Point Clouds Classafication Of LiDAR Based On Alpha Shapes Algorithm

Conventional marine remote sensing inversion technology has shortcomings about low timeliness and precision-challenged.Single beam and multi-beam can't measure offshore,reefs and unsafe ocean area.For overcoming their shortcomings,the paper consider using airborne Light Detection and Ranging system(Red &Green-Blue light,abbreviation LiDAR)to solve these questions.LiDAR can obtain high-precision water and topography features point cloud quickly.It can be used to map bathymetry and topography,target detection,marine chlorophyll concentration detection,sea surface oil monitoring,wave characteristics and other fields.In addition,as we can consider taking into account the national conditions,the area within 50 meters of china coastal has about 500000 square kilometers.Especially in the South China Sea,it has strategic island reef and other areas which has 130,000 square kilometers areas below 50 meters depth water.LiDAR has great advantages in its fast,wide coverage,high precision sounding for coastal protection,reclamation,port construction,defense and providing more accurate basic data.Nowadays,there are few methods for underwater point cloud but is mainly used for land point cloud.In this paper,3D Alpha shapes algorithm used to extract the characteristic points of underwater terrain feature points and other features,which can realize the fast processing of LiDAR data including water underwater point cloud.The paper analyzes the composition of the error source and the final point cloud data structure and its characteristics after stating the composition and principle of LiDAR.There are three methods to elvaluate precision of point cloud classification methods.They are visual method,random sampling method and crosstalk method.Based on the scientific and rigorous nature,this paper chooses the crosstalk method to evaluate the accuracy of the classification of 3D Alpha shapes.In addition,the paper states current point clouds classification algorithms,princeple of 3D Alpha shapes and its application in land point cloud classification with precision evaluation.Therefore,we analysis LiDAR's accuracy and applicability in marine surveying and mapping by comparing the measured cloud data and chart data.In addition,we invent a software AlphashapesEditor.The software integrates 2D&3D alpha shapes algorithms to process the land-water point clouds.The result can be output to txt or other format to further processing.We analysis the result of 3D alpha shapes processing and the result of commercial software processing by crosstalk methods comparision.We can know precision of 3D alpha shapes in extraction of underwater topography and other objects.In the paper,LiDAR system is systematically studied and compared.It is verified that LiDAR has the feasibility of rapid and high precision integrated waterborne measurement in difficult sea area,beach,island reef and so on.The paper analyzes the difference between the point cloud density and the reflection intensity.Through comparison of the domestic equipment to collect data and foreign equipment to collect data,we found that domestic research hardware equipment has been more mature,but there is a gap with foreign equipment,mainly in poor quality,poor data read-ability and so on.Based on the 2D Alpha shapes algorithm,the 3D Alpha shapes algorithm is compiled.Result of land point cloud by 3D alpha shapes perform well because the algorithm is based on objects' geometric shape.The article uses the Alpha shape algorithm for the first time underwater point cloud filtering.LiDAR obtain the underwater point cloud data and then the three-dimensional Alpha shapes algorithm operation and post-processing,which can quickly get rid of the singular point point cloud data.The results show that the algorithm has high precision and fast processing speed after after the deduction in accordance with ASPRS recommended accuracy assessment method.We invent a software which on the basis of C++ prepared on the basis of windows system ‘Alphashapeseditor'.The program has the functions of reading point cloud,displaying basic information of point cloud,3D&2D alpha shpes algorithm,2D/3D windows display,exporting alpha shapes algorithm running result.It has certain visibility and practicability.