the monitoring and control system also provides relevant operators with necessary real-time monitoring information. This information contains a lot of content, including the operating status of the global positioning system, inertial measurement devices, and sensors. The flight path of the aircraft platform. There are other types of digital scanning systems equipped in some airborne lidar systems. In order to achieve higher resolution of multi-spectral digital images, these images provide some information for the research of airborne lidar data.

2 The working principle of the airborne lidar system

If the airborne lidar system wants to measure the distance d between the sensor and the object to be measured, it needs to calculate, first measure the time t from the pulse sent from the sensor to the sensor receiving the pulse echo due to the ground object bounced back, and The propagation speed c of the laser during this time. So according to the formula, d is equal to half of the product of propagation velocity c and propagation time t. During data collection, measurement pulses are continuously emitted from the airborne lidar system, and the monitor records the three-dimensional coordinates of the echo generating points corresponding to the measurement pulses one by one.The coordinate information formed by the recorded three-dimensional point set is the machine. The survey results provided by the on-board lidar system. The airborne lidar is divided into two links, which are transmitting measurement pulses and receiving the echo signals that bounce back. After each beam of measurement pulse is emitted, due to the different composition of ground objects in its coverage area, this has an impact on the measurement and will cause different reflection strengths. Therefore, the strength of the signal received by the sensor and the echo The quantity is also different. Measuring the number of multiple echoes of a pulse is a function of many sensors. This technology is called single-beam multi-echo technology. Multi-echo airborne lidar data is very effective for the analysis of individual standing trees in areas with sparse trees, because in areas with sparse trees, the pulses emitted by the airborne lidar sensor can reach the ground without encountering obstacles. , The branches of different heights will bounce the echo signals, which describe the hierarchical information of the canopy. Therefore, the multi-echo technology is very suitable in the open forest area.

3 Application of airborne lidar data in forest resources

In the early days, the main function of airborne lidar was the main function of airborne lidar in the production of topographic maps using 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.

3.1  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 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 surveyed 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 that of the forest stand.Later, the average tree height weighted by the breast height of a single tree 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 used 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.

3.2 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 in 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 survey of the dominant and dominant species The highest accuracy.

3.3   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 for forest biomass modeling based on height information retrieved by lidar is the special constraint function of this tree. However, tree species are closely related to the geographical environment and 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 the measurement using only airborne lidar datawww.isurestar.net