A new method for shoreline extraction based on the mountain shadow index reveals detailed changes in the coastline of eastern Zhejiang coast, China

In recent years, coastlines have undergone significant changes due to human activities and environmental changes. Precise extraction of the coastline is especially necessary. However, in complex terrains such as mountainous coastal areas, the spectral similarity between mountain shadows and water bodies can lead to misclassification, reducing the accuracy of coastline extraction. To address this issue, this study proposed a visually-based Mountain Shadow Index and based on which we established a new coastline extraction method suitable for both high and low turbidity waters. This method achieves higher accuracy in areas with mountain shadows and turbid waters compared to the NDWI and MNDWI. Using this method, the study analyzed changes in the coastline of eastern Zhejiang over the past 20 years. The results show a clear seaward advancement of the coastline, with a shift from a jagged to a smoother profile. This transformation is primarily attributed to large-scale land reclamation projects, driven by increasing land-use pressures in the region. Moreover, the newly established method can be applied to other coastal areas worldwide.