Damage Assessment Of Buildings/Structures In The View Of High Resolution Satellite Stereo Images

High resolution satellite stereo images(HRSSIs)have the characteristics of wide amplitude and short response period,which is suitable for the building damage assessment.Gaofen-7(GF-7)is China’s first submeter stereo mapping satellite,and the laser altimetry system equipped on it is used to improve its elevation positioning accuracy.However,challenges still exist to assess building damage,such as difficulties in obtaining the ground control points in time,inconsistency of stereoscopic positioning accuracy between HRSSIs before and after the earthquake,etc.To solve these problems,a three-dimensional(3d)precise damage assessment method for building collapse is proposed using spaceborne laser altimetry data and Gao Fen-7(GF-7)stereo images before and after the earthquake and the key processing technology in this process is studied.The main researches of this paper are as follows:1.For the problems encountered in the application of GF-7 to the damage assessment of buildings/structures in the earthquake disaster,such as the lack of elevation positioning accuracy of HRSSIs,the inconsistency between threedimensional positioning accuracy of HRSSIs before and after the earthquake,and the difficulty of automatic extraction of building corners,etc.This paper presents a combined adjustment method using GF-7 and its laser altimetry points and its subsequent damage assessment method for buildings/structures.The method can be described as follows: Firstly,the ICESat-1 laser altimetry points(LAPs)and LAPs acquired by GF-7 serve as elevation control points(ECPs),and the tie points from HRSSIs before and after the earthquake were used as constraint to conduct combined adjustment to improve the consistency of stereoscopic positioning accuracy between HRSSIs before and after the earthquake.Subsequently,for the purpose of obtaining building corners which were used to finish building damage assessment using HRSSIs,a coarse-to-fine,sub-pixel building corners extraction method was performed.Finally,according to the height change of building corners calculated from HRSSIs,building damage assessment was completed.In this study,for the earthquake that occurred in Oaxaca,Mexico in 2020,HRSSIs were used to verify the 3d precise assessment of building damage.The results manifest root mean square error of the elevation positioning accuracy improved from 10.29 to 0.95 m.Three areas were selected for assessment in the disaster area,and the accuracy rates reached better than 93%.The elevation positioning accuracy of GF-7 may be limited by the absence of a laser altimetry system.Therefore,it is necessary to study the combined adjustment of external laser altimetry points(LAPs)and GF-7 stereo images.In this study,we proposed a method consisting of registering LAPs to GF-7 stereo images followed by combined adjustment.The registration method adopts a coarse-to-fine scheme: the LAPs are reprojected to stereo images to obtain coarse coordinates,then fine coordinates are acquired by least-squares matching and using the epipolar geometry constraint.The proposed method can effectively improve the registration accuracy: the root mean square error(RMSE)of the registration is sub-pixel,compared with about three pixels when only random sample consensus(RANSAC)is applied.At the same time,the number of LAPs is also increased tenfold when using the proposed method.In the subsequent combined adjustment,the RMSE of the elevation positioning is decreased from about 5.5 to 1.3 m.Based on the above researches,this paper completed the combined adjustment of the GF-7 HRSSIs and laser altimeter data before and after the earthquake to improve the elevation positioning accuracy of the HRSSIs before and after the earthquake and the consistency of the three-dimensional positioning accuracy between the two phases,and applied it to the subsequent damage assessment of buildings/structures.At the same time,combined with the characteristics of Gaofen-7 stereo image,Gaofen-7 laser altimeter data and external laser altimeter data and the core algorithm described above,a set of damage assessment system for buildings/structures in earthquake disasters is designed and developed.The system design is based on the needs of damage assessment of buildings/structures in earthquake disasters,and focuses on the requirements of timing,timeliness and light weight of data processing.According to the functions of the system,there are three main modules: data import module,data processing module and result analysis module.It provides a certain basis for the rapid assessment of earthquake disaster in the future and has a certain reference significance.