空間分解能の異なる2種の光学衛星データを用いた土砂流出範囲抽出手法におけるΔNDVI閾値の違い—2021年8月豪雨の長野県岡谷市および佐賀県神埼市の崩壊地の解析事例—

抄録

This study targets the surrounding areas of Okaya City, Nagano Prefecture (Okaya sites), and Kanzaki City, Saga Prefecture (Kanzaki sites), where heavy rainfalls caused landslides in August 2021, and uses optical satellite data from two types of optical satellites with different observation conditions and sensors. We extracted the sediment runoff range by calculating the difference value (ΔNDVI) and compared the accuracy, and investigated the difference in the ΔNDVI threshold for distinguishing new bare land from other areas.

The relationship between precision (x) and recall (y) can be approximated by the quadratic equation y＝ax²＋bx＋c, and there was a trade-off relationship between the two in that as the threshold rose, precision increased while recall decreased. Comparing the quadratic approximation curves at the Okaya and Kanzaki sites, the values at the Okaya sites were higher overall, suggesting that the value of the solar zenith angle in the satellite data may have had an effect. Furthermore, when comparing satellite data, the precision of the ΔNDVI threshold case with the maximum F-measure is 0.71 for Sentinel-2 and 0.72 for Planet, and the recall is 0.70 for Sentinel-2 and 0.75 for Planet. The average of the ΔNDVI threshold, which indicates the maximum F-measure, was 0.19 for Sentinel-2 and 0.22 for Planet, and differed depending on the satellite data used for analysis. The lower ΔNDVI threshold in Sentinel-2 was thought to be due to the lower recall due to coarser spatial resolution compared to Planet. These findings suggested that the ΔNDVI threshold that optimizes the extraction accuracy of the sediment runoff range differs depending on the satellite data used for analysis.