# Chart Datums¶

The chart datum is defined using the Lowest Astronomical Tide (LAT) : it is the lowest tide level which can be predicted to occur under average meteorological conditions and under any combination of astronomical conditions. [LAT]

The different water height are defined using this chart datum (see following figure):

- \(B(x,y)\) : bathymetry, varying in space,
- \(H(t)\) : tidal height, given by the tidal model,
- \(\eta(x,y,t)\) : wave elevation, given by the wave field model,
- \(H(t)+\eta(x,y,t)\) : free surface position, relatively to the LAT,
- \(H(t)-B(x,y)\) : ocean depth, relatively to the free surface.