一 The development status of airborne lidar at home and abroad
The development of airborne lidar measurement technology has a history of more than 20 years. From the early American aerospace laser ranging to the world's first commercial prototype laser profile measurement system born in Germany, it has developed in recent years with current technology With the rapid advancement of technology, the lidar system has developed rapidly, and its market share in the surveying and mapping market has grown rapidly year by year. At present, there are already many commercial systems in use around the world, such as TopScan, Optech, TopEye, Saab, Fli-map, TopoSys, Hawk2Eye and many other practical systems. Representative systems mainly include Lite Mapper 6800 jointly produced by German IGI and Austrian RIGEL, American Alpha's SHOLAS and Canadian OPTECH's ALTM3100T, German TopoSys' Falcon, and American Leica's Leica ALS50/60.
From the mid to late 1990s to the present, the United States, Germany, Canada and other countries have successfully applied this technology in experiments and engineering practices such as topographic surveying, forest resource investigation and assessment, and 3D city modeling. Especially in Finland and Germany, it is more widely used.
Domestically, related products have been put into actual production and application in the terrestrial 3D laser scanning system and vehicle-mounted laser radar system. However, there is a large gap between domestic and foreign in the hardware research and manufacture of airborne lidar technology, and the existing technology foundation is relatively weak. Although there is the development of the principle prototype, there is still a long way to go before practicality, especially the product formation. Therefore, there is no mature airborne lidar system in China.
二Application of airborne lidar in surveying and mapping
1. Main features and performance of airborne lidar measurement technology
High precision: the height accuracy can be less than 15 cm.
High-density: High-density point cloud data can truly reflect topography and landforms, replacing field elevation measurements. High efficiency: The data processing speed is fast, which is convenient for automated production. The digital surface model can be obtained within a few hours of the flight data, and the high-precision digital surface model and orthophoto can be obtained within a few days.
High resolution: High resolution digital images can be obtained. Measurement method: active measurement, can penetrate thin clouds and mist, greatly reduce the dependence on weather, and can work at night.
The high degree of automation: From flight design to data acquisition, to data processing of the final product, the degree of automation is very high. No need for extensive ground control work.
It has a certain ability to penetrate vegetation and can be measured to the ground below the vegetation.
2. Application of airborne lidar measurement in surveying and mapping
First of all, in traditional terrain mapping: airborne lidar can quickly obtain high-precision digital surface models (DSM), especially digital photogrammetry is difficult to obtain DEM in forest-covered areas, because laser pulses can penetrate the vegetation canopy and reflect back Wave signals can be used to generate DEM in forest areas and other vegetation-covered areas. After processing, an accurate digital elevation model (DEM) can be obtained. At the same time, the matching medium-format digital camera can obtain synchronized digital photos. After the correction of the digital elevation model (DEM), the orthophoto map (DOM) of the survey area can be obtained. ), you can also use the high-resolution digital stereo image pairs of these aerial photos to produce digital line drawing (DLG) through image control and air three encryption, and then convert DLG to digital imagery (DRG) if necessary. Therefore, the airborne lidar measurement can satisfy the production of 4D digital products on our traditional surveying and mapping.
Application in other industries: The laser point cloud data produced by airborne lidar and the processed 3D real scene animation are also great for emergency response, urban planning, homeland security, transportation, public utility management, and environmental monitoring departments. Application value. Use airborne lidar data combined with plot vector data to generate a three-dimensional city model, which can perform complex spatial queries and flood inundation analysis. After the Wenchuan earthquake, many landslides and dammed lakes appeared in the earthquake area. The DEM obtained by airborne lidar provided relevant departments with high-precision basic data of the dammed lake basin, assisting experts to accurately and quickly judge and read out in a short time. The location and volume of the landslide effectively supported the risk assessment of geological disasters and the formulation of risk elimination plans.
In terms of traffic pipeline design: LIDAR technology provides high-precision ground elevation model DEM for highway and railway design to facilitate line design and accurate calculation of construction earthwork. It also has important application value in the design of communication networks, oil pipes and gas pipe lines.
In particular, it should be noted that power transmission projects and power line patrols are currently a large number of domestic engineering projects that apply lidar technology, which greatly facilitates grid deployment and maintenance management projects. When designing power lines, LIDAR data can be used to understand the terrain and features of the entire line design area. Especially in densely wooded areas, the area and amount of wood that need to be felled can be estimated. During power line repair and maintenance, the height of any line from the ground can be calculated based on the LIDAR data points on the power line and the corresponding elevation of the exposed point on the ground, which can facilitate repair and maintenance.
Its derived types include site topographic maps, cross-section maps, digital orthophoto topographic maps, special surveying and mapping, automatic calculation of earthwork in engineering, calculation of slope and aspect, etc.; rapid construction of 3D virtual scenes, city modeling, etc.
3. Point cloud data processing of airborne lidar
Point cloud data processing is the decompression, difference, IPAS calculation, laser and camera calibration, point cloud data generation, etc. of raw data such as laser point cloud data, aerial image data, IPAS data and ground base station data obtained from aerial photography. The process to obtain parameters and data that meet the requirements of DSM, DEM, DOM, DLG production.
The ground elevation model data DEM only needs exposed points on the ground. The ground elevation model DEM is composed of the three-dimensional data of exposed points on the surface of the complete plot. In production, we use LIDAR software to automatically process most of the redundant laser pulse point data on the ground surface to obtain the three-dimensional data information of the exposed points and construct the triangulation TIN graphics. The LIDAR software divides into multiple height difference segments according to the elevation value of the pulse point and assigns different color values to render into a triangular mesh. The resulting LIDAR image has a very obvious color three-dimensional effect. Reuse the program to process and identify non-surface exposed points, such as most tree high points, building points, bridge deck points, etc., which are manually removed and classified. It can be roughly divided into the following types: exposed ground points, tree high points, bridge deck points, water points, building points, noise points (that is, gross points), and other unclassified points. The ground point data after data processing can not only generate high-precision DEM and topographic map contours but also can further correct aerial photos to produce orthophoto maps.
At present, there are many models of domestic equipment, basically all mainstream products. In addition to Finnish software, data processing software also uses software provided by equipment manufacturers. Among them, the most widely used and most typical LiDAR processing software is the TerraSolid series of third-party LIdar data processing software in Finland. It is the first commercial LiDAR data processing software, developed based on Microstation and running on the Microstation system, so users who are familiar with Microstation operation can use it well. It includes four important modules: TerraMatch, TerraScan, TerraModeler and TerraPhoto.
TerraSolid series software can quickly load lidar point cloud data for processing.
三. Huge changes and prospects brought by airborne lidar
Airborne Lidar (LIDAR) is a new type of sensor equipment. The laser is used for echo ranging and orientation to directly obtain high-precision digital surface models. It can also provide auxiliary data for target classification and recognition by collecting information such as position, radial velocity and object reflection characteristics (ie echo intensity data) . At the same time, the LIDAR system can carry an aerial multi-spectral CCD camera, and can directly register with the LIDAR point cloud through hardware or post-processing software, so it has the ability to obtain multi-spectral CCD images at the same time, providing rich data for subsequent applications Resources. Although the LIDAR system originated from the laser ranging technology in traditional engineering surveying, it is an emerging technology in the contemporary photogrammetry and remote sensing field due to its combination with modern laser and POS positioning technology. It not only provides new types of photogrammetry and remote sensing applications The means of data acquisition also poses new challenges to data post-processing methods and procedures.https://www.isurestar.net/