EO Low Tide Line

This product displays the latest low tide line (and previous years), derived from Landsat and Sentinel-2 optical images coupled with a tidal model. This contour delineates the low-tide bathymetry and can be used to:

• Provide near-real time information of the state of the intertidal bathymetry.
• Highlight the presence of transient features like sand banks, shoals and channels.
• Identify potential obstacles that can impair navigability in harbour entrances and delay operations.
• Support better planning and execution of dredging works by understanding past migration rates and direction of sand transport.
• Provide historical insights to better understand geomorphological changes in tidal bathymetry.

This product was funded by the European Space Agency (ESA) under the EO for the Port of the Future (EO4PORTS) initiative. This research and development project aims to employ Earth Observation data to provide useful insights for port and maritime operators in Europe.

Methods: EO Low Tide Line
The step-by-step methodology developed to extract the Low Tide line from publicly available satellite imagery is described below:

1. Retrieve imagery of AOI
All the available Landsat 5, 7, 8, 9 and Sentinel-2 images are retrieved from their respective collections.


2. Assign a tide level to each image
The global tide model developed by CNES FES2022 (Finite Element Solution) is used to assign a tide level to every satellite image.


3. Compute a low-tide NDWI composite
The satellite images captured at low tide (bottom 30% of the springs tidal range) are used to compute a composite NDWI (Normalized Difference Water Index) image. The 70th percentile is taken when calculating the composite to give more weight to dry pixels.


4. Extract the sand/water contours
Use the NDWI contrast between water and wet sand to extract the low-tide line from the composite as a vector line.

This process is repeated for each 10x10km grid cell shown on the map.

Downloads: EO Low Tide Line

Download the annual Low Tide Line for the North Sea region (as GeoJSON):

• 2020 Low Tide Line
• 2021 Low Tide Line
• 2022 Low Tide Line
• 2023 Low Tide Line
• 2024 Low Tide Line