Role of Photogrammetry and Remote Sensing in Wenchuan Earthquake

Deren Li

An earthquake, measured 8.0 on the Richter scale, struck Wenchuan County, Sichuan Province at 14:28 on May 12, 2008. The epicenter was at Yingxiu, a town in Wenchuan, as shown in Figure 1. As indicated in Figure 1, the middle segment of the Longmen Mountain earthquake zone encloses the epicenter, Yingxiu, with the Longmen Mountain zone
being part of the north-south earthquake zone in China. According to records, there have been 9 earthquakes larger than a grade of 8 occurring in the north-south earthquake zone from 1739, and 7 of them were after 1897, when the greatest one was at Haiyun in 1920 and at Chayu in 1950, both of them measured 8.5.

Figure 1. The Wenchuan earthquake in context

In Wenchuan earthquake, the worst hit areas include Beichuan and Qingchuan, with the earthquake intensity measured at 11, with the maximum being 12 by the Chinese seismic survey standards. By 12:00 pm on June 14, 2008, the death toll read 69,170, with 374,159 people injured, 17,428 people missing,and a population of 48,270,000 severely affected by this huge disaster. Figure 2 shows the beauty of Beichuan before the earthquake against the destruction and devastation after the earthquake.

Figure 2. Pictures showing Beichuan before and after the earthquake.

Under the leadership and organization of the Chinese government and Premier Wen Jiabao, and with the support of many countries and people around the world, the Chinese people braved against the devastating earthquake, and carried out timely and active disaster relief work. Photogrammetry and remote sensing, as high-tech, has played an important role in the fight against this natural disaster. In Phase I, rescue of people buried under the rubbles was the main goal. Highresolution aerial and satellite imagery were used to locate buildings collapsed in the worst hit areas so that rescuers were dispatched. In Phase II, prevention of and preparedness for secondary disasters, i.e., landslides and mudslides, especially, those in and around the quake lakes or barrier lakes that are formed when a landslide plugs a river, top the agenda. Air-borne and space-borne optical imagery and radar data are required for identifying, assessing, and decision-making regarding locations prone to such secondary disasters. Phase III is concerned with disaster assessment and reconstruction. It is then necessary to undertake topographic mapping at 1:10,000 scale in the region with an areal extent of 120,000 km2 based on the technique of aerial photogrammetric survey without ground control points, generating information products, such as DEMs, DOQs, and DLGs. Topographic mapping in urban areas is performed at a larger scale of 1:2,000. These will better serve the people in the disaster areas so that they can outline reconstruction of their homes.

Figure 3. Aerial photography flown with ADS40 (GSD: 0.3 m) showing what was left of Yingxiu Town in Wenchuan after the earthquake on May 15, 2008.

Figure 4. The image acquired by Cosmo (at a resolution of 1 m) indicating the locations (white segments) where buildings are likely collapsed after the earthquake; up to 14.8% of the areas were suspected to be results of collapsed buildings.

In the struggle against the destructive quake in Sichuan, Chinese photogrammetry and remote sensing professionals have, with supports from colleagues around the world, made various contributions as follows. For Phase I, rapid surveys and assessment of the disasters are important, as shown by Figures 3 and 4, with the former being aerial photography flown with ADS40 (GSD: 0.3 m) showing what was left of Yingxiu Town in Wenchuan after the earthquake on May 15, 2008, the latter being an image acquired by Cosmo (at a resolution of 1 m) indicating the locations (white segments) where buildings are likely collapsed after the earthquake; up to 14.8% of the areas were suspected to be results of collapsed buildings.

Figure 5. The change of river water ways due to the forming of quake lakes in Tangjiashan, as indicated by white regions generated by comparing SPOT5 (10 m resolution) images taken before (November 10, 2006) and after (May 16, 2008) the earthquake.

Figure 6. The DEM of the quake lake in Tangjiashan, Mianyang city, Sichuan Province, one of the area worst affected by Wenchuan earthquake, which was generated based on ALS50 II air-borne LiDAR data with a sampling interval of 2 m, flight date, May 31, 2008, by the State Bureau of Surveying and Mapping, Wuhan University, and Wuda Geo Information Company (the barrier dam is shown with a inset picture)

Deren Li

Phase II was marked by the prevention of and preparedness for secondary disasters, i.e., landslides and mudslides, especially, those in and around the quake lakes or barrier lakes. Figure 5 clearly highlights the change of river water ways due to the forming of quake lakes in Tangjiashan, as indicated by white regions generated by comparing SPOT5 (10 m resolution) images taken before (November 10, 2006) and after (May 16, 2008) the earthquake. Figure 6 shows the DEM of the quake lake in Tangjiashan, Mianyang city, Sichuan Province, one of the area worst affected by Wenchuan earthquake, which was generated based on ALS50 II air-borne LiDAR data with a sampling interval of 2 m, flight date, May 31, 2008, by the State Bureau of Surveying and Mapping and Wuhan University (the barrier dam is shown with an inset
picture) in collaboration. The boundaries of rivers in the quake lakes, Tangjiashan, Mianyang city, Sichuan Province, are shown in Figure 7, as indicated by the green lines against the backgroundRadarsat imagery at a resolution of 7 m, acquired on May 17, 2008.

Figure 7. The boundaries of rivers in the quake lakes, Tangjiashan, Mianyang city, Sichuan Province, as indicated by the green lines against the background Radarsat imagery at a resolution of 7 m, acquired on May 17, 2008.

In addition to disaster relief efforts, D-inSAR can also be used for scientific research on earthquake. Preliminary results derived from ALOS PALSAR satellite acquisition on June 11, 2008 are released at www.gmat. unsw.edu. au/LinlinGe/ Earthquake, as shown in Figure 8, thanks to the efforts of the team led by Dr Linlin Ge. Dr Ge used predicted orbit data in order to produce and deliver the result in NEAR REAL-TIME: 17 hours and 46 minutes, from image capture to results posting on web. The Japanese ALOS PALSAR sensor acquired the Path
477 image on 11 June 2008 1:37 Sydney Time AEST (10/06/2008 15:37:07 UTC; 10/06/2008 23:37 Bejing Time). Image data were available to download from ERSDAC at 11:24AM. Image data (using predicted orbit) were downloaded to UNSW on 11/06/2008 at 16:55. D-InSAR results were generated on 11/06/2008 at 19:05. Post-processed D-InSAR results were uploaded on 11/06/2008 at 19:23.

Figure 8. The image showing the aftershocks, generated by D-inSAR technology based on ALOS PALSAR sensor acquisition, Path 477 image on 11 June 2008 1:37 Sydney Time AEST.

Some concluding remarks are as follows. It has been demonstrated that photogrammetry and remote sensing has played a crucial role in the aftermath of the earthquake in Wenchuan, Sichuan. The rapid data acquisition and information services, especially, those featured with fully automatic, near realtime remote sensing systems without ground control, have contributed greatly to the rescue work and disaster relief
efforts. The specialty of photogrammetry and remote sensing is becoming increasingly visible and its roles are growingly recognized. It is important to develop a national disaster rapid response system that corroborates the work by different agencies and facilitates data and resources sharing. It is necessary to further develop China’s National Spatial Data Infrastructure. Moreover, it should be on the agenda to enhance the capability of high-resolution earth observation systems. Last but not the least, it is necessary to strengthen international cooperation in spatial information science and technology, like International Charter and CEOSS from GEO.

Acknowledgements

Many institutions and people have helped with data acquisition and provision of materials during the project concerning rapid response to Wenchuan earthquake. The diligent work by and kind helps from colleagues home and abroad are gratefully acknowledged.