Inundation Mapping using time series RADASART-SAR images In the Lower Making River Basin
inundation mapping through image analysis
The Thabok area near Vientiane in the Lao People's Democratic Republic is selected for the investigating, and acquired observation data over four different times. it is found that peak inundation occurred on August 17 in 1997, and observed 10 days later and again 40 days later in order to track the change in distribution on inundated area as the inundation receded. In addition, we obtained an image acquired in December 1996 to identify the water boundary in the dry season and thus re-create the conditions before inundation occur. These SAR images were processed for each timing of capture and finally combine to from an image, or inundation map (table 2).
| Data of capture | Beam mode | Orbit direction | Situation |
| August 17, 1997 | Wide 1 | Ascending | Occurrence of inundation |
| August 27, 1997 | Standard 5 | Ascending | 10 days after occurrence |
| September 28,1997 | Wide 1 | Ascending | 40 days after occurrence |
| December 20,1997 | Wide 1 | Ascending | Dry season |
Table 2 Specification of RADARSET-SAR images used in the investigation
Procedure of inundation mapping
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Obtain SAR images .
- Apply geometrical correction to the SAR images so that those taken at different times cab be overlapped.
- Define the submerged area from the SAR images.
- Remove speckle-noise from the identified submerged area.
- Creator a mask of the water area throughout the year from the SAR image taken in the dry season
- Combine the SAR images taken at four different times.
- Conduct site investigation to check whether the inundation map matches actual conditions.
- Conduct site investigation to check whether the inundation map matches actual conditions.
A SAR images is corrected geometrically using GPS site observations to allow accurate overlapping with other SAR images taken at different times as well as with the LANDSAT/TM image taken in the dry season (Imazato, Rikimaru, et al., 1998) . in order to divide a SAR image into submerged and non-submerged areas , the areas of the main channel of the Mekong River and Lake Nam Ngum lying to the north of Vientiane are first identified from the image s typical, identifiable water areas. From an image histogram of these, water areas are defined for the whole image . by using data of water area obtained from the image taken during the dry season in December 1996, we created mask data the at shows the regular water area throughout the year. The mask data is used to exclude from an image taken during the rainy season those water areas considered to have existed before the inundation. This processing is applied to all of the SAR images taken at different times. consequently, a time series inundation is mapped as shown in figure 1.
Figure.1 Inundation map basded on times series RADARSAT-SAR images
verificaitn by site investigation
We conducted a series of site investigation at four different times before and after the SAR observations. In the site investigations, we surveyed the actual inundation conditions in the field, and compared the results with the inundation map. The site investigations showed that the actual distribution of t inundated area almost matched the distribution given by the inundation map. However , much of the area was inundated even before August 17 that were considered as the peak of the inundation in this study. We also showed that the inundated area then was larger than that of the peak inundation in august. Therefore, the inundation mapped in this study did not cover all of the inundated area in the investigated area.
Conclusions
We developed a method of assessing the actual conditios of inundatin that occurs frequently in Lower Mekong River Basin by usintg time series RADARSET-SAR images. Ground investigations showed that the inundated area identified by the SAR observations almost matched the actual inundated area at the same timing. However, another large inundation had occurred before we conducted the SAR observation, and we did not succeed in inundation mapping that could cover all the inundated area throughout the rain season.
In future we shall attempt to completely cover the maximum inundated area. we must build a system that allows SAR observations to be conducted promptly after acquiring reliable inundation information from the site itself, and then perform proper analysis.
Acknowledgments
This investigation was made possible with the Expert of the Japan International Cooperation Agency (JICA) in Lao, Mesrs. Hashimoto, Hirodo and Nakai. Thanks are also due to the staff of the Dept. of irrigation, and the Dept. of Hydro-meteorology, of the Ministry of Agriculture and Forestry of the Laotian Government, and the Mekong River Commission.
Reference:
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IMAZATO, Akihiko, RIKIMARU, Atsushi, et al. (1988): "Geographical correction for satellite images by matching path data obtained through mobile GPS observation," Japan Society of Photogrammetry and Remote Sensing, Papers for lectures in academic year 1998, pp. 195-198, 1998 ( in Japanese ).
