1 Composition of airborne LiDAR system
The airborne LiDAR system mainly includes:
(1) GPs (global positioning system): Calculate the position of a high-speed moving aircraft based on the ground base station GPS and airborne GPs.
(2) INS (i nerti al navigation system) is an inertial measurement system for acquiring aircraft attitude and acceleration.
(3) A device used by the laser scanning ranging system to obtain the distance between the laser launch point and the ground measurement point.
(4) Imaging device (CCD digital camera): used to obtain color digital images corresponding to the ground for the final production of digital orthophotos.
(5) Working platforms Fixed-wing aircraft, airships or helicopters are working platforms. Airborne LiDAR technology to obtain terrain data and data processing procedures and methods
2.1 LiDAR data collection and acquisition of digital images
The LIDAR system can receive multiple reflections, record the intensity information of each reflection, and provide dense lattice data with a horizontal spacing of 0.5-2m and an accuracy of 0.15-1m, a vertical spacing of 2-12m, and an accuracy of about 10-20cm. The attached high-precision cCD camera can simultaneously acquire high-definition image data.
(1) Make a flight plan. The flight plan should include the division of the flight zone to determine the flight height, speed, laser pulse frequency, flight zone width, laser mirror rotation speed, digital camera orientation elements and positioning, camera shooting time interval, etc.
(2) Place GPs base station. In order to ensure the accuracy of the three-dimensional coordinate data at each time of the flight, a certain number of GPs base stations must be deployed along the route on the ground, and the GPs rover stations must be placed on the aircraft.
(3) Laser scanning measurement. The distance from the sensor to the ground can be accurately measured according to the time interval from laser emission to reception.
(4) Inertial measurement. When the airplane is flying, the inertial measurement device also measures the airplane's flight attitude and records the scanning angle of the laser-related data on the magnetic tape.
(5) Obtain high-definition digital images. The function of LiDAR to directly obtain the three-dimensional coordinates of the point provides the height information that traditional two-dimensional data lacks. At the same time, it also uses the high-resolution digital camera to synchronously obtain the true color or infrared digital image information of the ground features and geomorphology to generate DEM products. It can be used as a data source to classify and identify targets or as a texture data source.
2 Airborne LiDAR data processing
The airborne LiDAR raw data undergoes a preprocessing stage to generate a digital surface model DSM, and then through data filtering and feature extraction, the terrain and ground features related to modeling can be obtained before subsequent applications.
(1) Li DAR raw flight data. Airborne GPS and ground base station GPS spatial position data, inertial navigation system data, laser scanning data, laser reflection intensity information and echo data, raw digital images.
(2) Route reconstruction. Route reconstruction provides data support for the later flight belt splicing and side check. Through the joint difference calculation of the ground base station GPS data and the airborne GPS data, the aircraft flight trajectory can be accurately determined.
(3) Elimination of systematic errors and abnormal values of laser data. When processing the raw data of laser ranging, anomalous points must be eliminated, that is, singular points whose range is far greater than the flying height or invalid data with a very small range value.
(4) Calculate the three-dimensional space coordinates of the laser point. Using random commercial software provided by LiDAR hardware manufacturers, the aircraft GPS trajectory data, INS aircraft attitude data, laser ranging data and laser scanning mirror swing angle data are jointly processed, and finally (X, Y, Z) of each measurement point is obtained. Three-dimensional coordinate data.
(5) Coordinate conversion. The positioning information provided by the dynamic positioning of the GPS/I NS combined system belongs to the WGS-84 coordinate system. If the measurement result belongs to other coordinate systems, the coordinate conversion problem of the positioning result must be solved.
(6) Point data read and write. The data generated by the LiDAR system is initially processed by the hardware manufacturer and then delivered to the user. The format of the original data generated is different due to different product manufacturers.
(7) Splicing of flight belts. Using high-definition ground images acquired by flight synchronization can determine and eliminate systematic errors between flight belts. The purpose of flight belt splicing is to improve the data accuracy of the overlapping area and meet the continuity of the edge features.
(8) Multi-source data registration. The imaging modes of remote sensing images are diversified, and data of different sensors, different scales, and different time phases can usually be obtained in the same area. Therefore, when fusing these multi-source data, image registration technology must be applied to correct various images. The difference between.
(9) Filtering. The basic principle of LiDAR point cloud filtering is based on the sudden changes in elevation between adjacent laser foot points, which are generally not caused by the abrupt undulations of the terrain. It is more likely that higher points are located on certain ground objects. The methods currently used for airborne Li DAR data filtering can be roughly divided into mathematical morphology filtering method, moving window method, iterative linear least square interpolation method, and terrain-based slope filtering.
(10) Manual editing. The purpose of manual interactive editing is to eliminate automatic filtering, automatic classification of the unfiltered parts of the rough search and unclassified laser points.
(11) Side check. In order to ensure the completeness and accuracy of the features in the border area, algorithms and visual interpretation methods based on features are needed.
(12) Generate DEM/DSM. The LiDAR data processed above is subjected to operations such as interpolation to generate DEM and DSM that can meet engineering standards.
3 Application of Airborne LiDAR Technology
Airborne LiDAR technology can not only be used to directly generate DSM data, but also can be widely used in forestry, electric power, urban feature extraction, water conservancy, coastal topographic mapping, geological disaster survey, national security, etc.
(1) Digital city application. LiDAR systems can better reflect the advantages of not being restricted by altitude and shadow occlusion in cities, and have more distinctive data processing. They are widely used for large-scale terrain data acquisition in urban planning.
(2) Transportation and pipeline design and monitoring applications. LiDAR technology provides high-precision DEM for highway and railway design to facilitate line design and accurate calculation of construction earthwork.
(3) Power design, survey, line selection and line monitoring application. When designing power lines, LiDAR data can be used to understand the terrain and features of the entire line design area.
(4) Disaster investigation and environmental monitoring. Mainly used in post-disaster assessment and response to natural disasters (such as hurricanes, earthquakes, floods, landslides, etc.).
(5) Coastal engineering. Airborne LiDAR is an active sensing technology that can perform routine basic coastline surveys in high dynamic environments at low cost, and has a certain underwater detection capability. It can measure underwater terrain within 70m of coastal waters and can be used in coastal zones and seasides. Three-dimensional measurement and dynamic monitoring of sand dunes, coastal dikes and coastal forests.
(6) Forestry applications. According to LiDAR data, analyze the coverage and coverage of forest trees, understand the density of trees, the coverage area of older trees and the coverage area of young trees. Through LiDAR data, it is possible to estimate the area occupied by the forest, the average height of the trees, and the amount of wood, which is convenient for relevant departments to carry out macro-control.
(7) Protection of cultural heritage. Large-scale cultural relics and historical relics that cannot be moved outdoors need to measure their three-dimensional data for restoration and protection. For cultural relics that are in harsh measurement environments or cannot be directly touched, LiDAR technology has become a good solution for directly obtaining 3D data.www.isurestar.net