Comparative Evaluation of NDWI, NDVI, MNDWI, AWEI and EWI for Sentinel-2 Shoreline Delineation in Klungkung, Indonesia: KDE-Valley vs Otsu Thresholding Sakina Hasan(a*), Abd Rahman As-syakur(b), Xuan Truong Trinh(c), and Masahiko Nagai(d)
a*, b) Faculty of Marine and Fisheries, Udayana University, Indonesia
*hasan.2213511062[at]student.unud.ac.id
c) Faculty of Engineering, Yamaguchi University, Japan
d) Center for Research and Application of Remote Sensing, Yamaguchi University, Japan
Abstract
Shoreline detection is essential for coastal monitoring amid accelerating climate change and intensified human activities. This study examines the coastal area of Klungkung, which experiences recurrent erosion and abrasion. We evaluated six spectral indices-NDVI, NDWI, Modified NDWI (MNDWI), Automated Water Extraction Index-shadow (AWEI-SH), AWEI-non-shadow (AWEI-NSH), and the Enhanced Water Index (EWI)-using Sentinel-2 imagery to delineate the shoreline. Thresholds were derived with Otsu method and a kernel density estimation (KDE) valley approach applied to manually selected samples. Shorelines from each index were validated against the Last Vegetation Line (LVL) surveyed with a Garmin GPS, and mean distances to the LVL were compared. Results (mean distance error, meters) show that NDWI (6.26 m) and NDVI (6.45 m) outperformed MNDWI (10.50 m) and EWI (9.70 m). For NDWI, KDE-based thresholding yielded higher accuracy than Otsu method (6.26 m vs. 10.23 m). The lower performance of MNDWI and EWI likely reflects the use of 20-m Sentinel-2 bands, which reduced delineation precision. NDVI performed on par with NDWI, plausibly because the validation shoreline is defined by the vegetation line. Overall, NDWI and NDVI-particularly when paired with appropriate thresholding-provide high-accuracy shoreline detection in this setting. Future work should test higher-resolution sensors and extend the approach to region-wide shoreline monitoring.
Keywords: Coastal erosion- Last Vegetation Line (LVL)- Field validation (GPS)- Water indices- Shoreline Monitoring
